causes and effects of obesity essay conclusion

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Obesity: causes, consequences, treatments, and challenges.

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Obesity: causes, consequences, treatments, and challenges, Journal of Molecular Cell Biology , Volume 13, Issue 7, July 2021, Pages 463–465, https://doi.org/10.1093/jmcb/mjab056

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Obesity has become a global epidemic and is one of today’s most public health problems worldwide. Obesity poses a major risk for a variety of serious diseases including diabetes mellitus, non-alcoholic liver disease (NAFLD), cardiovascular disease, hypertension and stroke, and certain forms of cancer ( Bluher, 2019 ).

Obesity is mainly caused by imbalanced energy intake and expenditure due to a sedentary lifestyle coupled with overnutrition. Excess nutrients are stored in adipose tissue (AT) in the form of triglycerides, which will be utilized as nutrients by other tissues through lipolysis under nutrient deficit conditions. There are two major types of AT, white AT (WAT) and brown AT, the latter is a specialized form of fat depot that participates in non-shivering thermogenesis through lipid oxidation-mediated heat generation. While WAT has been historically considered merely an energy reservoir, this fat depot is now well known to function as an endocrine organ that produces and secretes various hormones, cytokines, and metabolites (termed as adipokines) to control systemic energy balance. Studies over the past decade also show that WAT, especially subcutaneous WAT, could undergo ‘beiging’ remodeling in response to environmental or hormonal perturbation. In the first paper of this special issue, Cheong and Xu (2021) systematically review the recent progress on the factors, pathways, and mechanisms that regulate the intercellular and inter-organ crosstalks in the beiging of WAT. A critical but still not fully addressed issue in the adipose research field is the origin of the beige cells. Although beige adipocytes are known to have distinct cellular origins from brown and while adipocytes, it remains unclear on whether the cells are from pre-existing mature white adipocytes through a transdifferentiation process or from de novo differentiation of precursor cells. AT is a heterogeneous tissue composed of not only adipocytes but also nonadipocyte cell populations, including fibroblasts, as well as endothelial, blood, stromal, and adipocyte precursor cells ( Ruan, 2020 ). The authors examined evidence to show that heterogeneity contributes to different browning capacities among fat depots and even within the same depot. The local microenvironment in WAT, which is dynamically and coordinately controlled by inputs from the heterogeneous cell types, plays a critical role in the beige adipogenesis process. The authors also examined key regulators of the AT microenvironment, including vascularization, the sympathetic nerve system, immune cells, peptide hormones, exosomes, and gut microbiota-derived metabolites. Given that increasing beige fat function enhances energy expenditure and consequently reduces body weight gain, identification and characterization of novel regulators and understanding their mechanisms of action in the beiging process has a therapeutic potential to combat obesity and its associated diseases. However, as noticed by the authors, most of the current pre-clinical research on ‘beiging’ are done in rodent models, which may not represent the exact phenomenon in humans ( Cheong and Xu, 2021 ). Thus, further investigations will be needed to translate the findings from bench to clinic.

While both social–environmental factors and genetic preposition have been recognized to play important roles in obesity epidemic, Gao et al. (2021) present evidence showing that epigenetic changes may be a key factor to explain interindividual differences in obesity. The authors examined data on the function of DNA methylation in regulating the expression of key genes involved in metabolism. They also summarize the roles of histone modifications as well as various RNAs such as microRNAs, long noncoding RNAs, and circular RNAs in regulating metabolic gene expression in metabolic organs in response to environmental cues. Lastly, the authors discuss the effect of lifestyle modification and therapeutic agents on epigenetic regulation of energy homeostasis. Understanding the mechanisms by which lifestyles such as diet and exercise modulate the expression and function of epigenetic factors in metabolism should be essential for developing novel strategies for the prevention and treatment of obesity and its associated metabolic diseases.

A major consequence of obesity is type 2 diabetes, a chronic disease that occurs when body cannot use and produce insulin effectively. Diabetes profoundly and adversely affects the vasculature, leading to various cardiovascular-related diseases such as atherosclerosis, arteriosclerotic, and microvascular diseases, which have been recognized as the most common causes of death in people with diabetes ( Cho et al., 2018 ). Love et al. (2021) systematically review the roles and regulation of endothelial insulin resistance in diabetes complications, focusing mainly on vascular dysfunction. The authors review the vasoprotective functions and the mechanisms of action of endothelial insulin and insulin-like growth factor 1 signaling pathways. They also examined the contribution and impart of endothelial insulin resistance to diabetes complications from both biochemical and physiological perspectives and evaluated the beneficial roles of many of the medications currently used for T2D treatment in vascular management, including metformin, thiazolidinediones, glucagon-like receptor agonists, dipeptidyl peptidase-4 inhibitors, sodium-glucose cotransporter inhibitors, as well as exercise. The authors present evidence to suggest that sex differences and racial/ethnic disparities contribute significantly to vascular dysfunction in the setting of diabetes. Lastly, the authors raise a number of very important questions with regard to the role and connection of endothelial insulin resistance to metabolic dysfunction in other major metabolic organs/tissues and suggest several insightful directions in this area for future investigation.

Following on from the theme of obesity-induced metabolic dysfunction, Xia et al. (2021) review the latest progresses on the role of membrane-type I matrix metalloproteinase (MT1-MMP), a zinc-dependent endopeptidase that proteolytically cleaves extracellular matrix components and non-matrix proteins, in lipid metabolism. The authors examined data on the transcriptional and post-translational modification regulation of MT1-MMP gene expression and function. They also present evidence showing that the functions of MT1-MMP in lipid metabolism are cell specific as it may either promote or suppress inflammation and atherosclerosis depending on its presence in distinct cells. MT1-MMP appears to exert a complex role in obesity for that the molecule delays the progression of early obesity but exacerbates obesity at the advanced stage. Because inhibition of MT1-MMP can potentially lower the circulating low-density lipoprotein cholesterol levels and reduce the risk of cancer metastasis and atherosclerosis, the protein has been viewed as a very promising therapeutic target. However, challenges remain in developing MT1-MMP-based therapies due to the tissue-specific roles of MT1-MMP and the lack of specific inhibitors for this molecule. Further investigations are needed to address these questions and to develop MT1-MMP-based therapeutic interventions.

Lastly, Huang et al. (2021) present new findings on a critical role of puromycin-sensitive aminopeptidase (PSA), an integral non-transmembrane enzyme that catalyzes the cleavage of amino acids near the N-terminus of polypeptides, in NAFLD. NAFLD, ranging from simple nonalcoholic fatty liver to the more aggressive subtype nonalcoholic steatohepatitis, has now become the leading chronic liver disease worldwide ( Loomba et al., 2021 ). At present, no effective drugs are available for NAFLD management in the clinic mainly due to the lack of a complete understanding of the mechanisms underlying the disease progress, reinforcing the urgent need to identify and validate novel targets and to elucidate their mechanisms of action in NAFLD development and pathogenesis. Huang et al. (2021) found that PSA expression levels were greatly reduced in the livers of obese mouse models and that the decreased PSA expression correlated with the progression of NAFLD in humans. They also found that PSA levels were negatively correlated with triglyceride accumulation in cultured hepatocytes and in the liver of ob/ob mice. Moreover, PSA suppresses steatosis by promoting lipogenesis and attenuating fatty acid β-oxidation in hepatocytes and protects oxidative stress and lipid overload in the liver by activating the nuclear factor erythroid 2-related factor 2, the master regulator of antioxidant response. These studies identify PSA as a pivotal regulator of hepatic lipid metabolism and suggest that PSA may be a potential biomarker and therapeutic target for treating NAFLD.

In summary, papers in this issue review our current knowledge on the causes, consequences, and interventions of obesity and its associated diseases such as type 2 diabetes, NAFLD, and cardiovascular disease ( Cheong and Xu, 2021 ; Gao et al., 2021 ; Love et al., 2021 ). Potential targets for the treatment of dyslipidemia and NAFLD are also discussed, as exemplified by MT1-MMP and PSA ( Huang et al., 2021 ; Xia et al., 2021 ). It is noted that despite enormous effect, few pharmacological interventions are currently available in the clinic to effectively treat obesity. In addition, while enhancing energy expenditure by browning/beiging of WAT has been demonstrated as a promising alternative approach to alleviate obesity in rodent models, it remains to be determined on whether such WAT reprogramming is effective in combating obesity in humans ( Cheong and Xu, 2021 ). Better understanding the mechanisms by which obesity induces various medical consequences and identification and characterization of novel anti-obesity secreted factors/soluble molecules would be helpful for developing effective therapeutic treatments for obesity and its associated medical complications.

Bluher M. ( 2019 ). Obesity: global epidemiology and pathogenesis . Nat. Rev. Endocrinol . 15 , 288 – 298 .

Google Scholar

Cheong L.Y. , Xu A. ( 2021 ). Intercellular and inter-organ crosstalk in browning of white adipose tissue: molecular mechanism and therapeutic complications . J. Mol. Cell Biol . 13 , 466 – 479 .

Cho N.H. , Shaw J.E. , Karuranga S. , et al.  ( 2018 ). IDF Diabetes Atlas: global estimates of diabetes prevalence for 2017 and projections for 2045 . Diabetes Res. Clin. Pract . 138 , 271 – 281 .

Gao W. , Liu J.-L. , Lu X. , et al.  ( 2021 ). Epigenetic regulation of energy metabolism in obesity . J. Mol. Cell Biol . 13 , 480 – 499 .

Huang B. , Xiong X. , Zhang L. , et al.  ( 2021 ). PSA controls hepatic lipid metabolism by regulating the NRF2 signaling pathway . J. Mol. Cell Biol . 13 , 527 – 539 .

Loomba R. , Friedman S.L. , Shulman G.I. ( 2021 ). Mechanisms and disease consequences of nonalcoholic fatty liver disease . Cell 184 , 2537 – 2564 .

Love K.M. , Barrett E.J. , Malin S.K. , et al.  ( 2021 ). Diabetes pathogenesis and management: the endothelium comes of age . J. Mol. Cell Biol . 13 , 500 – 512 .

Ruan H.-B. ( 2020 ). Developmental and functional heterogeneity of thermogenic adipose tissue . J. Mol. Cell Biol . 12 , 775 – 784 .

Xia X.-D. , Alabi A. , Wang M. , et al.  ( 2021 ). Membrane-type I matrix metalloproteinase (MT1-MMP), lipid metabolism, and therapeutic implications . J. Mol. Cell Biol . 13 , 513 – 526 .

Author notes

Shanghai Diabetes Institute, Shanghai Key Laboratory of Diabetes Mellitus, Shanghai Clinical Center for Diabetes, Shanghai Jiao Tong University Affiliated Sixth People's Hospital, Shanghai 200233, China E-mail: [email protected]

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Obesity in America: Cause and Effect Essay Sample

It is clear that the American lifestyle has contributed to the increasing prevalence of obesity. With estimates from the Washington-based Centers for Disease Prevention in the Department of Health and Human Services indicating that one in three American adults is overweight, it is evident that the country is facing an obesity epidemic. To better understand the causes and effects of obesity, research is needed to further explore the issue. For those struggling with obesity, coursework assistance may be available to help them make the necessary lifestyle changes in order to live a healthier life.

Writing a thesis paper on the topic of obesity can be extremely challenging. It requires extensive research and time to adequately cover the subject. However, there are services available that can provide assistance with the writing process. Pay for a thesis allows for the benefit of having an experienced professional provide guidance and support throughout the entire process.

Causes of Obesity

Every phenomenon must have a reason. In order to write a cause and effect essay , you need to analyze the topic carefully to cover all aspects. Obesity is considered to be a complex illness, with a number of factors contributing to its development. These can be:

• hereditary;

As you may have guessed, it is the latter category of causes and effects that we are interested in. At this point, we care about the five ones that have made the biggest contribution.

Product Range

The main cause of obesity is junk food and an unbalanced diet rich in simple carbohydrates, fats, and sugars, plus a bunch of additives. Manufactured, processed, refined, and packaged meals are the most popular. Thanks to advances in technology, Americans have come to mass-produce meals that keep fresh longer and taste better. It takes less time to prepare unhealthy, processed foods in the microwave than it does to cook them yourself.

Lack of a work-life balance, high-stress levels, insufficient sleeping hours contribute to body weight gain. Not only do these factors contribute to this, but failing to take the time to do your homework can also have a negative impact on your physical health. Without a healthy, balanced approach to work, rest, and play, you may find yourself increasingly dependent on a sedentary lifestyle that can lead to overweight consequences. Many Americans work 50, 60, or more hours a week and suffer from a deficit of leisure hours. Cooking processed foods saves them hours and money, even though they end up costing them a lot more – by causing cardiovascular disease. In addition, obese people feel stressed on a regular basis in the United States metropolitan areas. Many of them are simply binge eating under the influence of negative emotions. Chronic overeating leads to a disturbance in the appetite center in the brain, and the normal amount of food eaten can no longer suppress hunger as much as necessary, affecting the body mass.

Food Deserts

The term ‘ food desert ‘ refers to poor areas (urban, suburban and rural) with limited access to fresh fruit, grains, and vegetables – places where it is much easier to access junk food. A grocery shop in a food desert that sells healthy foods may be 10-15 miles away, while a mini-market or cheap shop that sells harmful snacks is close to the house. In such a world, it takes much more effort to eat healthier, form eating habits, and stay slim.

Everyone’s Passion for Sweets

Consuming sweets in large quantities is addictive: the more and easier we give the body energy, the more the brain uses serotonin and dopamine to encourage it – it will make obese people want sweets again and again during the day. Cakes and pastries are fast carbohydrates that easily satisfy hunger and increase body mass. Despite the harm of sweets, obese people experience the need for them to satiate. Sweetened carbonated drinks are one of the main sources of sugar in the American diet. Moreover, some individuals may be more adversely affected by such diets than others: patients with a genetic predisposition to obesity gain body mass faster from sugary drinks than those without it. This leads to childhood obesity.

The Harm of Tolerance

Every year, the body positive movement is becoming more and more popular all over the world. It would seem that this major trend should have freed us from the problems associated with the cult of thinness and society’s notorious standards. In many ways, a positive attitude towards the body has proved fruitful. For example, the notion of beauty has clearly broadened. Now on fashion shows and magazine covers, you can see not only a girl with perfectly retouched skin and without a single hint of body fat but also an ordinary person with its inherent features: overweight, wrinkles, hair, and individual skin features. In general, all the things that we are all so familiar with in real life.

Does it really make that much sense? Is this a positive thing in terms of the cause and effect topic regarding obesity? In short, opinions are divided. Extremes aren’t easy to overcome. Not everyone manages to do it. Researchers have concluded that due to plus size having become positioned as a variant of the norm, more persons have become obese. Many obese Americans have formed the opinion that it is really quite normal, and they have become oblivious to the damage it does to their health. This is what we are going to focus on next.

Effects of Obesity

We all know that obesity is dangerous to health. However, medical studies show that most adults are unaware of the number of complications and diseases that obesity in America entails. So they are fairly comfortable with becoming gradually fatter. But indifference is replaced by concern when obesity related diseases begin to occur.

For interesting examples of students writing that also reveal the causes and effects of other phenomena, consult the custom essay service offering essays by professionals. In this way, you will realize the importance of highlighting the effects right after the causes.

Is obesity an aesthetic disadvantage, an inconvenience, a limitation in physical activity or is it an illness after all? How does it affect health, and what are the consequences? The visible signs of obesity are by no means the only complication associated with this condition. Obesity creates a high risk of life-threatening diseases such as atherosclerosis, hypertension, heart attack, myocardial infarction, and kidney and liver problems. Moreover, it can also lead to disability.

Cardiovascular Disease

This is the most serious and damaging impact on the body and blood vessels in particular. Every extra kilo is a huge additional load on the heart. Obesity increases the risk of heart attacks. Experts from the American Heart Association have developed a paper on the relationship between obesity and cardiovascular disease, which discusses the impact of obesity on the diagnosis and outcomes of patients with atherosclerotic cardiovascular disease, heart failure, and arrhythmias. Childhood obesity aggravates the course of cardiovascular disease from a very early age. The fact that even kids and adolescents are obese is associated with high blood pressure, dyslipidemia, and hyperglycemia.

The result is excessive insulin production in the body. This, in turn, leads to an overabundance of insulin in the blood, which makes the peripheral tissues more resistant to it. As a consequence of the above, sodium metabolism is disturbed, and blood pressure rises. It is important to remember that excessive carbohydrate food intake leads to increased production of insulin by the pancreas. Excess insulin in the human body easily converts glucose into fat. And obesity reduces tissue sensitivity to insulin itself. This kind of vicious circle leads to type 2 diabetes.

Effects on Joints

Obesity increases the load on joints to a great extent, especially if one undergoes little or no physical activity. For instance, if one lives in a megalopolis, where all physical activity consists of getting off the sofa, walking to the car, and plumping up in an office chair at work. All this leads to a reduction in muscle mass, which is already weak, and all the load falls on the joints and ligaments.

The result is arthritis, arthrosis, and osteochondrosis. Consequently, a seemingly illogical situation is formed – there is practically no exercise, but joints are worn out harder than in the case of powerlifters. In turn, according to a study by the University of California, reducing body weight reduces the risk of osteoarthritis.

Infertility

In most cases, being obese leads to endocrine infertility, as it causes an irregular menstrual cycle. Women experience thyroid disease, polycystic ovarian syndrome, problems with conception, and decreased progesterone hormone. Obese men are faced with erectile dysfunction, reduced testosterone levels, and infertility. It should be noted that the mother’s obesity affects not only her health but also the one of her unborn child. These children are at higher risk of congenital malformations.

Corresponding Inconveniences

Public consciousness is still far from the notion that obese people are sick individuals. The social significance of the issue is that people who are severely obese find it difficult to get a job. They experience discriminatory restrictions on promotion, daily living disadvantages, restrictions on mobility, clothing choices, discomfort with adequate hygiene, and sexual dysfunction. Some of these individuals not only suffer from illness and limited mobility but also have low self-esteem, depression, and other psychological problems due to involuntary isolation by watching television or playing video games. Therefore, the public has to recognize the need to establish and implement national and childhood obesity epidemic prevention programs.

Society today provokes unintentional adult and childhood obesity among its members by encouraging the consumption of high-fat, high-calorie foods and, at the same time, by technological advances, promoting sedentary lifestyles like spending time watching television or playing video games. These social and technological factors have contributed to the rise in obesity in recent decades. Developing a responsible attitude towards health will only have a full impact if people are given the opportunity to enjoy a healthy lifestyle. At the level of the community as a whole, it is therefore important to support people in adhering to dieting recommendations through the continued implementation of evidence-based and demographic-based policies to make regular physical activity and good nutrition both affordable and feasible for all. It is recommended to cut down on the food consumed.

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The impact of obesity: a narrative review

Benjamin chih chiang lam.

1 Family and Community Medicine, Khoo Teck Puat Hospital, Singapore

2 Integrated Care for Obesity and Diabetes, Khoo Teck Puat Hospital, Singapore

Amanda Yuan Ling Lim

3 Singapore Association for the Study of Obesity, Singapore

4 Division of Endocrinology, Department of Medicine, National University Hospital, Singapore

Soo Ling Chan

5 Division of Endocrinology, Department of Medicine, Ng Teng Fong General Hospital, Singapore

Mabel Po Shan Yum

6 Psychology Service, Khoo Teck Puat Hospital, Singapore

Natalie Si Ya Koh

7 Department of Cardiology, National Heart Centre, Singapore

Eric Andrew Finkelstein

8 Health Services and Systems Research, Duke-NUS Medical School, Singapore

Obesity is a disease with a major negative impact on human health. However, people with obesity may not perceive their weight to be a significant problem and less than half of patients with obesity are advised by their physicians to lose weight. The purpose of this review is to highlight the importance of managing overweight and obesity by discussing the adverse consequences and impact of obesity. In summary, obesity is strongly related to >50 medical conditions, with many of them having evidence from Mendelian randomisation studies to support causality. The clinical, social and economic burdens of obesity are considerable, with these burdens potentially impacting future generations as well. This review highlights the adverse health and economic consequences of obesity and the importance of an urgent and concerted effort towards the prevention and management of obesity to reduce the burden of obesity.

INTRODUCTION

The prevalence of obesity has increased significantly over the last two decades worldwide, including in Singapore, a multiethnic Southeast Asian country.[ 1 , 2 ] Recent national health surveys of adult Singaporeans suggest a continuation of this rising trend after a brief period of stabilisation.[ 3 ] Obesity, which is characterised by excessive adiposity, is not benign.

Obesity predisposes affected individuals to a large array of diseases that are often interconnected, leading to an increased risk of simple (two comorbid diseases) and complex (four or more comorbid diseases) multimorbidity in these individuals, when compared to people with healthy weight.[ 4 ] For example, in a large Finnish cohort of 114,657 people aged 16–78 years, with a mean follow-up of 12.1 years, people with obesity were five times more likely to develop simple multimorbidity and 12 times more likely to develop complex multimorbidity, with stronger associations found in people with more severe obesity.[ 4 ] This dose–response relationship between obesity and multimorbidity is also observed in other populations, including Asian populations.[ 5 , 6 ] In Singapore, the proportion of disability-adjusted life years, a composite measure of all health loss within a population, contributed by overweight and obesity, increased from 3.9% in 1990 to 6.4% in 2017, making it the fifth leading risk factor affecting health in Singapore.[ 7 ] Hence, obesity is a disease with a major negative impact on human health and has become a major global and regional health problem.

However, according to international surveys and interviews, people with obesity may not perceive their weight to be a significant problem,[ 8 ] with evidence also suggesting that less than half of patients with obesity are advised by their physicians to lose weight.[ 9 , 10 ] Hence, the purpose of this narrative review, as part of a series on obesity, is to highlight the importance of managing overweight and obesity by presenting and summarising the latest evidence on the adverse consequences and impact of obesity [ Figure 1 ]. The causal role of excess adiposity on obesity-related conditions (as established by the many epidemiological evidence already described in literature) will be explored. In addition, data from Singapore, if any, will be included in the review for each section, with the review on the economic burden of overweight and obesity focusing on Singapore.

Diagram shows a summary of the diseases and conditions associated with obesity and the potential impacts. *Supported by mendelian randomisation studies. T1DM: type 1 diabetes mellitus, T2DM: type 2 diabetes mellitus

EVIDENCE FOR CAUSAL ROLE OF EXCESS ADIPOSITY ON OBESITY-RELATED DISEASES

The hypertrophy of adipose tissue is associated with proinflammatory adipokine production and macrophage infiltration. In addition, the failure of adipose tissue to continually expand leads to lipotoxicity and ectopic fat deposition in lean tissues such as the heart, liver, pancreas and kidneys.[ 11 , 12 ] These phenomena contribute to a proinflammatory and insulin-resistant milieu and, together with increased mechanical stress due to increased adipose tissue mass,[ 11 , 13 ] are the main pathophysiological mechanisms responsible for the development of multiple medical conditions. Hence, there are reasonable pathways to link outcome to exposure (i.e. plausibility), one of the principles useful for establishing a causal relationship.[ 14 ]

This causal role of obesity is further supported by evidence from Mendelian randomisation (MR) studies. MR is an analytical technique involving genetic variants that are associated with exposures (risk factors such as obesity) as instrumental variables to investigate the effects of these exposures on an outcome of interest (e.g. a disease).[ 15 ] Since these genetic variants are fixed, randomly allocated at conception and temporally precede the outcome, MR is less likely to be subject to bias, confounding and reverse causation, which are frequent in conventional observational studies.[ 15 ] ’BMI’, ’obesity’, ’Mendelian randomisation’, ’Mendelian randomization’ and the outcome of interest (e.g. ’diabetes’) were used as search terms, and all studies relevant to this review were considered. These obesity-related diseases, with evidence for the causal role of obesity (plausible biological mechanisms and MR studies), and the various impacts of obesity will be discussed next.

Diabetes mellitus, dyslipidaemia and hypertension

As highlighted, obesity leads to insulin resistance. Additionally, elevated free fatty acid from the adipose tissues and ectopic fat deposition cause pancreatic β-cell dysfunction.[ 16 , 17 ] Hence, the various pathological mechanisms synergistically exacerbate the onset of type 2 diabetes mellitus (T2DM). In particular, visceral fat deposition plays an important role in the development of T2DM.[ 18 ] Hepatic fat worsens hepatic insulin resistance, while pancreatic fat affects insulin secretion and glucose tolerance.[ 18 ] In a meta-analysis of MR studies, genetically predicted higher body mass index (BMI) was consistently associated with T2DM, with a combined odds ratio (OR) of 2.03 (95% confidence interval [CI] 1.88–2.19) per 1 standard deviation increase in BMI.[ 19 ] In Singapore, multiple observational studies confirm the increased risk of T2DM with increasing BMI in all major ethnic groups,[ 20 , 21 , 22 ] although there might be ethnic-specific sensitivity to the effects of increasing adiposity.[ 23 ]

There is also increasing evidence for the role of obesity in the increased incidence of type 1 diabetes mellitus (T1DM), with studies from North America and the UK suggesting an increasing prevalence of overweight and obesity in people with T1DM.[ 24 ] It has been hypothesised that obesity-induced insulin resistance may be responsible for the accelerated loss of pancreatic β cells through excessive stimulation[ 25 ] and a chronic proinflammatory state.[ 26 ] This causal role is supported by MR studies, although the evidence is not as consistent (compared to obesity and T2DM), with high heterogeneity between the studies.[ 19 ]

Obesity is associated with dyslipidaemia, which is characterised by increased triglycerides (TGs) and free fatty acids, decreased high-density lipoprotein-cholesterol (HDL-C) with HDL dysfunction and increased low-density lipoprotein cholesterol (LDL-C), in particular, small dense LDL-C, which is particularly atherogenic.[ 27 ] The underlying mechanisms involve hepatic fat accumulation, insulin resistance and chronic inflammation.[ 27 , 28 ] In a recent MR study based on participants from the UK Biobank, genetically predicted higher BMI was significantly associated with dyslipidaemia (low HDL-C levels).[ 29 ] In Singapore, in a multiethnic sample of 4,723 adult participants, elevated TG was more common in people with obesity, with the prevalence of elevated TG increasing with higher BMI and waist-to-hip ratio (WHR),[ 20 ] consistent with the evidence.

Hypertension is more than twice as prevalent in people with obesity compared to people with normal weight.[ 30 ] The mechanisms for obesity-induced hypertension are varied (involving adipokines, cytokines, free fatty acids, insulin, the rennin–angiotensin–aldosterone system) and interconnected, with the final common pathways being endothelial dysfunction, extracellular fluid overload and sympathetic nervous system activation.[ 30 , 31 ] Hence, obesity is an established risk factor for hypertension, and its causal role is supported by MR studies,[ 19 , 32 ] particularly that of higher adiposity with a more unfavourable metabolic profile (higher visceral and ectopic fat).[ 32 ] In Singapore, the increased prevalence of hypertension with higher BMI and WHR is consistent, especially in males.[ 20 ]

Nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is a condition in which fat accumulates in the liver in the absence of excessive alcohol consumption. Genetic variants that increase hepatic fat content have been shown to be associated with increased liver enzymes, hepatocellular damage and fibrosis, suggesting that hepatic fat accumulation mediates the development of liver fibrosis, independent of inflammation.[ 33 ] Hence, NAFLD may lead to nonalcoholic steatohepatitis (NASH), and ultimately, cirrhosis or hepatocellular carcinoma.

The prevalence of NAFLD has risen in tandem with the global epidemic of obesity, with NAFLD now being the most common cause of chronic liver disease worldwide.[ 34 ] A meta-analysis of MR studies confirms the causal effect of obesity on NAFLD,[ 19 ] with central adiposity (waist circumference) having the strongest relationship (OR 2.93, 95% CI 1.85–4.63) among the various obesity measures.[ 35 ] In Singapore, studies consistently show that participants with evidence of NAFLD have significantly higher BMI and waist circumference.[ 36 , 37 ]

Cardiovascular diseases

The association between obesity and increased incidence of cardiovascular diseases such as heart failure, coronary heart disease and stroke has long been established.[ 38 ] For example, based on pooled data from 97 prospective cohort studies involving 1.8 million participants, the hazard ratio (HR) for each 5 kg/m 2 higher BMI was 1.27 (95% CI 1.23–1.31) for coronary heart disease and 1.18 (95% CI 1.14–1.22) for stroke after adjustment for potential confounders.[ 39 ] Additional adjustment for diabetes mellitus, hypertension and dyslipidaemia reduced the HRs to 1.15 (95% CI 1.12–1.18) and 1.04 (95% CI 1.01–1.08) for coronary heart disease and stroke, respectively, suggesting that 46% (95% CI 42%–50%) of the excess risk of BMI for coronary heart disease and 76% (95% CI 65%–91%) for stroke were mediated by these conditions,[ 39 ] which are common in people with obesity.[ 4 , 30 ]

Obesity itself leads to an increased risk of these cardiovascular events, likely via mechanisms such as the secretion of adipokines, proinflammatory cytokines and hypofibrinolytic factors, that together could lead to increased oxidative stress and endothelial dysfunction resulting in atherosclerosis.[ 40 ] Additionally, excessive adiposity results in haemodynamic alterations via various neurohormonal and metabolic abnormalities, causing left ventricular (LV) hypertrophy and subsequent dysfunction, leading to LV failure. LV failure, facilitated by pulmonary arterial hypertension from hypoxia due to obstructive sleep apnoea (OSA) and/or obesity hypoventilation syndrome (OHS), may subsequently lead to right ventricular failure.[ 41 ] This causal role of obesity is supported by multiple MR studies,[ 19 , 29 , 32 ] with the strongest association between BMI and heart failure, followed by BMI and coronary artery disease, then BMI and stroke.[ 19 , 32 ]

Another obesity-related cardiovascular disease is atrial fibrillation (AF), with evidence suggesting that obesity is an independent risk factor for AF, even after accounting for OSA.[ 41 ] Also, studies have demonstrated a strong graded association between higher BMI and the risk of persistent AF and higher BMI, with increased risk of postablation AF.[ 41 ] The mechanisms linking obesity and AF are complex and incompletely understood, with increased left atrial and ventricular abnormalities, altered haemodynamics, increased epicardial and pericardial fat, inflammation, and metabolic and neurohormonal abnormalities being the potential causal mechanisms.[ 41 ] This causal relationship is similarly supported by MR studies which consistently show that genetically predicted BMI is associated with AF.[ 19 , 32 , 42 ]

In Singapore, a longitudinal study involving 2,605 Chinese participants found that the adjusted HR for cardiovascular and stroke mortality was highest in the group with obesity (BMI ≥30 kg/m 2 ) among those aged ≥65 years,[ 43 ] which is consistent with the association between obesity and increased incidence of cardiovascular diseases.

Obstructive sleep apnoea and hypoventilation syndrome

The increased intra-abdominal and intrathoracic pressure as a result of excessive adiposity impedes inflation of the lung, which can significantly affect the lung function, thereby leading to hypoventilation and ventilation–perfusion imbalance.[ 44 ] A constellation of obesity, daytime hypoventilation characterised by hypercapnia and hypoxaemia, and sleep-disordered breathing, without an alternative cause for hypoventilation, is known as OHS, with an estimated prevalence of 8%–20% in patients with obesity who were referred to sleep centres for evaluation of sleep-disordered breathing.[ 45 ]

The most common sleep-disordered breathing in such patients and people with obesity is OSA, as fat accumulation around the upper airways predisposes to the collapse of these airways.[ 44 ] About 50% of people with OSA have obesity, and approximately 40%–90% of people who are overweight suffer from OSA.[ 44 , 45 ] Consistent with epidemiological observations and genetic correlation (between OSA and BMI), an MR study shows that genetically predicted BMI is strongly associated with OSA, supporting the causal effect of BMI on OSA.[ 46 ] In Singapore, a study based on 587 Chinese participants reported that people with OSA had significantly higher BMI, and also BMI remained an important predictor of OSA after adjusting for hypertension and smoking,[ 47 ] in line with the overall evidence.

Polycystic ovary syndrome

Obesity is strongly associated with polycystic ovary syndrome (PCOS),[ 48 ] which is characterised by reproductive dysfunction (oligo-amenorrhoea, infertility), hyperandrogenism (hirsutism, acne, androgenic alopecia and biochemical hyperandrogenism) and a polycystic ovarian morphology (high antral follicle counts or increased ovarian volume).[ 49 ] Up to 88% of women with PCOS are overweight or obese,[ 48 ] with a meta-analysis showing that women with obesity had a twofold to threefold higher risk for PCOS when compared to women without obesity.[ 50 ] The pathogenesis of PCOS involves primarily insulin resistance, with the ensuing secondary hyperinsulinaemia resulting in enhanced steroidogenesis in the ovaries, particularly androgen production.[ 48 ] Hence, the insulin-resistant milieu associated with obesity can lead to the development of PCOS. This causal effect of obesity on PCOS is supported by MR studies,[ 32 , 51 ] with one MR study suggesting that this effect is predominantly metabolic in nature.[ 32 ] In Singapore, a study based on a multiethnic population of 389 participants reported that women with PCOS had significantly higher BMI compared to women without PCOS,[ 52 ] consistent with the evidence.

Cognitive impact and dementia

Experimental studies have shown that cellular mechanisms such as oxidative stress and inflammation can affect the brain structure and function.[ 53 ] Obesity is an established risk factor for dementia,[ 54 ] and has been associated with cognitive impairment[ 55 , 56 ] and decreased grey matter volume linked with executive functioning.[ 53 , 57 ] However, while MR studies have found causal relationships between BMI and grey matter volumes,[ 58 ] evidence for BMI and dementia has so far not been significant.[ 32 , 58 , 59 ] In Singapore, a longitudinal analysis of 1,519 cognitively normal older persons (>55 years) of Chinese ethnicity showed that central obesity was associated with a higher risk of developing mild cognitive impairment,[ 60 ] consistent with the overall evidence that higher adiposity has a negative impact on the brain.

Chronic kidney disease

Excess adiposity results in pathological processes such as lipotoxicity, inflammation, oxidative stress and activation of the renin–angiotensin–aldosterone system, leading to glomerular and tubular injuries (obesity-induced nephropathy).[ 61 ] Multiple MR studies have confirmed this causal relationship between obesity and kidney disease,[ 32 , 62 , 63 ] including one study conducted in an East Asian population using BMI-associated variants validated in East Asia.[ 63 ] In Singapore, longitudinal gain in adiposity was associated with progressive renal decline in a prospective multiethnic cohort with T2DM, suggesting that increasing adiposity would lead to adverse renal outcomes over time.[ 64 ]

Obesity is known to be associated with 13 types of cancers: oesophageal adenocarcinoma, renal cell carcinoma, postmenopausal breast cancer, endometrial cancer, colorectal cancer, meningioma, multiple myeloma, and cancer of the gastric cardia, liver, gallbladder, pancreas, ovary and thyroid.[ 65 ] In Singapore, these obesity-associated cancers make up four out of the top five cancers affecting women (breast, colorectal, endometrial and ovarian) and two of the top five cancers affecting men (colorectal and liver).[ 66 ] Potential mechanisms of increased cancer risk in obesity include hyperinsulinaemia, chronic inflammation and oestrogen excess.[ 67 ] This causal role of obesity in cancer is supported by MR studies,[ 19 , 32 ] particularly for cancers of the digestive system, with all of them (oesophageal, colorectal, gastric, liver, gallbladder, pancreas) positively associated with genetically predicted BMI based on a meta-analysis of MR studies.[ 19 ]

Depression and anxiety

The prevalence of depression is much higher among people with obesity than that in the general population.[ 68 , 69 ] Similarly, anxiety occurs more frequently in people who are overweight or obese compared to people with normal weight, and the relationship is stronger among those who are more severely obese.[ 70 , 71 , 72 ] Research findings consistently show that people with obesity frequently suffer from psychological issues ranging from stress associated with weight-related issues, perceived weight discrimination and stigmatisation to body image dissatisfaction.[ 68 , 69 ] Additionally, there is evidence that the dysfunctional adipose tissues present in obesity result in metabolic abnormalities, such as altered glucocorticoid, adipokine, insulin, leptin and inflammatory signalling, which either directly or indirectly impact the control of emotions and mood.[ 73 , 74 ] Hence, the causal relationship between obesity and depression/anxiety is likely to have both psychological and biological components. This is supported by MR studies which demonstrate the relationship between genetically predicted BMI (and fat mass) and depression,[ 75 , 76 , 77 ] even when using a genetic instrument that omits the metabolic consequences of higher BMI.[ 75 ]

In Singapore, a study based on 83 patients with obesity at a weight management clinic reported that the prevalence of anxiety symptoms and depressive symptoms was 28% and 11%, respectively,[ 78 ] suggesting that symptoms of depression and anxiety are highly prevalent in people with obesity in Singapore and are higher than the national prevalence of depression and anxiety,[ 79 ] consistent with international data.

Severe coronavirus disease-19

The dysfunctional physiological milieu of obesity has been associated with altered lymphoid tissue integrity, shifts in leukocyte populations and proinflammatory profiles, such that immune responses and pathogen defence are impaired.[ 80 ] This is demonstrated in previous influenza outbreaks[ 81 ] and the current coronavirus disease 2019 (COVID-19) pandemic, where obesity (high adiposity) is a major risk factor for severe COVID-19 (death and hospitalisation), as supported by multiple epidemiological studies worldwide[ 82 ] and MR studies[ 83 , 84 , 85 , 86 ] using data from the COVID-19 Host Genetics Initiative (an international collaboration that aims to uncover the genetic determinants of outcomes related to COVID-19 susceptibility and severity). In Singapore, where the COVID-19 mortality rate is low (<0.001),[ 87 ] a subgroup analysis of younger (<60 years) COVID-19 patients found that a BMI ≥25 kg/m 2 was significantly associated with the need for low-flow supplemental oxygen and mechanical ventilation,[ 88 ] consistent with observations internationally.

Other diseases

Other diseases with established epidemiological and strong MR evidence include asthma,[ 32 ] gastro-oesophageal reflux disease,[ 19 , 32 ] diverticular disease,[ 19 , 32 ] gallstone disease,[ 19 , 32 ] Crohn's disease,[ 19 , 89 ] osteoarthritis,[ 32 , 90 ] intervertebral degeneration (including back pain and sciatica),[ 91 , 92 ] peripheral arterial disease,[ 19 , 32 ] venous thromboembolism,[ 19 , 32 ] deep vein thrombosis,[ 19 , 32 ] aortic valve stenosis,[ 19 ] atopic dermatitis,[ 93 ] psoriasis,[ 32 ] gout[ 32 ] and rheumatoid arthritis.[ 32 ]

IMPACT ON MOTHER AND CHILD

Maternal obesity has been associated with adverse outcomes, including increased mortality, for both mother and child.[ 94 ] Mothers with obesity are more likely to develop pregnancy complications such as gestational hypertension, preeclampsia, gestational diabetes mellitus and thromboembolic disease, with a higher risk of preterm delivery, caesarian section, stillbirth, intrauterine growth retardation and foetus that is large for gestational age.[ 94 , 95 , 96 ] In an MR study, genetically elevated maternal BMI was associated with higher offspring birthweight, supporting a causal relationship.[ 97 ] These adverse outcomes can lead to complications and disability, with increased birth weight being associated with childhood adiposity and metabolic disorders during life.[ 94 ] Hence, the impact of obesity may extend beyond the current generation (mother) to the next generation (child). In Singapore, findings from the Growing Up in Singapore Towards Healthy Outcomes (GUSTO) study, a prospective mother–offspring birth cohort, showed that pre-pregnancy BMI and maternal obesity were associated with child size and adiposity[ 98 ] and childhood obesity,[ 99 ] respectively, confirming the generational impact of obesity.

PSYCHOSOCIAL IMPACT

Obesity negatively impacts health-related quality of life, with greater degrees of obesity associated with greater impairments.[ 100 , 101 , 102 ] While most individuals often report significant difficulties with physical and occupational functioning, many also experience problems with social functioning, such as social withdrawal[ 103 ] and social isolation.[ 104 ] This could be due to perceived negative attitudes and discrimination towards people with obesity, increased self-consciousness and self-blame for being overweight.[ 69 , 104 ] People with obesity, especially females, are often dissatisfied with their body image, which is exacerbated by society's expectation of thinness, with the degree of dissatisfaction positively correlated with the amount of excess weight.[ 102 , 105 , 106 ] Taken together, obesity can negatively impact an individual's self-esteem,[ 69 ] thereby affecting self-efficacy,[ 107 ] possibly resulting in a vicious circle and downward spiral.

ECONOMIC IMPACT

There are substantially higher healthcare utilisation and medical costs among people who are overweight or obese due to treatment of medical conditions caused by excess adiposity and for direct obesity treatments, which may include weight loss surgeries and medications.[ 108 ] In addition to direct medical costs, there is also an increase in indirect costs as a result of increased absenteeism (workdays missed due to illness or injury) and presenteeism (reduced productivity while working).[ 109 ]

The economic burden of overweight and obesity has been well described and quantified in North America,[ 108 , 110 ] Europe,[ 111 , 112 , 113 , 114 ] Brazil,[ 115 ] Australia,[ 116 , 117 ] China[ 118 ] and Saudi Arabia,[ 119 ] with an estimated cost of 0.8%–2.4% of gross domestic product (GDP) in 2019 based on eight countries.[ 120 ] As for Singapore, a recent study has attempted to quantify the economic burden of overweight and obesity.[ 121 ] Using econometric methods and cross-sectional data from the Singapore Epidemiology of Eye Diseases (SEED) cohort, which includes measured height and weight, self-reported healthcare utilisation and absenteeism/presenteeism (based on a modified version of the Work Productivity Activity Impairment questionnaire), the incremental per capita and aggregate direct and indirect costs of excess weight among a multiethnic population of older adults (aged 40–80) were estimated.

Among Chinese, individuals who were overweight missed one additional workday per year compared to those who were of normal weight. Individuals in the obese category had SGD720 per year greater medical expenditures, but missed workdays were not statistically different from those in the normal weight category. Among Indians, differences were not significant between normal and overweight categories, but Indians in the obese category incurred an additional SGD310 per year in absenteeism costs than those of normal weight. For Malays, no significant differences by BMI category were identified.[ 121 ]

In aggregate, the predicted total medical expenditures attributable to overweight and obesity in Singapore were estimated to be SGD178 million, representing 1.6% of Singapore's total healthcare expenditures (SGD11,300 million in 2019). This figure is on the low end of published estimates, which range between 2.9% and 9.7% of the total healthcare spending.[ 121 ] Including absenteeism increases this estimate to SGD261 million, although this estimate is likely to be conservative because it does not include costs for presenteeism (reduced productivity while working), retraining, injuries or other costs resulting from excess weight in the workplace.[ 121 ] By ethnicity, Malays are responsible for 19% of the total costs of excess weight, even though they make up only 12% of the Singapore population.[ 121 ] This disproportionate burden is consistent with the disproportionately higher rates of overweight and obesity among Malays.[ 3 ]

LIMITATIONS OF REVIEW

First, this review discussed mainly the impact of adult obesity, with a focus on diseases that have shown a strong relationship with obesity, especially those supported by MR studies. Hence, the impact of paediatric obesity is not discussed and not all diseases have been covered. Second, not all the diseases supported by evidence from MR studies were discussed in detail. Nonetheless, they are listed in the section ’Other diseases’ and included in Figure 1 . Third, interventional studies which show significant weight loss-improving health outcomes were not discussed. These studies, by showing the positive impact of weight loss, would have further supported the negative impact of obesity. Lastly, while there are some MR studies based on Asian cohorts, the majority of MR studies were based on genetic data derived from individuals of European ancestry or large cohorts that were predominantly European. Hence, the inference of causality may be limited in non-European populations.

As discussed, obesity is strongly related to more than 50 medical conditions [summarised in Figure 1 ], with evidence from MR studies to support causality for many of these conditions. Based on the 2017 Global Burden of Diseases study, the top four causes (cardiovascular diseases, cancers, musculoskeletal disorders and mental disorders) of disability-adjusted life years in Singapore account for more than 50% of the total burden,[ 7 ] with many of the common conditions in these four causes related to obesity. Hence, the clinical, social and economic burdens of obesity are considerable, potentially impacting future generations as well. This review, therefore, highlights the importance of an urgent and concerted effort towards the prevention and management of obesity to reduce the burden of obesity.

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Essay on Obesity : Causes, Effects, Prevention & Solution

February 25, 2018 by Study Mentor Leave a Comment

We all have heard a very common phrase- “ Health is wealth ”. Well, health is the most important asset and wealth that we have in our life.

A proper health will help us be confident and successful and we can perform our assigned tasks with full dedication.

There are severe disturbances in one’s eating behavior that causes eating disorders. The common ones are anorexia nervosa and bulimia nervosa.

One is about turning overweight and fat while the other talks about choosing to stay thin. Along with these two, it is important to know what obesity is all about.

Table of Contents

What is obesity?

Obesity is not called an eating disorder. In general, obesity does not include proper eating patterns or habits. Irregular eating patterns ultimately contribute to the cause of eating disorders, not obesity.

Anorexia nervosa is the scientific name given to the disorder in which a person is afraid of gaining weight and turning fat.

They may be underweight by nature, but the phobia of gaining unnecessary weight pushes them into a stress zone.

Generally women are known to be suffering heavily from such phobias and in most cases; it may not be a serious issue.

On account of this disorder, people tend to show specific signs of anxiety. People may suffer from two types of anorexia nervosa.

One is binge eating or purging or we can also call it over-eating and the other is the restrictive type wherein people feed very limited meals to themselves.

In binge eating or purging, the person consumes large quantity of food compared to normal intakes.

On the other hand, people who restrict themselves to limited quantities of food, people consider many factors like calorie intake, the carbohydrate intake factor etc. They control the intake of calories as well.

In bulimia nervosa, the person does not have any control over binge eating. Binge eating is involved in both anorexia nervosa and bulimia nervosa.

The difference between the two is that in anorexia nervosa the person is underweight while in bulimia nervosa the person has a normal weight or is overweight.

Binge eating is also one very common eating disorder. People who suffer from this disorder generally think a lot about their shape and weight.

Causes of obesity

In the modern world there is easy supply and availability of food and people do not have too many difficulties to get food.

With easy availability of food, people fail to distinguish between what is healthy and what is not.

They fail to glance through what food suits their body type and what does not. Consumption of excess food gets stored as fat in our bodies.

Increased fat makes our body look bulky, without proper shape and leads to excess storage of lipids in muscles. This has become a major cause for all health problems in the world today.

Obesity is measured by body mass index (BMI) which is calculated by taking the relative ratio of a person’s weight and height.

By calculating the BMI, we can easily conclude whether a person is underweight, normal or overweight.

Effects of obesity on health

Due to obesity, a person’s life is at risk along with many other associated health issues.

There could be health complications ranging from cancer, diabetes, high cholesterol issues, heart diseases, hypertension and arthritis.

Nowadays these diseases are termed under the name ‘lifestyle diseases, because directly or indirectly they are caused by the type of lifestyle we lead.

Whether we have a sedentary or active lifestyle goes a long way in deciding the extent of these diseases in our body.

Blood pressure (Hypertension) and diabetes are like very common ailments in people these days.

Obesity may be caused due to genetic factors too. Body hormones are also responsible for weight regulation. Family history and socio-cultural influences also play a significant role in attaining obesity.

Sometimes it is in the family genes due to which a person does not gain weight no matter how much they eat.

Fast food joints are present all over cities and at every corner of the street. We are also influenced by the food shown to us through advertisements.

The food is advertised in such a way that will tempt us to consume it. We will not think whether the food is healthy or not.

The type of food we eat is also affected by our culture. We consume foods which are commonly consumed in our culture and our region.

Family also plays an important role in the choice of foods. The families which always prefer healthy food remain fit. But those who consume high calorie foods have higher chances of getting obesity.

Solution of obesity

Obesity can be treated by suitable lifestyle modifications, proper medication and bariatric surgery.

Modifications in our lifestyle have a lot of positive effect but it may take some time to adapt to new lifestyle changes.

Good changes in diet teamed with regular exercise and proper amount of calorie intake will control and prevent obesity.

But it is not very easy to achieve this as shown through advertisements.  A good determination and a positive state of mind will help achieve good results at a faster rate.

Some people prefer medications to treat obesity. This can be carried out by taking medications which will either absorb the nutrients or reduce the eating.

Bariatric surgery is the most efficient treatment for obesity. The storage capacity of the stomach becomes less.

But a decision for the same should be taken after expert doctor consultations and the risks associated should definitely be discussed.

Prevention of obesity

As obesity has become a major problem in the world, people these days teach their children about side effects of obesity from their childhood itself.

It is a lifelong ailment. It should be treated once you know your body is gaining additional weight. If the child suffers from obesity from childhood itself, it may become dangerous for the child during adulthood.

So, proper knowledge must be given at the right time. There should be proper eating habits and also regular physical exercises.

Eating disorders can be treated through proper consultations and doctor advises. But people suffering from these disorders will be conflicted with many ideas and thoughts regarding their health.

They may hesitate visiting a doctor to get treated. Even though there is treatment for these disorders, people should try to stay away from improper lifestyle and irregular and imbalanced food eating habits.

At present, people have easy access to internet and they should make good use of it. They can easily read from blogs about nutritious and balanced food items that are good for the body. They will keep a person healthy.

Proper food intake along with exercises and a positive state of mind are essential elements to remain fit. If the person only prefers eating and there is no physical activity in the body, then the body becomes lazy.

This will further give rise to many health problems which may turn into huge complications in due course of time. Thus, it is better to prevent them than to cure them when one becomes a victim of lifestyle diseases.

Our mind, body and soul will be in a balanced state when we are hale and hearty.

There are many ways to maintain a proper health. Some of them are- exercise, eating nutritious food, maintaining a balanced diet, regular intake of water and staying happy.

Our society is developing at a very fast pace due to which no one has proper time to consume the right food. This is the reason why people adapt to eating of junk food, fast food which is totally unhealthy.

If a person does not maintain proper health, there are chances they will develop heart problems, obesity, too much loss or gain in weight and many other common diseases and ailments.

In order to avoid this, one should choose and prefer to have a healthy life.

Only with a healthy life, one can gain anything in life.

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Home — Essay Samples — Nursing & Health — Obesity — The Most Common Causes of Obesity

The Most Common Causes of Obesity

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• Published: 11 March 2024

Effects of dietary intervention on human diseases: molecular mechanisms and therapeutic potential

• Yu-Ling Xiao   ORCID: orcid.org/0000-0002-3684-0816 1 , 2   na1 ,
• Yue Gong 1 , 2   na1 ,
• Ying-Jia Qi   ORCID: orcid.org/0009-0006-9878-4019 1 , 2   na1 ,
• Zhi-Ming Shao 1 , 2 &
• Yi-Zhou Jiang 1 , 2  

Signal Transduction and Targeted Therapy volume  9 , Article number:  59 ( 2024 ) Cite this article

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• Cancer imaging
• Cancer metabolism

Immunotherapy

Diet, serving as a vital source of nutrients, exerts a profound influence on human health and disease progression. Recently, dietary interventions have emerged as promising adjunctive treatment strategies not only for cancer but also for neurodegenerative diseases, autoimmune diseases, cardiovascular diseases, and metabolic disorders. These interventions have demonstrated substantial potential in modulating metabolism, disease trajectory, and therapeutic responses. Metabolic reprogramming is a hallmark of malignant progression, and a deeper understanding of this phenomenon in tumors and its effects on immune regulation is a significant challenge that impedes cancer eradication. Dietary intake, as a key environmental factor, can influence tumor metabolism. Emerging evidence indicates that dietary interventions might affect the nutrient availability in tumors, thereby increasing the efficacy of cancer treatments. However, the intricate interplay between dietary interventions and the pathogenesis of cancer and other diseases is complex. Despite encouraging results, the mechanisms underlying diet-based therapeutic strategies remain largely unexplored, often resulting in underutilization in disease management. In this review, we aim to illuminate the potential effects of various dietary interventions, including calorie restriction, fasting-mimicking diet, ketogenic diet, protein restriction diet, high-salt diet, high-fat diet, and high-fiber diet, on cancer and the aforementioned diseases. We explore the multifaceted impacts of these dietary interventions, encompassing their immunomodulatory effects, other biological impacts, and underlying molecular mechanisms. This review offers valuable insights into the potential application of these dietary interventions as adjunctive therapies in disease management.

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Introduction

Nutrients play a crucial role in regulating various physiological processes. 1 The main source of nutrients is usually considered to be diet. The quantity, quality, and composition of the food consumed, as well as the timing of meals, directly impact human health by influencing the availability of nutrients. 2 Although there have been advancements in understanding the link between diet and disease in recent years, there is still much to learn about how specific dietary components affect disease risk and prevention. 3

Epidemiological studies have linked various dietary patterns to cancer and other diseases. 4 For instance, diets high in saturated fats and sugars have been associated with an increased risk of cardiovascular diseases (CVD) and type 2 diabetes. 5 Conversely, diets rich in fiber, fruits, and vegetables are associated with a lower risk of these conditions. 6 Similarly, conditions such as osteoporosis and certain neurological disorders have also shown links to dietary patterns, highlighting the broad influence of diet on overall health. 7 , 8 In the context of cancer, increased consumption of alcohol and red or processed meat is associated with a heightened risk of cancer, whereas adherence to a Mediterranean dietary pattern—characterized by high intake of fruits, vegetables, whole grains, legumes, fish, and olive oil, along with moderate consumption of dairy products such as yogurt—may confer protective effects against carcinogenesis. 9 , 10 Similarly, a strong adherence to the plant-based Paleolithic diet and a Paleolithic-like lifestyle has been found to significantly reduce the risk of colorectal cancer (CRC), especially in individuals with a body mass index (BMI) less than 30. 11 Although many cancer patients are interested in using dietary intervention to improve cancer therapy outcomes or even using it as a key component of the therapeutic process, 12 there is currently no solid evidence showing that any nutrition-related regimen can be a primary treatment for cancer. 13 However, preclinical studies suggest that calorie and energy restrictions can hinder tumor growth and progression and increase the efficacy of chemotherapy and radiotherapy. 14 , 15 A rising number of clinical trials are exploring the impact of dietary interventions or nutritional supplements in conjunction with standard antitumor therapies, with some showing clinical benefits. 16 , 17

Diet is a crucial source of nutrients for tumors and has emerged as a key component in determining whole-body metabolism. 18 The nutrients in the tumor microenvironment (TME) largely regulate tumor cell and immune cell metabolism. 19 Recent evidence suggests that metabolic reprogramming, a crucial hallmark of cancer, involves several metabolic adaptations by tumor cells to sustain proliferation and metastasis in the TME. 19 , 20 , 21 The TME constitutes a multifaceted and dynamic ecosystem comprising an assortment of cell types, including tumor cells, immune cells, and stromal cells, in addition to components of the extracellular matrix. The interplay among these constituents, along with the challenging environmental conditions, exerts a significant influence on the growth trajectory and progression of tumors. 22 For example, oxygen levels within the TME can vary due to increased metabolic demand from rapidly proliferating tumor cells, resulting in low oxygen tension, known as hypoxia, in tissues. In addition, nutrient availability, including the availability of glucose, fatty acids, and amino acids, can vary within the TME, impacting metabolic processes and energy production. The accumulation of metabolic waste products and alterations in pH can further contribute to a hostile TME, which can impair immune function and promote tumor progression. 23 These factors, along with dynamic interactions within the TME, play crucial roles in influencing tumor proliferation and the effectiveness of antitumor immune responses. 24

As our understanding of the complex relationships between diet, metabolic reprogramming, and various diseases continues to evolve, it becomes increasingly evident that dietary components and patterns significantly influence disease risk, prevention, and progression. This review delves into the unique metabolic characteristics and nutrient availability of tumors. Furthermore, we investigate recent evidence and emerging trends concerning the effects of dietary interventions on both cancer and other diseases, underscoring the potential therapeutic benefits these dietary strategies may offer to a wide range of patients (Fig. 1 ).

Overview of the relationship between dietary interventions and diseases. The cellular microenvironment, including the tumor microenvironment (TME), plays a crucial role in disease biology, and diet serves as a vital source of nutrients that can influence these microenvironments. Metabolic reprogramming, a prominent feature associated with disease progression, can affect cell metabolism and immune function. Dietary interventions, such as caloric restriction (CR), fasting-mimicking diet (FMD), and ketogenic diet (KD), can modulate the progression and treatment sensitivity of various diseases, including cancer. Additionally, dietary interventions can alter the composition and functional capacity of the gut microbiome, thereby indirectly influencing the progression and treatment of diseases. These direct and indirect effects of dietary interventions can influence metabolic reprogramming, modulate immune responses, and potentially enhance the clinical efficacy of treatments for various diseases. This figure was created with BioRender.com

Metabolic characteristics and nutrient availability in the tumor

Cellular metabolism encompasses a complex array of biochemical reactions that utilize specific nutrients, including carbohydrates, fatty acids, and amino acids. These nutrients are the primary sources for maintaining energy homeostasis and synthesizing macromolecules. 25 Our focus here is on cancer metabolism, which differs from that in corresponding healthy tissues in terms of nutrient levels and metabolic demands. 26 Within the TME, cancer cells can establish an immunosuppressive metabolic microenvironment by depriving immune cells of vital metabolites such as glucose and oxygen while also elevating the levels of mediators such as lactate and adenosine that limit the function of immune cells. 27 Therefore, different subsets of immune cells undergo metabolic reprogramming in tumors, and specific nutrients are required for these metabolic programs. 28 , 29 Generally, the metabolic programs that play vital roles in immune cells include glycolysis, the tricarboxylic acid (TCA) cycle, oxidative phosphorylation (OXPHOS), the pentose phosphate pathway (PPP), fatty acid oxidation (FAO), fatty acid synthesis (FAS) and the amino acid metabolic pathway 30 (Fig. 2 ).

Major metabolic pathways associated with different immune cell subtypes within the tumor microenvironment (TME). Summary of the main metabolic pathways of immune cells, highlighting the distinctive metabolic characteristics and requirements of different subsets of immune cells. This figure was created with BioRender.com

Glucose metabolism

Glucose serves as a vital energy source, facilitating the functioning of immune cells. Once transported across the plasma membrane, glucose is metabolically processed via three distinct pathways: glycolysis, the PPP, and the TCA cycle. Glycolysis, which occurs in the cytosol, transforms glucose into pyruvate and lactate, simultaneously generating adenosine triphosphate (ATP). Under aerobic conditions, pyruvate is channeled into the TCA cycle, where OXPHOS occurs, yielding additional ATP. Moreover, glucose-6-phosphate, a derivative of glycolysis, fuels the PPP, culminating in the production of ribose-5-phosphate and nicotinamide adenine dinucleotide phosphate (NADPH). Recent research has indicated a marked disparity in energy consumption between immune cells in resting and activated states. 18 Although glycolysis does not generate as many ATP molecules as OXPHOS, glycolysis produces ATP more rapidly, which is important to metabolically active immune cells.

Cancer cells are characterized by their rapid proliferation, primarily fueled by the consumption of glucose as an energy source. Intriguingly, these cells continue to rely on glycolysis for energy production even in the presence of ample oxygen, a phenomenon referred to as the “Warburg effect”. 31 This unique phenomenon leads to glucose depletion and lactic acid (LA) accumulation in the microenvironment, ultimately inhibiting antitumor responses. 32 High glycolytic rates in triple-negative breast cancer cells promote the infiltration of myeloid-derived suppressor cells (MDSCs) and suppress T-cell function, while suppressing glycolysis inhibits tumor colony-stimulating factor (CSF) expression and MDSC development. 33 Cancer cells produce LA through glycolysis, which reduces the antitumor activity of CD8 + T cells and natural killer (NK) cells. However, the activation of LA metabolism pathways in regulatory T cells (Tregs) is increased, and these cells adapt to high-LA conditions. 34 , 35 Furthermore, cancer cells can take advantage of immune cells by utilizing their metabolic byproducts. LA can shift tumor-associated macrophages (TAMs) from a proinflammatory (M1-like) to an anti-inflammatory (M2-like) phenotype in the TME. Notably, lactate-activated TAMs enhance cancer cell adhesion, migration, invasion in vitro, and promote metastasis in vivo. 36

T cells play crucial roles in the TME. Upon activation, these cells undergo metabolic reprogramming, which subsequently yields diverse functional outcomes. Naïve T cells, which are metabolically quiescent, exhibit basic nutrient intake rates and low glycolysis rates. They primarily generate ATP through TCA cycle-fueled OXPHOS. 37 The activation of specific membrane receptors triggers the differentiation of naïve T cells into effector T cells, also known as T eff cells. This process is accompanied by a pronounced increase in both energy demand and biosynthetic activity within T eff cells. In T eff cells, the metabolic state is changed to increasingly rely on glycolysis, as these cells upregulate GLUT1, increase glucose intake. 38 , 39 , 40 , 41 Simultaneously, this metabolic alteration benefits T eff cells by reducing their reliance on oxygen for energy production, which enables them to maintain cytokine production and cytolytic activity even when they migrate into microenvironments within solid tumors that have low oxygen levels. 42 In contrast to naïve and T eff cells, memory T cells undergo a metabolic rewiring process that leads them to enter a quiescent state characterized by elevated OXPHOS rates compared to the glycolysis rate. 43 Tregs, known for their suppressive function, exhibit decreased glycolysis rates and primarily rely on OXPHOS to support their function, while glycolysis is crucial for their migration. 44 It has been reported that the Treg-specific transcription factor FOXP3 reprograms Treg metabolism by suppressing Myc expression and glycolysis while promoting OXPHOS and NAD(H) oxidation. This adaptation enables Tregs to be more adaptable to low-glucose and/or lactate-rich microenvironments. 45

There are several other types of cells within the TME that exhibit distinct metabolic functions. In the case of NK cells, glycolysis and OXPHOS play important roles in maintaining their cytotoxicity, as indicated by the inhibition of these processes leading to diminished expression of IFNγ and Fas ligands. 46 Researchers have shown that transcription factor-controlled glucose metabolism, specifically by sterol regulatory element-binding proteins (SREBPs), which conventionally control lipid synthesis, is essential for metabolic reprogramming in activated NK cells. 47 Dendritic cells (DCs), on the other hand, rely on glycolysis and the PPP for energy production to sustain their function, including cytokine production, antigen processing and presentation, and the stimulation of T cells. 48 Furthermore, different subsets of macrophages present distinct metabolic functions. M1-like macrophages predominantly utilize anabolic metabolism, specifically glycolysis and the PPP, to generate energy and synthesize cellular components, whereas M2-like macrophages are more reliant on OXPHOS, particularly through the enhancement of FAO. 49

Lipid metabolism

Lipids, such as fatty acids, triglycerides, cholesterol, phospholipids, and sphingolipids, play crucial roles as precursors to many important biological molecules. 50 Lipids, including substances such as cholesterol and fatty acids that are widely distributed in organelles, are key components of internal cellular membranes. Moreover, lipids are essential biological molecules that provide energy during nutrient deficiency, participate in the synthesis of complex fat-containing substances, and aid in cellular signal transmission as second messengers. 51 Lipids within the microenvironment profoundly influence the proliferation of cancer cells and regulate the functional activity of immune cells.

Cancer cells undergo metabolic reprogramming of lipids in the tumor niche. The activation of adipocytes triggers the lipolysis of stored triglycerides and secretion of fatty acids. Cancer cells can then take up these fatty acids to fulfill their lipid requirements for rapid growth. 52 Research has also demonstrated that ovarian cancer cells stimulate membrane cholesterol efflux from TAMs, fostering an environment that promotes tumor growth by enhancing interleukin (IL)-4-mediated reprogramming and suppressing IFNγ-induced gene expression. The deletion of ABC transporters, responsible for cholesterol efflux, reversed the tumor-promoting functions of TAMs, leading to reduced tumor progression. 53

Furthermore, elevated cholesterol levels in the microenvironment stimulate the expression of immune checkpoints, including PD-1, 2B4, TIM-3, and LAG-3, in T cells, driving T-cell exhaustion via the activation of the endoplasmic reticulum stress response. 54 In contrast to the negative effects of reprogramming T-cell lipid metabolism on antitumor immunity, the inhibition of ACAT1, a pivotal enzyme responsible for cholesterol esterification in CD8 + T cells, results in elevated cholesterol levels in the plasma membrane. This increase subsequently amplifies TCR signaling and promotes antitumor activity. These findings highlight the complex mechanisms through which cholesterol regulates T-cell function. 55

For efficient tumor antigen processing and presentation to T cells, activated DCs need high rates of cell surface or secretory protein biosynthesis, which is partly regulated by FAS-induced increases in cytokine production. 56 T eff cells depend mainly on FAS to support inflammatory cytokine secretion and proliferation, while naïve T cells and memory T cells maintain their basic functions by increasing the FAO rate. 57 , 58 , 59 Although T eff cells rely mainly on glycolysis for energy, CD8 + T cells that undergo enhanced FAO exhibit stable antitumor functions even under conditions of low glucose and oxygen levels. By promoting fatty acid catabolism, CD8 + T cells exhibit increased functionality, and the efficacy of immunotherapy in patients with melanoma can thus increase. 60

While these studies indicate a positive influence of lipids on the functionality and metabolism of CD8 + T cells in the TME, it is important to note that alterations to T-cell lipid metabolism might attenuate their antitumoral effects. In obesity-related breast cancer murine models, the activation of STAT3 triggered an increase in FAO in CD8 + T cells, which suppressed glycolysis and weakened their tumor-suppressing ability. 61 Moreover, enhanced lipid uptake and peroxidation can result in high oxidative stress, which leads to CD8 + T cell dysfunction. CD36, a fatty acid scavenger receptor, facilitates the incorporation of arachidonic acid into CD8 + T cells. This process subsequently triggers lipid peroxidation and ferroptosis, events that cumulatively attenuate the antitumor immune response and reduce the efficacy of immunotherapy. 62 , 63 , 64

Lipid metabolism also plays an active role in regulating Treg function. Fatty acid synthase (FASN)-mediated FAS contributes to the proliferation and maturation of Tregs, and FAO provides the energy crucial for Treg infiltration into the TME. 65 Research has shown that OX40 plays a role in modifying the lipid composition of Tregs, leading to the proliferation of OX40 + Tregs in the TME. This effect is achieved through increased FAS expression and glycolysis rate in Tregs. 66 CD36, via the peroxisome proliferator-activated receptor-β (PPAR) signaling pathway, maintains the mitochondrial fitness of Tregs, promoting Treg viability and inhibitory functions. 67 SREBPs have been found to show increased activity in Tregs that infiltrate tumors. Inhibiting FAS and metabolic signaling by targeting SREBPs has been shown to effectively activate the antitumor immune response without causing autoimmune toxicity. When the SREBP-SCAP axis was inhibited, in addition to tumor growth attenuation, immunotherapy effectiveness was boosted. These findings suggest that SREBPs may be promising targets for cancer therapy. 68

High expression of FASN in TAMs promotes the accumulation of fatty acids, leading to enhanced tumor immune tolerance via the FAO pathway. 69 Notably, lipid metabolism differs between M1-like and M2-like macrophages. M1-like macrophages prevalently engage the FAS pathway, while M2-like macrophages predominantly utilize the mitochondrial FAO pathway for their bioenergetic demands. 70 , 71 Receptor-interacting protein kinase 3 (RIPK3), which is crucial for necroptosis, is found to be diminished in hepatocellular carcinoma (HCC)-associated macrophages, leading to inhibited caspase1-mediated cleavage of PPAR, a process vital for enhancing fatty acid metabolism, including FAO. This metabolic shift results in increased accumulation and polarization of M2-like macrophages in the TME, contributing to accelerated HCC growth. 72

MDSCs also exert a substantial influence in suppressing antitumor immunity in the microenvironment, and they can be categorized into monocytic MDSCs (M-MDSCs) and granulocytic MDSCs (PMN-MDSCs). 73 Tumor-infiltrating MDSCs increase fatty acid uptake and induce FAO. 74 The accumulation of lipids in MDSCs increases oxidative metabolism, resulting in MDSC acquisition of an immunosuppressive and anti-inflammatory phenotype. 75

Amino acid metabolism

Amino acids are the primary substrates for protein biosynthesis, and recent evidence emphasizes the critical role of amino acid availability and metabolism in the regulation of antitumor immunity.

Glutamine is the most abundant amino acid and a crucial energy substrate, as well as an important nitrogen and carbon donor for various biosynthetic precursors. 76 T eff cells require higher levels of glutamine than naïve T cells due to their rapid proliferation and demand for sufficient raw materials for macromolecule synthesis and cytokine secretion. 77 Cancer cells have been shown to exhibit the highest glutamine uptake capacity and consume most of the glutamine in the microenvironment. 76 In turn, elevated glutamine consumption by cancer cells diminishes the glutamine supply necessary for T cells, consequently impeding the antitumor immune response. 78 In the microenvironment, cancer cells consume glutamine to synthesize γ-aminobutyric acid (GABA) via glutamate decarboxylase 1 (GAD1). By activating the GABA B receptor, GABA inhibits GSK-3β activity, which enhances β-catenin signaling, promoting cancer cell proliferation while suppressing intratumoral infiltration of CD8 + T cells. 79 Furthermore, elimination of glutaminase, a vital enzyme for glutamine metabolism, within tumor cells stimulates T-cell activation and augments the efficacy of antitumor immune responses. The compound V-9302, an inhibitor of the glutamine transporter, selectively impedes glutamine uptake in cancer cells while simultaneously enhancing both glutamine assimilation and glutathione synthesis in T eff cells, ultimately enhancing their function. 80

Tryptophan is another essential amino acid. Following its entry into eukaryotic cells via the transport proteins SLC1A5 or SLC7A5, tryptophan is primarily subjected to three primary metabolic pathways: incorporation into protein synthesis, metabolism via the kynurenine (Kyn) pathway, or conversion through the serotonin pathway. 81 Notably, a substantial fraction of tryptophan is directed through the Kyn pathway, culminating in the production of a suite of metabolites with significant physiological implications. 82 Tryptophan plays a crucial role in determining the strength and effectiveness of the T cell response by affecting its availability in the microenvironment. 83 However, within the tumor niche, cancer cells, MDSCs, TAMs, suppressive DCs, and cancer-associated fibroblasts, among other cell types, exhibit upregulated expression of indoleamine 2,3-dioxygenase (IDO), which metabolizes tryptophan into suppressive kynurenine to promote Tregs and suppress CD8 + T cell function. 84 , 85 , 86 Most cancer cells overexpress IDO, and the level of kynurenine in the microenvironment is associated with poor prognosis in multiple solid and hematological malignancies. 87 Kynurenine has been found to bind to the aryl hydrocarbon receptor (AHR) in naïve CD4 +  T cells, which promotes Treg differentiation. 87

An additional metabolite generated through the Kyn pathway is the essential redox cofactor nicotinamide adenine dinucleotide (NAD+), a molecule of fundamental importance for the maintenance of cellular homeostasis. 88 In particular, cancer cells heavily depend on NAD+ to promote metabolic reprogramming and meet higher demands for ATP. Elevated NAD+ levels have been demonstrated to promote the proliferation of cancer cells. 89 Although the majority of studies suggest that an increase in NAD+ drives cellular proliferation, prior investigations have proposed that a decrease in NAD+ levels can lead to genomic instability, subsequently instigating liver tumorigenesis. 90 Moreover, tryptophan metabolism mediated by IDO affects not only the Kyn pathway but also other pathways, such as the purine, nicotinamide, and pyrimidine metabolism pathways, ultimately leading to decreased T-cell function. 91 In addition to IDO, another enzyme, tryptophan 2,3-dioxygenase (TDO), is involved in tryptophan catabolism. High TDO expression has been shown to impair T-cell antitumor immunity and to be correlated with poor clinical prognosis. Suppressing TDO expression can increase the antitumor efficacy of immune checkpoint inhibitors (ICIs). 92

In addition to the aforementioned amino acids, other amino acids play crucial roles in regulating tumor metabolism. T-cell proliferation relies heavily on arginine consumption. L-arginine supplementation has been shown to facilitate the metabolic shift from glycolysis to OXPHOS, enhancing T-cell survival and boosting antitumor responses of CD8 + tumor infiltrating lymphocytes (TILs). 93 Notably, the functional differences resulting from TAM polarization partially depend on arginine metabolism. In macrophages with the M1-like phenotype, arginine is converted into nitric oxide (NO) and citrulline via inducible nitric oxide synthase (iNOS), and this anabolic pathway is closely associated with macrophage cytotoxicity and antitumor effects. Conversely, in macrophages with the M2-like phenotype, arginine is hydrolyzed to yield ornithine and urea through arginase 1 (Arg1). 94 This metabolic shift affects arginine availability, which in turn impacts the activation and proliferation of T cells and NK cells, leading to immune suppression within the microenvironment. Notably, Arg1 expression in MDSCs contributes to arginine depletion in the microenvironment, further inhibiting T-cell antitumor function and reducing their survival. 95 , 96 In addition, depletion of cystine and cysteine is also linked to the immunosuppressive effect of MDSCs. T cells are unable to synthesize the essential amino acid cysteine from substances such as cystine or methionine, necessitating its import from external sources for their functionality. 97 MDSCs import cystine but do not release cysteine, thus the levels of cysteine in the microenvironment are regulated, inhibiting T-cell activation. 98 Asparagine is another amino acid that significantly boosts CD8 + T-cell activation and antitumor responses. Restricting dietary asparagine or inhibiting its uptake impaired T-cell activation and differentiation into memory-like cells. 99 Cancer cells consume higher levels of methionine due to increased expression of its transporter (SLC43A2), which inhibits methionine metabolism and function in CD8 + T cells by altering histone methylation patterns. 100

Organ-specific metabolic profiles

Understanding the metabolic differences between various organs is critical for developing targeted therapeutic strategies in cancer treatment. Each organ has unique metabolic demands and pathways that can be dysregulated in cancer, leading to distinct metabolic profiles for different types of tumors. 101 This organ-specific metabolic reprogramming plays a key role in cancer progression and survival, and its understanding could be leveraged for therapeutic benefits.

Consider primary brain tumors as an example. These tumors, often found nestled within the intricate neural networks of the brain, exhibit a remarkable metabolic flexibility. 102 They are known to express elevated levels or alternative isoforms of glycolytic enzymes, a trait that points towards a potential therapeutic opportunity. 103 Specifically, the therapeutic strategy of glucose deprivation could selectively starve brain tumor cells while sparing healthy neurons, which are capable of surviving on alternative fuels such as ketone bodies. 104 Similarly, HCC cells undergo a significant metabolic shift from glucose production (a state known as gluconeogenesis) to glucose usage. 105 HCC cells also exhibit a marked increase in amino acid metabolism, particularly in the metabolism of glutamine. 106 Additionally, studies have shown that HCC cells often exhibit abnormal lipid accumulation, increased FAS, and enhanced cholesterol metabolism. These changes contribute to the aggressive and metastatic behaviors of HCC. 107

Moreover, hormone-sensitive tissues such as the breast, endometrium, and prostate also exhibit significant metabolic fluctuations in response to hormone levels. 101 Hyperactivation of the PI3K pathway, a lipid kinase that promotes proliferation and nutrient uptake in response to growth signals, has been implicated in breast and endometrial cancers, providing a possible mechanism for hormonal therapy evasion. 107 This pathway could be a potential target for therapeutic interventions, particularly in hormone therapy-resistant cancers.

In summary, understanding organ-specific metabolic profiles and their dysregulation in cancer can open up new avenues for targeted cancer therapy. By exploiting these unique metabolic dependencies of tumors, more effective and personalized treatment strategies can be developed.

Targeted dietary interventions and mechanistic insights into their impact on cancer

Understanding the metabolic pathways of glucose, lipids, and amino acids lays a crucial foundation for exploring the effects of various dietary restrictions. Macronutrients, including carbohydrates, fats, and proteins, are the primary sources of energy for our bodies, and they each follow distinct metabolic pathways. By manipulating the relative intake of these macronutrients, we can influence the metabolic pathways they utilize and thereby exert control over our systemic metabolism. This concept forms the basis for various dietary restrictions and special diets, such as caloric restriction (CR), fasting or fasting-mimicking diet (FMD), ketogenic diet (KD), high-fat diet (HFD), or amino acid-defined diet. Moreover, high-salt diet (HSD), although not directly involving macronutrients, is noteworthy due to its potential impact on tumor biology. Therefore, an in-depth discussion on the role of HSD in cancer research and treatment is included in our exploration.

The connections between various dietary patterns and cancer risk are likely rooted in several biological mechanisms, such as inflammation and immune function; specific factors, such as the gut microbiota and their metabolites; unfavorable events, such as certain epigenetic changes and metabolic or hormonal disruptions; and stress, such as oxidative stress. 108 Alterations in dietary composition impact not only the availability of nutrients within tumor cells but also the surrounding microenvironment, thereby offering potential opportunities to impede tumor growth 109 (Table 1 ).

Calorie restriction

Effective CR is a dietary intervention that reduces energy intake by approximately 15–30% while maintaining a balanced proportion of macronutrients and preventing malnutrition. 110 CR has been shown to prolong life and reduce age-related diseases, including cancer, in experimental models. 111

Although the antitumor effect of CR has been confirmed, the underlying mechanism remains unclear. Nonetheless, it is believed that the tumor-inhibiting effect is partially mediated by several biological changes, such as increased apoptosis rates in cancer cells, decreased circulating blood glucose levels, inhibited insulin-like growth factor 1 (IGF-1) signaling, reduced insulin levels, and mediators that regulate metabolic pathway activation and inhibit angiogenesis. 112 In particular, controlling IGF-1 signal transduction is a critical component underlying the antitumor effects of CR. The IGF-1 signaling pathway is frequently activated in cancer cells, and it shifts metabolic resources toward growth and proliferation. Therefore, the reduction in IGF-1 levels in response to CR leads to attenuated tumor growth and progression. 113 The impact of CR on cancer is also interconnected with mutations and oncogenic pathways. A study showed that CR results in a reduction of insulin levels, thereby diminishing tumor PI3K signaling. 114 CR has also been found to suppress xenograft tumor growth by upregulating the aldolase A (ALDOA)/DNA-PK/p53 pathway, with ALDOA acting as a potential oncogene that can also activate the tumor suppressor p53. 115 Moreover, CR has been shown to modify the cancer stem cell (CSC) phenotype, reducing their carcinogenic and metastatic potential. Notably, in MMTV-ErbB2 transgenic mice, the CSC subpopulation was most affected by CR, as shown by a reduction of luminal cells (CD24 high /CD49f low ), putative mammary reconstituting unit subpopulations (CD24 high /CD49f high ) and luminal progenitor cells (CD61 high /CD49f high ). These effects were largely attributed to the concurrent inhibition of estrogen receptor and ErbB2 signaling. 116

CR has been shown to shape the TME in several ways, including through the specific reduction in the number of TAMs, increase in the formation of CD8 + cytotoxic T cells and memory T cells, and negative modulation of immunosuppressive Treg cell activity and immunosuppressive cytokine levels. 117 Additionally, CR promotes favorable changes in the immune signature, providing enhanced protection against tumor growth and metastasis, possibly in part by remodeling the TME. In mice, no impact of a CR diet was observed on the number of CD4 + or CD8 + cells in the TME; however, the cytotoxic killing potential of these cells was elevated. Notably, higher expression of CD103 + , a marker of crucial tissue-resident memory T cells that possess enhanced cytotoxic capacity and can contribute to tissue protection against tumor cell invasion, was found. Additionally, a downward trend in the frequency of Tregs was observed, and a significant reduction in the total number of MDSCs was detected. 118 Hence, it was concluded that CR not only inhibits cancer cell proliferation but also helps maintain antitumor immunity.

Furthermore, research has shown that fasting, CR, and caloric restriction mimetics (CRMs) can promote T-cell-mediated tumor cytotoxicity, alter NK cell function, and potentially trigger immunogenic cell death, thereby stimulating cancer immunosurveillance pathways. 119 CRMs are pharmacological agents or natural compounds that imitate the biochemical effects of CR by reducing the lysine acetylation rates of cellular proteins. 120 Examples of CRMs include hydroxycitrate (an inhibitor of ATP citrate lyase), spermidine (an inhibitor of EP300 acetyl transferase activity), and resveratrol (an activator of sirtuin-1 deacetylase activity). 121 Treatment with CRMs has been found to decrease the concentration of free IGF-1, promote autophagy in cancer cells, and improve the antitumor immune response, resulting in a reduction in tumor growth when combined with immunogenic chemotherapeutics. 119 CRM hydroxycitrate has been found to stimulate autophagy in U2OS osteosarcoma cells in vitro, thereby increasing antitumor immunosurveillance and reducing tumor mass in mice with autophagy-competent mutant KRAS-induced lung cancers. 122 Moreover, in vitro treatment with resveratrol inhibits mitochondrial respiration in breast cancer cell lines through a SIRT1-dependent mechanism, diminishes the expression of markers associated with breast CSCs, and promotes their differentiation. 123 Collectively, these findings suggest that CRMs may enhance antitumor immunosurveillance in preclinical models.

Moderate physical activity, energy restriction, and their combination can also affect tumor growth. In fact, the combined effects of moderate physical activity and 10% energy restriction (PA + ER) have been shown to significantly delay primary tumor growth, reduce spontaneous metastases, and prolong survival. These effects on tumor progression and survival are accompanied by beneficial changes in immune cell infiltrates within the microenvironment. Specifically, the PA + ER combination leads to an increase in the percentage of CD8 + T cells and a decrease in the percentage of total MDSCs and MDSC subsets within tumors. 124

Nevertheless, it is crucial to emphasize that there are established nutritional recommendations for cancer care, and the weight loss or reduction in protein intake often associated with CR may conflict with these guidelines. 125 These dietary practices could exacerbate the risk of malnutrition, sarcopenia, fatigue, delayed wound healing, and impaired immunity, particularly in cancer patients who are already at an increased age-associated risk for these conditions. 126 Therefore, while exploring dietary interventions for cancer treatment, the potential adverse effects on overall patient health and nutritional status must be carefully considered.

Fasting or fasting-mimicking diet

In addition to CR, alternative approaches such as intermittent fasting (IF), including short-term fasting (STF), intake of an FMD, and time-restricted feeding (TRF), which limits food consumption to a specific time window each day, are being condisered. 127 , 128 The term “fasting” has a broad definition, encompassing a range of eating patterns, including complete and voluntary deprivation of food with no restriction on drinking water. 129 An FMD is based on a regimen of low-calorie and low-protein foods that mimics the effects of fasting but induces fewer side effects. This approach retains the benefits of traditional fasting methods while minimizing their potential drawbacks. 130

Fasting or intake of an FMD can cause various metabolic changes, including alterations in the systemic levels of hormones and growth factors such as insulin, glucagon, growth hormone, IGF-1, glucocorticoids or adrenaline. 131 In response to these changes, normal cells activate protective mechanisms against stress and toxic insults, thereby reducing their metabolic requirements and cell division rate. On the other hand, because fasting or FMDs reduce tumor growth-promoting nutrients and factors, cancer cells struggle to manage metabolite deprivation and thus develop greater sensitivity to cancer therapies. 132 In obesity-driven postmenopausal cancer mouse models, TRF was shown to delay the onset of tumors and reduce lung metastasis. Moreover, TRF was found to increase systemic insulin sensitivity and decrease hyperinsulinemia. Importantly, TRF could also restore the circadian rhythm of gene expression within tumors while attenuating both tumor growth and insulin signal transduction. 133 Fasting can cause an “anti-Warburg effect” by reducing aerobic glycolysis and glutaminolysis while increasing OXPHOS uncoupled from ATP synthesis. 134 In cancer cells, OXPHOS increases reactive oxygen species (ROS) production and leads to oxidative stress, activation of p53 signaling and DNA damage, particularly when combined with chemotherapy or other cancer therapies. 135 Therefore, the unique metabolic vulnerabilities of cancer cells, which differ from those of normal cells, can be strategically targeted to develop novel and effective therapeutic interventions. According to a recent study, the combination of chemical treatment with an FMD reduces the expression of heme oxygenase-1 (HO-1), which is a stress-responsive enzyme that protects cancer cells against oxidative damage and apoptosis in vivo. Interestingly, this combination treatment resulted in upregulated HO-1 expression in normal cells. The downregulation of HO-1 production in cancer cells, in part, facilitated FMD-induced chemosensitization of cancer cells by boosting CD8 + TIL-dependent cytotoxicity, which was possibly facilitated by decreased Tregs. 136 A separate study conducted with mouse models of colon cancer indicated that alternate day fasting for 2 weeks triggered autophagy in cancer cells, which in turn downregulated CD73 expression. As a result, the production of immunosuppressive adenosine in cancer cells was reduced, ultimately preventing macrophages from acquiring an M2 immunosuppressive phenotype. 137

Clinical experiments have suggested that intake of an FMD can induce metabolic changes and increase antitumor immunity in cancer patients. In fact, the final outcomes of an FMD-treated clinical trial (NCT03340935) demonstrated that a severely calorie-restricted, five-day FMD regimen was well tolerated and resulted in substantial systemic metabolic changes in patients with different tumor types who were concurrently receiving antitumor therapies. 138 , 139 In another clinical trial called DigesT (NCT03454282), a five-day FMD regimen was found to broadly reshape intratumor immunity in breast cancer patients. Specifically, the FMD was shown to promote the infiltration of activated and cytotoxic immune cell populations, including total and activated intratumoral CD8 + T cells, M1-like macrophages, aDCs, and NK cells. These changes were paralleled by an increase in immune signatures associated with improved clinical outcomes in cancer patients. 138

Ketogenic diet

A KD comprises a high-fat component, very low carbohydrate levels, and low to moderate protein levels, as explained in a recent study. 140 A traditional KD is typically formulated at a 4:1 ratio of fat:carbohydrate plus protein. 141 In this classical formulation, 80–85% of calories are derived from fat, 10–15% from protein, and less than 5% from carbohydrates. 142 A KD is known to be effective at treating epilepsy, lowering glucose levels, and producing ketone bodies in vivo. 143 There is increasing evidence to support the use of KD as a potential tumor treatment or prevention method, either as a standalone approach or in combination with other medicines. 144

The Warburg effect indicates that lower intratumoral glucose levels can impede tumor growth, which can be achieved through pharmacological intervention and dietary changes such as a KD. Cancer cells, unable to utilize ketone bodies produced by KD for energy due to their aberrant mitochondrial function and diminished enzyme activity, can essentially be “starved” of glucose. Hence, KD emerges as a potentially promising strategy for cancer prevention. 145 One of the primary ways in which a KD potentially promotes potential anticancer effects is by increasing the levels of β-hydroxybutyrate (β-HB), which is the most abundant ketone body. 146 For instance, β-HB has been proven to inhibit CRC by activating the transcriptional regulator Hopx through the surface receptor Hcar2, thereby reducing the proliferation of colonic crypt cells and suppressing tumor growth. 147 Another antitumoral effect of KD is upregulating the expression of the circadian clock gene Per (Period) by activating AMPK and upregulating SIRT1 (Sirtuin1), resulting in enhanced apoptosis and growth delay in tumor cells. 148 KD also decreases insulin-regulated PI3K-Akt-mTOR signaling, which is overactivated in pancreatic neuroendocrine tumors (PanNETs), resulting in decreased blood glucose levels and a suppressive effect on the development and progression of PanNETs. 149

Emerging evidence suggests that a KD may be a valuable clinical tool to enhance T-cell-mediated antitumor immune responses. In vitro and in vivo studies have shown that KD intake markedly increased the specific responses of human T cells, resulting in enhanced CD4 + , CD8 + , and Treg capacity, as well as augmented T memory cell formation. Under conditions of KD intake, CD8 + T cells undergo metabolic reprogramming to rely on OXPHOS in response to increased ketone bodies, leading to enhanced cellular energy and respiratory reserve, potentially improving their functionality. 150 In addition, KD intake prevented the progression of colon tumors by inducing tumor cell oxidative stress, inhibiting MMP-9 expression, and promoting M2 to M1 TAM polarization. 151 In a mouse model of malignant glioma, KD feeding led to significantly enhanced innate and adaptive tumor-specific immune responses. Mice fed a KD showed increased cytokine production (IFNγ, TNF, and IL-2) and greater tumor-reactive CD8 + T-cell cytotoxicity. Moreover, the mice maintained on a KD presented with a higher number of immune cells and a higher ratio of CD4 + T cells to Tregs, while the functionality of the Tregs was weakened. Feeding mice with the KD resulted in a noteworthy decrease in the expression of immune inhibitory receptors (PD-1 and CTLA-4) on CD8 + TILs, as well as a reduction in the expression of inhibitory ligands (CD86 and PD-L1) on cancer cells. 152 These findings suggest that a KD has the potential to attenuate tumor-induced T-cell suppression by decreasing the population of cells susceptible to the inhibitory PD-1 pathway.

Although KD has shown various potential benefits to tumor patients with its promising effects of inhibiting tumor cell growth and activating immune response, there is still limitation in its clinical application owing to its inevitable side effects. 153 It should be considered that KD also presents some risks, as they are typically high in saturated fats and may lack a substantial amount of nutrients, specifically carbohydrates and dietary fiber, as well as micronutrients such as calcium, magnesium, potassium and vitamins A, B and B6. 154 , 155 According to a recent research, KD delayed tumor growth but meanwhile accelerated cachexia onset, therefore shortening survival in a mouse model of IL-6-producing cancer. Excitingly, the same research group found that applying dexamethasone during KD treatment might delay cachexia onset without affecting the inhibition of tumor growth, providing fundamental insight into reversing the limitations of the clinical application of KD. 156

Protein restriction diet

The prevailing notion suggests that high protein intake, particularly among individuals under the age of 65, potentially escalates the risk of overall and cancer-related mortality. 157 To establish a protein restriction diet, either dietary protein intake or the number of amino acids can be reduced. 140 Recent research has demonstrated that dietary protein restriction is linked with a reduced incidence of tumor occurrence and a decreased risk of mortality. 158

Dietary restriction of protein and certain amino acids, including serine, methionine, and branched-chain amino acids (BCAAs) such as leucine, isoleucine, and valine, has been shown to impede tumor growth. 159 One mechanism through which protein restriction may inhibit tumor growth is via the IGF-1 signaling pathway. In melanoma and breast cancer mouse models, it has been observed that mice fed a low-protein diet (4% kcal protein) exhibit reduced IGF-1 levels and slower tumor progression compared to those fed a high-protein diet (18% kcal protein). A low-protein diet has been associated with reduced IGF-1 levels in patients aged 50–65 years, subsequently decreasing their risk of death from cancer. Conversely, a low-protein diet has been linked with an increased mortality rate in older patients (aged 65 and above), suggesting that a life-stage-specific approach to protein intake could optimize healthspan and longevity. 157 Other potential mechanisms for cancer prevention that are mediated by protein restriction could involve mTOR signaling, amino acid metabolic programming, FGF21, and autophagy. 158 In addition to these general effects, specific dietary restrictions on certain amino acids, such as serine and glycine, have been associated with prolonged survival in mouse models of various tumor types. The mechanisms underlying this observed survival benefit could include the correction of abnormal cellular nucleotide, protein, and lipid synthesis; improved mitochondrial function; and changes in epigenetic modifications. 160 , 161

The antitumoral effect of a low-protein diet also hinges on promoting immunosurveillance against cancer, while the dietary restriction of amino acids may adversely affect the metabolic reprogramming of the TME in various ways. In multiple mouse models, reducing dietary methionine inhibited tumor growth and boosted antitumor immunity by increasing the quantity and cytotoxicity of tumor-infiltrating CD8 + T cells. 162 Moreover, restricted intake of dietary protein or methionine/cystine has been shown to modify the infiltration and tumoricidal capacity of TAMs, leading to a significant increase in tumor-infiltrating CD8 + T cells and a decrease in the number of infiltrating MDSCs. Mechanistically, a protein-restricted diet inhibited mTOR pathway activation and increased macrophage acquisition of an antitumor phenotype by increasing the number of macrophages undergoing polarization to the M1 type. 163 Macrophages might sense diet-derived cytosolic amino acids via the GTPase Rag, which subsequently regulates the expression of TFEB, TFE3 and mTORC1 when activated. 164 Furthermore, an isocaloric diet that moderately reduced protein intake (by 25%) was shown to trigger an unfolded protein response (UPR) that depended on IRE1α in cancer cells. The increase in UPR activation, in turn, led to an increase in the recruitment of CD8 + T cells and enhanced antitumor immunosurveillance. Notably, intake of a low-carbohydrate diet did not exert the same effect. 165 Although a low-protein isocaloric diet has been proven to reduce the concentration of amino acids in tumor tissues, it remains uncertain whether this reduction is limited to certain amino acids. Thus, further research is needed to explore the correlation between a low-protein isocaloric diet and the decrease in the levels of specific amino acids in tumors.

Interestingly, several studies have shown that high-protein diets may also benefit the restriction of tumor growth or clinical outcoming of cancer patients, which seem contradictory to the findings of the protein restriction diet discussed above. However, the underlying mechanisms are totally different. A high-protein diet increased the production of urinary urea in a tumor protein 53 (TP53)-mutated orthotopic bladder tumor mouse model, leading to the cascade modulation of ammonia in tumor cells, which induces tumor apoptosis. 166 These findings challenge the former hypothesis that high urinary urea concentrations caused by a high-protein diet might serve as a potential carcinogenic factor in the bladder, suggesting the urgent need for further investigation. 167 Applying a high-protein diet may improve the overall survival of older outpatients with advanced gastrointestinal cancer, which may improve the nutritional state of these patients with poor digestive system function. 168

Moreover, there have been efforts to develop a series of drugs that mimic amino acid restriction. One focus of researchers in the cancer therapy field has been on glutamine metabolism, as cancer cells rely heavily on glutamine. Glutaminase inhibitors, for instance, have been shown to decrease tumor burden. 169 , 170 The use of 6-diazo-5-L-oxo-norleucine (DON) promoted antitumor immunity by greatly favoring OXPHOS over glycolysis in CD8 + T cells while disrupting the metabolism of cancer cells. 171 Notably, DON showed the ability to significantly inhibit the generation and recruitment of MDSCs and to reprogram M2-like TAMs into proinflammatory TAMs, which increased tumor antigen cross-presentation to T cells and enhanced the efficacy of immune checkpoint blockade (ICB). 172 In addition, CB-839, which is considered the most effective glutaminase inhibitor, can be utilized alone or in combination with PD-1 inhibitors to treat solid or hematological malignancies. 173 , 174 , 175 As previously mentioned, IDO and TDO are tryptophan catabolism enzymes, and inhibitors of these enzymes have been developed and evaluated in various clinical trials. 176 For example, epacadostat is a novel compound that serves as an IDO1 inhibitor, suppressing systemic tryptophan catabolism. 177 Both in vitro and in vivo studies have demonstrated that epacadostat can reduce tumor growth and promote the proliferation of T cells and NK cells. 178 Furthermore, cyst(e)inase, a glutathione inhibitor that degrades cysteine and cystine, reduces tumor progression by elevating ROS levels and inducing tumor cell-selective ferroptosis. 179 , 180

High-salt diet

HSD has long been considered as a risk factor and trigger of malignancies. However, recent studies have provided new insights into the effect of sodium intake. As research continues, it is becoming increasingly clear that salt can accumulate in the interstitium and modulate immune cell differentiation, activation, and function through the effects of extracellular hypersalinity. 181 In addition, consumption of a HSD can lead to elevated tissue sodium concentrations and affect immune responses within microenvironments, ultimately impacting the development of immune-regulated diseases such as infections and cancer. 182

HSD, comprising 4% sodium chloride (NaCl), is recognized as a robust immunomodulator that is capable of eliciting a substantial inflammatory response. 183 Indeed, research has shown that high salt conditions can inhibit tumor growth by enhancing antitumor immunity, particularly through the modulation of MDSC functions. 184 According to a recent study, an HSD reduced the production of cytokines essential for the expansion of MDSCs and thus attenuated the accumulation of MDSCs within the tumor niche. As a result, the two primary types of MDSCs acquired different phenotypes: M-MDSCs differentiated into antitumor macrophages, and PMN-MDSCs adopted a proinflammatory phenotype, which led to the reactivation of T-cell antitumor functions. 185 Furthermore, a high salt level has been found to induce the transformation of anti-inflammatory Tregs into proinflammatory Th1 cells, which led to the secretion of the inflammatory cytokine IFNγ. 186 In another study, salt functioned as an adjuvant that enhanced the effectiveness of anti-PD-1 immunotherapy in tumor regression. Specifically, an HSD induces NK cell-mediated tumor immunity by suppressing PD-1 expression while increasing IFNγ levels and the serum hippurate concentration. Notably, hippurate is a microbial benzoate metabolism product that has been identified as a metabolic marker of effective PD-1 immunotherapy in responsive patients. 183 Although the major antitumoural effect of HSD is modulating immune cell function, mechanisms other than immunomodulation have also been discovered. For instance, HSD suppressed tumor growth and lung metastasis in a murine model of breast cancer, possibly by inducing hyperosmotic stress or through mimicking CR. 187

Nevertheless, despite the potential benefits of salt intake on cancer treatment effectiveness, high salt intake can also lead to the development of a proinflammatory state, which can negatively impact cancer outcomes. 188 High salt intake is a risk factor for various types of cancer in humans, including lung, testicular, bladder, renal cell, pancreatic, esophageal, and gastric cancer. 182 HSD has been shown to induce chronic inflammation, which may in turn incite continuous cell proliferation, DNA damage, or cancer transformation. However, whether there is a connection remains uncertain. 188 IL-17, specifically IL-17A, plays an important role in the mechanism of action of HSD. Evidence suggests that high salt intake can induce the differentiation of Th17 cells, a prominent source of IL-17A. 189 The overproduction of IL-17A can lead to inflammation and other immune responses that contribute to various pathologies. Furthermore, in the case of breast cancer, an HSD has been found to promote tumor progression and lung metastasis, increase the proportion of Th17 cells, and activate the MAPK/ERK signaling pathway in breast cancer cells through the secretion of IL-17F. The increase in the secreted IL-17F level results in the unregulated expression of protumor genes and the induced inflammatory responses, ultimately accelerating the proliferation, migration and invasion of breast tumors. 190 In addition, the combination of high NaCl concentrations with subeffective IL-17 has been proven to reduce reactive nitrogen and oxygen species (RNS/ROS) levels and enhance the growth of breast cancer cells. 191 , 192 Recent research has also demonstrated that intake of an HSD can disrupt the development and function of NK cells in mice. 193 Therefore, it can be concluded that dietary salt may exert dual effects on tumorigenesis, and the contradictory results obtained may be due to variations in the effects of high salt concentrations on tumors in different tissues and during different phases of tumor development.

Obesity and high-fat diet

Obesity, a serious health issue characterized by excessive body fat, is a known risk factor for multiple types of cancer. It can be induced or exacerbated by HFD, characterized by the consumption of foods rich in saturated fats and cholesterol. 194 Obesity can induce systemic metabolic disruptions within the body, leading to dyslipidemia, hypercholesterolemia, insulin resistance, alterations in hormone levels, and changes in the baseline inflammation status. 195 Conversely, a low-fat diet, typically associated with reduced total fat intake, can potentially lower the risk of certain types of cancer. 196 , 197 Given that both HFD and obesity are major factors influencing cancer risk, the forthcoming discussion will primarily focus on these aspects. By diving deeper into the mechanisms by which HFD and obesity affect cancer development and progression, we aim to provide a more comprehensive understanding of this intricate relationship.

Dietary obesity is associated with multiple factors related to cancer occurrence and exacerbation of immune suppression in tumor niches. 198 In the context of obesity, increased hepatic expression of the unconventional prefoldin RPB5 interactor (URI) has been shown to couple nutrient surplus with inflammation, leading to nonalcoholic steatohepatitis (NASH) and consequent HCC. This process involves URI-induced DNA damage in hepatocytes triggering Th17 lymphocyte-mediated inflammation, and subsequent IL-17A-induced adipose tissue neutrophil infiltration, which promotes insulin resistance and hepatic fat accumulation, thereby inducing NASH and HCC. 199 Notably, obesity also accelerates Helicobacter felis -induced gastric carcinogenesis by enhancing the trafficking of immature myeloid cells and the Th17 response. This exacerbates proinflammatory immune responses, characterized by cross-talk between inflamed gastric and adipose tissues, thereby contributing to a protumorigenic gastric microenvironment. 200

Diet-induced obesity has been shown to elevate nitric oxide (NO) production, which enhances tumor growth. This is primarily due to the recruitment of macrophages and the overexpression of inducible NO synthase as a result of HFD. 201 Additionally, in response to HFD intake, IL-6-mediated inflammation has been shown to accelerate prostate cancer tumor growth and increase the fraction of MDSCs and the M2/M1 macrophage ratio. 202 The effects of diet-induced obesity extend to the microenvironment of colitis-associated CRC. Here, diet-induced obesity has been shown to increase IL-6 expression and promote the polarization of macrophages into M2-like macrophages, enhancing the production of CC-chemokine-ligand (CCL) 20. CCL20 recruits CC-chemokine receptor 6 (CCR6)-expressing B cells and γδ T cells, ultimately leading to colitis-associated CRC progression. 203 In animal models of HFD-induced obesity, the infiltration rate of TAMs and the expression of cytokines in M2-like macrophages were increased, enhancing tumor growth and metastasis. However, ablation of VEGFR-1 signaling can reverse the abnormal TME associated with obesity and reprogram TAMs to promote their acquisition of the M1 phenotype. 204

The intake of an HFD has been shown to significantly increase the incidence of oral squamous cell carcinoma (OSCC) by expanding MDSCs within the local immune microenvironment. 205 Obesity induced by diet can also trigger the accumulation of PMN-MDSCs, leading to Fas/FasL-mediated apoptosis of tumor-infiltrating CD8 + T cells and causing resistance to immunotherapy in breast cancer treatment. 206 Obesity has been shown to suppress the infiltration and function of CD8 + T cells, which was linked to decreased chemokine production, reduced fatty acid availability, and alterations in amino acid metabolism. 207 , 208 Moreover, based on findings from mouse models, obesity reduced the number and function of CD4 + T cells in the TME of CRC, leading to a compromised antitumor response of both CD4 + and CD8 + T cells and ultimately accelerating disease progression. 209 Furthermore, considerable evidence shows that obesity-associated adipocytes in pancreatic ductal adenocarcinoma can secrete IL-1β to attract tumor-associated neutrophils (TANs), which subsequently activate pancreatic stellate cells and contribute to tumor growth. 210

HFD or diet-induced obesity may induce tumor metastasis. HFD has been proven to increase palmitate secretion from alveolar type 2 cells and nuclear factor-kappaB subunit p65 acetylation in the lung to prepare a premetastatic niche. 211 HFD-induced fatty liver may promote liver metastasis by facilitating the secretion of hepatocyte-derived extracellular vesicles (EVs), which transfer Yes-associated protein (YAP) signaling-regulating microRNAs, hence elevating nuclear YAP expression, CYR61 expression, and M2-like macrophage infiltration. 212 Another mechanism of HFD-induced liver metastasis is the upregulation of NOD-like receptor C4 (NLRC4), which further induces M2-like macrophage activation and IL-1β processing. An alteration from an indolent to a metastatic state may be stimulated by HFD-induced lipid accumulation in prostate tumors, the mechanism of which may be related to the sterol regulatory element-binding protein (SREBP)-related prometastatic lipogenic program. 213 In addition, it is widely acknowledged that the fatty acid receptor CD36 plays an important role in HFD-related metastasis promotion by enhancing the metastatic potential of CD36 + metastasis-initiating cells. 214 However, a recent study revealed that CD36 may prevent palmitate-induced lipotoxicity rather than facilitating HFD-driven metastasis, suggesting that further investigations of the dual effects of CD36 are needed. 215

An elevated cholesterol level is an obesity comorbidity, and studies suggest that the effects of obesity on cancer may be partly mediated by increased cholesterol levels. 216 In fact, a high-cholesterol diet (HCD) alone has been shown to promote macrophage infiltration and significantly enhance the growth of CRC tumors. 217 One mechanism by which HCD promotes CRC progression is through the inhibition of the CD8 + T-cell response. Specifically, macrophages with infiltration driven by HCD can secrete CCL5, which obstructs the activation of CD8 + T cells, thereby facilitating the evasion of immune system surveillance by CRC cells. 218 27-Hydroxycholesterol (27-HC) is a crucial mediator of the effects of dietary cholesterol on cancer metastasis. This oxysterol is synthesized through the action of the CYP27A1 enzyme and is present at high levels in the circulatory system. 219 Oxysterol has been shown to modulate the TME by recruiting immunosuppressive neutrophils to the metastatic niche, facilitating cancer progression. 220 However, some studies have reported conflicting findings regarding the effects of high serum cholesterol levels on cancer progression. For instance, one study showed that high serum levels of cholesterol attributed to HCD intake increased the accumulation of NK cells and promoted their effector functions to reduce the growth of liver tumors in mice. 221 However, further studies are needed to understand these conflicting findings.

In expanding on the relationship between HFD and tumor promotion, it is worth noting that the tumor-promoting effect of HFD is not universal and depends largely on the subtype of fatty acids involved. Mouse models of breast cancer developed comparable obesity levels from an HFD of either cocoa butter or fish oil. However, the consumption of the cocoa butter HFD, which is high in saturated fatty acids, led to faster mammary tumor growth and increased protumor macrophages and IL-10 expression while reducing B-cell and CD8 + T-cell infiltration. On the other hand, the fish oil HFD, which is rich in omega-3 fatty acids, disrupted the typical obesity-tumor growth link and reduced the number of protumor macrophages. 222 This effect of dietary omega-3 fatty acids is mediated by host GPR120 and has also been shown to inhibit prostate cancer. 223 Moreover, oleic acid (OA) and linoleic acid (LA) are the most common unsaturated fatty acids in dietary oils. While both an HFD rich in OA and an HFD rich in LA can similarly induce obesity in mice, a diet high in LA specifically encourages the growth of mammary tumors. Furthermore, an LA-rich HFD can impair antitumor T-cell responses via the induction of mitochondrial dysfunction. 224 Based on these findings, it appears that modulating dietary oil composition may constitute a promising strategy for enhancing immune function in both the prevention and treatment of obesity-associated cancers. By carefully selecting and balancing the types of fatty acids in HFDs, it may be possible to reduce the tumor-promoting effects of obesity while simultaneously increasing immune responses against tumors. Further research in this area may help to identify more precise dietary interventions that can ultimately improve outcomes for individuals at risk of developing obesity-associated cancers.

Potential role of dietary factors in cancer treatment

Recent studies have highlighted the pivotal influence of the TME on the efficacy of immunotherapy in cancer treatment. 225 Immunotherapy, recognized as a substantial advance in cancer treatment, has revolutionized the field of oncology by augmenting the body’s innate defenses to effectively target and eliminate malignant cells. 226 Various forms of cancer immunotherapy have been developed, including oncolytic virus therapies, cancer vaccines, cytokine therapies, adoptive cell transfer, and ICIs, all of which have shown promise in clinical practice. 227 Among these therapies, ICIs are perhaps the most important, as they are antibody-based drugs that can eliminate the influence of tumor-specific CD8 + T cells. 228 In particular, ICIs targeting PD-1 or its ligand PD-L1 have demonstrated notable clinical efficacy in the treatment of various advanced cancers. 229

Extensive research has been conducted to identify the effects of various dietary substances and patterns on tumor growth, metastasis and TME reprogramming, which has led to the consideration of nutritional intervention as a possible strategy for increasing the efficacy of tumor treatment 230 , 231 (Tables 2 , 3 ). The decline in T-cell functionality with aging, a widely documented phenomenon, is linked to a reduced efficacy of anti-OX40 immunotherapy in murine models. 232 CR not only preserves T-cell function but also improves the response of aged CD4 + T-cell populations to anti-OX40 therapy. 233 When used in combination with immunogenic cell death (ICD)-inducing chemotherapy and immunotherapy, CRMs potentially enhance the efficacy of cancer treatments through synergistic effects. 234 Preclinical studies have shown that STF, which serves as an adjunct to various cancer treatments, may bolster antitumor immunity by attenuating immunosuppressive conditions and amplifying CD8 + T-cell cytotoxicity. 235 For example, an experimental study of non-small cell lung cancer demonstrated that STF sensitized cancer cells to anti-PD-1 therapy. The antitumor efficacy of combination therapy was achieved by inhibiting IGF-1-IGF-1R signaling in cancer cells, boosting the intratumoral CD8 cell: Treg ratio in the TME. 132 Furthermore, intake of an FMD has been shown to enhance the effectiveness of immunotherapy against triple-negative breast cancer with low immunogenicity by affecting the TME. Specifically, intake of an FMD has been shown to reactivate T eff cells that underwent early exhaustion, shift cancer metabolism from glycolytic to OXPHOS, and reduce the collagen deposition rate. 236 These effects led to the increased efficacy of anti-PD-L1 and anti-OX40 immunotherapy. These results suggest that combining immunotherapy with dietary restriction may lead to profound synergistic effects.

KD also enhances the antitumor effects of PD-1 blockade alone or in combination with anti-CTLA-4 antibodies. Mechanistically, the principal ketone body 3-hydroxybutyrate (3HB) in a KD prevented the ICB-mediated upregulation of PD-L1 on myeloid cells while simultaneously promoting the expansion of CXCR3 + T cells. 237 Similarly, KD enhanced the effectiveness of anti-CTLA-4 immunotherapy by reducing PD-L1 protein levels and augmenting the expression of interferons and antigen presentation-related genes. When combined with immunotherapy, the intake of a KD can reshape the TME by increasing the population of CD8 + TILs, macrophages and CD86 + DCs. Mechanistically, the activation of AMPK via KD intake is the key molecular event that promotes immunotherapy efficacy. This activated AMPK phosphorylates PD-L1 on Ser283, which interrupts its association with CMTM4 and results in PD-L1 degradation. Furthermore, AMPK phosphorylates EZH2, which impedes polycomb repressive complex 2 (PRC2), leading to an increase in interferons and antigen-presenting gene expression. 238

Combining a protein-restricted diet with a vaccine or anti-PD-1 therapy has been shown to significantly inhibit tumor growth and prolong survival. 239 Notably, treatment with a methionine-/cystine-restricted diet significantly increased the number of tumor-infiltrating CD8 + T cells and cytotoxic granzyme B + CD8 + T cells, which was further enhanced when combined with immunotherapy. 163 Another study confirmed the inhibitory effect of dietary methionine restriction on tumor growth and its ability to synergize with PD-1 blockers to increase tumor control. Mechanistically, this dietary approach reduced the number of metabolites, such as S-adenosylmethionine (SAM), which controls N6-methyladenosine (m6A) methylation reactions, in cancer cells. A reduction in the SAM level altered the m6A modification rate and decreased the expression of PD-L1 and V-domain Ig suppressor of T-cell activation (VISTA) in cancer cells. 162 Moreover, the enzyme cyst(e)inase breaks down cystine and cysteine, thereby bolstering T-cell-mediated antitumor immunity and inducing ferroptosis in tumor cells when combined with PD-L1 blockade. 240 IDO1 is a critical enzyme in the tryptophan–kynurenine pathway and has been identified as a promising immunomodulatory target. 241 A phase 1/2 (ECHO-202/KEYNOTE-037) trial evaluating the effectiveness of the IDO1 inhibitor epacadostat combined with pembrolizumab on advanced solid tumors showed a high objective response rate (ORR) of 40.3% overall and 61.9% in malignant melanoma patients, demonstrating promising antitumor efficacy. 242 Unfortunately, phase 3 trials failed to confirm these benefits. The ECHO-301/KEYNOTE-252 trial showed that combining epacadostat with pembrolizumab failed to prolong progression-free survival (PFS) or overall survival (OS) compared to pembrolizumab alone in patients with advanced melanoma. 243

Despite being linked to T-cell dysfunction and poor cancer prognosis, obesity has paradoxically been shown to enhance the response to anti-PD-1/PD-L1 immunotherapy. 244 Recent research suggests that immunotherapy yielded superior outcomes in obese patients, evidenced by an improved response rate and extended PFS and OS, in comparison to lean patients. 245 However, obesity also promoted tumor growth and T-cell exhaustion, leading to increased PD-1 expression and dysfunction, partly due to high leptin levels. Despite this outcome, PD-1-mediated T-cell dysfunction in individuals with obesity was found to significantly enhance tumor responsiveness to PD-1/PD-L1 inhibitors, as confirmed by preclinical and clinical data. 246 Therefore, obesity seems to be a double-edged sword for cancer immunotherapy, and the underlying mechanisms remain unclear and require further investigation.

Chemotherapy

Chemotherapy, a cornerstone of traditional cancer treatment, employs drugs to destroy rapidly dividing cells, a defining characteristic of cancer. 247 Despite its widespread use and undeniable efficacy in many cases, chemotherapy often has substantial side effects due to its impact on healthy cells. 248 Additionally, individual responses to chemotherapy can vary greatly and are influenced by a multitude of factors, including genetics, tumor characteristics, and, intriguingly, diet. 15 A growing body of research now highlights the role of dietary interventions in modulating the effectiveness of chemotherapy, emphasizing the need to further understand these interactions for improved therapeutic outcomes.

Due to their expression of oncogenes, cancer cells are more susceptible to the effects of fasting and CR than are normal cells, an effect termed ‘differential stress resistance’. 14 , 249 , 250 Based on this characteristic, CRM hydroxycitrate has been shown to increase sensitivity to chemotherapy by eliciting an adaptive cellular immune response, resulting in a decrease in the number of tumor-infiltrating Tregs into the tumor niche in various tumor models. 122

Emerging research also suggests a profound influence of fasting or FMD on the efficacy of chemotherapy. In vitro studies indicate that fasting cycles not only retard tumor growth but also sensitize a wide array of cancer cell types to chemotherapy. 14 This heightened sensitivity has been observed in various contexts, including the enhancement of gemcitabine efficacy in mice with prostate cancer xenografts and the increased efficacy of chemotherapy in triple-negative breast cancer via the upregulation of ROS. 251 , 252 FMD combined with vitamin C can potentially increase the effectiveness of chemotherapy for treating KRAS-mutant cancer cells by reversing the vitamin C-induced upregulation of HO-1 and ferritin. 253 Furthermore, when combined with a ferroptosis inducer, FMD can effectively eliminate slow-cycling, chemotherapy-resistant cells, suggesting a potential strategy for enhancing the sensitivity of certain difficult-to-treat cancers to chemotherapy through dietary interventions. 254 Interestingly, fasting can also counteract certain adverse effects of chemotherapy. For instance, it has been demonstrated to enhance self-renewal in hematopoietic stem cells and mitigate the immunosuppression induced by cyclophosphamide chemotherapy in mice. 255 In tumor-bearing mice, both prolonged fasting and FMDs can induce specific stress resistance responses, enhancing chemotoxicity in cancer cells while protecting normal cells. 256 This dual action is partly mediated by the reduction in IGF-1 and glucose levels, thus shielding normal cells and organs from chemical toxicity. 250 The potential of FMD in clinical settings has been supported by the ‘DIRECT’ study involving HER2-negative stage II/III breast cancer patients. This study revealed that treatment with FMD, administered three days prior to and during neoadjuvant chemotherapy, enhanced therapeutic efficacy without increasing toxicity or reducing chemotherapy-induced DNA damage in T cells. 257 Collectively, these findings highlight the potential of fasting and FMD as adjuncts to chemotherapy, warranting further exploration and clinical testing.

In addition to slowing tumor growth, KD also sensitizes tumor cells to classic chemotherapy. For example, the combination of KD with metronomic cyclophosphamide significantly enhances antitumor effects, resulting in the regression of neuroblastoma tumors. 258 , 259 Similarly, in pancreatic cancer, cotreatment with KD and cytotoxic chemotherapy substantially elevates tumor NADH levels, synergistically suppressing tumor growth and tripling survival benefits compared to chemotherapy alone. 260

Radiotherapy

Dietary interventions have emerged as promising strategies for enhancing the efficacy of radiotherapy in cancer treatment. For instance, CR combined with radiotherapy, has been shown to modulate the TME in a triple-negative breast cancer model by decreasing the number of intratumoral Tregs, increasing the CD8 + cell: Treg ratio, and upregulating PD-1 expression on CD8 + T cells. Furthermore, compared with patients who received radiotherapy alone, breast cancer patients who underwent CR concurrently with radiotherapy exhibited a significant reduction in the serum levels of immunosuppressive cytokines, suggesting potential benefits of CR in mitigating radiation-induced immunosuppression. 261

When combined with radiation or radiochemotherapy, KD slows tumor growth in lung cancer xenografts, potentially through a mechanism involving increased oxidative stress. 262 Additionally, KD was shown to enhance radiation sensitivity in a pancreatic cancer xenograft model, suggesting potential improvements in therapeutic outcomes. However, phase 1 clinical trials in patients with locally advanced non-small cell lung cancer and pancreatic cancer showed suboptimal compliance with the diet, indicating challenges in practical application. 263

Moreover, other dietary restrictions, such as methionine deprivation, have shown promising results in enhancing the efficacy of radiation and antimetabolite chemotherapy. In patient-derived xenograft and autochthonous tumor mouse models, methionine restriction sensitized tumor cells to these treatments, possibly via alterations in one-carbon metabolism. 264

Other therapies

In hormone receptor-positive breast cancer mouse models, periodic fasting or an FMD can enhance the therapeutic effects of endocrine agents such as tamoxifen and fulvestrant. This enhancement is believed to occur through a reduction in circulating IGF1, insulin, and leptin levels and suppression of AKT-mTOR signaling. Concurrent administration of these dietary strategies with a therapeutic regimen of fulvestrant and palbociclib has been associated with prolonged tumor regression and reversal of treatment resistance. Analogous metabolic alterations found in patients on an FMD during estrogen therapy suggest the potential of diet as an adjuvant in treating hormone receptor-positive breast cancer. 265

In addition to their effects on hormone-driven cancers, fasting or FMD has also been shown to enhance the efficacy of tyrosine kinase inhibitors (TKIs) across different cancer cell lines. Mechanistically, these effects are attributed to the increased ability of TKIs to block cancer cell growth and inhibit the MAPK signaling pathway under starvation conditions. 266 Another study reported that in HCC cells, xenografts, and patient-derived organoids, fasting improved the therapeutic response to sorafenib through the regulation of glucose transporters and proapoptotic protein expression by p53. 267

KD has also shown promise in supporting the effectiveness of phosphatidylinositol 3 kinase (PI3K) inhibitors and overcoming drug resistance in various mouse cancer models, including pancreatic, bladder, endometrial, and breast cancer models, as well as acute myeloid leukemia. 145 KD appears to enhance this effectiveness by decreasing hyperglycemia and reducing insulin secretion, actions correlated with a decrease in mTORC1 signaling within the tumor. 268

Finally, the combination of serine deprivation and biguanide treatment, such as phenformin and metformin, can lead to metabolic stress in cancer cells. This stress arises from the forced upregulation of glycolysis due to the biguanide-induced reduction in OXPHOS. Under conditions of serine deficiency, this stress may exceed the metabolic flexibility of cancer cells, leading to their potential death and, consequently, enhanced anticancer effects. 269

In summary, these findings underscore the potential of dietary interventions to modulate the therapeutic landscape of cancer treatment, enhancing the effectiveness of drugs and potentially overcoming resistance mechanisms. However, it should be viewed with cautious optimism. The biological plausibility of diet modifying treatment efficacy and resistance is compelling; however, the translation of this concept into clinical practice requires rigorous validation. It is critical to remain grounded in evidence-based medicine, recognizing that dietary strategies are adjuncts, not replacements, for established therapeutic regimens. Further exploration and clinical validation are necessary to fully understand these interactions and to integrate dietary strategies into standard cancer care effectively and safely.

Diet changes the gut microbiome in conjunction with antitumor effects and cancer treatment

The gut microbiome encompasses the genetic makeup of all species within the gut, such as bacteria, viruses, yeasts, protozoans, fungi, and archaea, and can be affected by a range of internal and external factors. 270 The gut microbiota plays a significant role in influencing the health and disease status of the host. The constituents of the gut microbiome and their interactions with the host immune system can impact the development of tumors and carcinogenesis. 271 Various dietary patterns have been found to significantly influence the composition and functionality of the gut microbiome. 272 , 273 It is through these changes in the gut microbiome that dietary patterns can indirectly influence the outcomes of cancer patients. 274

In recent early studies, several interventional strategies, ranging from dietary interventions to fecal microbiome transplant (FMT) and prebiotic, probiotic and antibiotic treatments, have shown promise in altering the composition or functional capacity of the gut microbiome. 275 Two prospective cohort studies have suggested that diet-related inflammation can alter the gut microbiome, leading to the development of CRC by suppressing adaptive antitumor immune responses. 276 , 277 Other prospective cohort studies have revealed the associations between prudent diets (rich in whole grains and dietary fiber) and Western diets (rich in red and processed meat, refined grains, and desserts) with CRC risk and indicated that the effect of these diets may differ based on the presence of Fusobacterium nucleatum in tumor tissue. 278 , 279 Specifically, these studies showed that, compared with a Western diet, adhering to a long-term prudent diet is associated with a reduced risk of F. nucleatum -positive CRC; however, it does not appear to mitigate the risk of F. nucleatum -negative CRC. 278 A recent study investigated the impact of the gut microbiota and dietary patterns on the response to ICIs in patients with melanoma. The present study revealed that patients with microbiomes dominated by the Ruminococcaceae family had greater response rates than did those with microbiomes dominated by the Bacteroidaceae family. Furthermore, another finding revealed that a poor response was associated with decreased intake of fiber and omega-3 fatty acids. 280 These results suggest that dietary interventions may be promising for improving cancer treatment outcomes.

Accumulating data suggest that alterations in the gut microbiome primarily contribute to the progression, prognosis, and treatment of cancer, primarily through interactions with the immune system. Metabolites produced by the microbiota play important roles in modulating antitumor immunity. 281 , 282 Microbiota-derived metabolites have been demonstrated to influence the efficacy of tumor immunotherapy. Short-chain fatty acids (SCFAs) are produced primarily by the fermentation of nondigestible carbohydrates, such as dietary fiber, by the microbiota. The main SCFAs include acetate, propionate, and butyrate. 283 , 284 The gut microbiota, which is mediated by SCFAs, can potentiate the antitumor activity of CD8 + T cells, thereby influencing the efficacy of tumor immunotherapy both in vitro and in vivo. 285 Metabolic and epigenetic reprogramming enables pentanoate and butyrate to enhance the effectiveness of cancer immunotherapy by boosting the antitumor activity of antigen-specific cytotoxic T lymphocytes and ROR1-targeting chimeric antigen receptor (CAR)-T cells. 286 Inosine is another important metabolite produced by the microbiome and is closely associated with immunotherapy. Intestinal Bifidobacterium pseudolongum promoted Th1 cell transcriptional differentiation and antitumor activity to increase the efficacy of immunotherapy, mainly through the action of inosine. 287 Inosine is instrumental in enhancing antitumor therapy by serving as a carbon source for CD8 + T cells in glucose-restricted microenvironments, facilitating their growth and optimal functioning. 288 Moreover, engineered bacteria can modify the concentration of metabolites in the microenvironment, thereby altering the composition of the TME. For instance, the genetically engineered probiotic strain Escherichia coli Nissle 1917 colonizes tumor sites and continuously converts ammonia metabolites into L-arginine. When injected into the tumor, this strain has been shown to increase the concentration of L-arginine within the microenvironment, leading to increased infiltration of tumor-infiltrating T cells, sustained effector T-cell functions, increased tumor-specific T-cell memory formation, and enhanced efficacy of PD-L1-blocking antibodies. 289

Recent research has highlighted the role of the gut microbiota in the antitumor effects of dietary intervention (Fig. 3 ). Specifically, enrichment of Bifidobacterium bifidum after CR increases acetate levels, which in turn elevates IFNγ + CD8 + T cells in the TME. In contrast, the antitumor effect of IF was not mediated by the gut microbiome, as it was not abrogated after the microbiota was depleted. 290 Similarly, recent studies have revealed that KD significantly influences the gut microbiota, inducing a shift from a population dominated by tolerogenic bacteria ( Lactobacilli spp., Clostridium asparagiforme ) toward a population dominated by an increase in immunogenic bacteria (such as Akkermansia muciniphila ). 237 It has been reported that a shift in the gut microbiota is partially attributable to the host’s production of ketone bodies due to the intake of a KD. Among these ketone bodies, β-HB selectively suppresses the proliferation of Bifidobacterium . This suppression subsequently leads to a reduction in intestinal Th17 immune cells. 291 Dietary methionine/cystine restriction has been shown to alter the gut microbiota and potentially contribute to immune system alterations. Specifically, this type of diet restriction promoted a significant decrease in the relative abundance of multiple Ruminococcaceae and Prevotellaceae families while increasing the presence of members of the Lactobacillaceae family. 163 Consumption of an HSD promotes an increase in the abundance of Bifidobacterium , which, due to enhanced gut permeability, infiltrates tumors, subsequently augmenting the functionality of NK cells and ultimately contributing to tumor regression. These results suggest that HSD intake modulates the gut microbiome, which may stimulate NK cell-dependent tumor immunity, thereby providing potential implications for the development of novel therapeutic interventions. 183 The intake of HSD has also been shown to inhibit enterotoxigenic Bacteroides fragilis (ETBF)-promoted colon carcinogenesis by decreasing the expression of IL-17A and iNOS, thereby inhibiting inflammation. 292 However, intake of an HSD can exacerbate Helicobacter pylori infection, contributing to gastric carcinogenesis. 293 In a mouse model of Barrett’s esophagus, feeding an HFD was observed to promote dysplasia and carcinogenesis by modulating the esophageal microenvironment and gut microbiome, thereby inducing inflammation and promoting stem cell proliferation. 294 The bile salt hydrolase (BSH) enzyme expressed by Bacteroides was also found to play a crucial role in CRC progression in overweight patients and in model mice with HFD-induced CRC. High BSH activity activates the β-catenin/CCL28 axis, resulting in an increase in immunosuppressive Tregs and accelerated CRC progression. 295 Moreover, HFD feeding can reduce the level of SCFA-producing bacteria and the rate of SCFA production, leading to decreased levels of SCFAs that can activate the MCP-1/CCR2 axis. This effect promotes M2 TAM recruitment and polarization, ultimately contributing to CRC progression. 296

Mechanisms by which diet modulates antitumor effects and cancer treatment via modulation of the gut microbiome. a Calorie restriction (CR) elevates IFNγ + CD8 + T cells in the tumor microenvironment (TME) by enriching Bifidobacterium bifidum and increasing acetate levels. b Ketogenic diet (KD) induces a shift from tolerogenic ( Lactobacilli spp., Clostridium asparagiforme ) toward immunogenic bacteria (such as Akkermansia muciniphila ) driven by host production of ketone bodies, of which β-HB selectively inhibits the growth of bifidobacteria, resulting in KD-associated decreases in intestinal Th17 cell levels. c High-salt diet (HSD) increases the abundance of Bifidobacterium and leads to intratumoral localization of Bifidobacterium , further enhancing NK cell functions and tumor regression. HSD decreases the expression of IL-17A and iNOS and inhibits inflammation, which reduces enterotoxigenic Bacteroides fragilis (ETBF)-promoted colon carcinogenesis. HSD exacerbates Helicobacter pylori infection and promotes gastric carcinogenesis. d High-fat diet (HFD), through augmentation of queuosine-producing gut bacteria, can incite chemotherapy resistance in pancreatic cancer patients. HFD reduces SCFA-producing bacteria and SCFA production, leading to decreased levels of short-chain fatty acids (SCFAs) that activate the MCP-1/CCR2 axis, which promotes M2 TAM recruitment and polarization, ultimately contributing to colorectal cancer (CRC) progression. High bile salt hydrolase (BSH) enzyme activity in an HFD mouse model activates the β-catenin/CCL28 axis, further inducing immunosuppressive Tregs and accelerating CRC progression. e Dietary intake rich in tryptophan stimulates certain Bacteroides to produce the metabolite indole-3-acetic acid (3-IAA). Increased levels of 3-IAA enhance the efficacy of chemotherapy treatment. Dietary intake rich in tryptophan, through the action of the probiotic Lactobacillus reuteri (Lr), leads to the production of the metabolite indole-3-aldehyde (I3A). This metabolite promotes the production of IFNγ from CD8 + T cells, thereby enhancing antitumor immunity and the efficacy of immune checkpoint inhibitors (ICIs). f High-fiber diet enriches Akkermansia muciniphila which produces the microbiota-derived STING agonist c-di-AMP, inducing type I interferon (IFN-I) production by intratumoural monocytes, resulting in various TME modulation pathways, including reprogramming of mononuclear phagocytes into immunostimulatory monocytes and DCs, promoting macrophage polarization toward an antitumor phenotype and stimulating crosstalk between NK cells and DCs, further enhancing the therapeutic effect of immunotherapy. Dietary fiber inulin can enhance the effectiveness of anti-PD-1 therapy by increasing the abundance of beneficial commensal microbes (e.g., Akkermansia , Lactobacillus and Roseburia ) and SCFAs, further increasing the number of stem-like T-cell factor-1 (Tcf1) + PD-1 + CD8 + T cells numbers. Dietary fiber pectin can improve the effectiveness of anti-PD-1 therapy by increasing the abundance of butyrate-producing bacteria, further promoting T-cell infiltration and activation in the TME. This figure was created with BioRender.com

Studies suggest that the gut microbiota plays a crucial role in modulating the therapeutic response to immunotherapy. 297 , 298 In fact, specific gut microbial signatures have been shown to differentiate responders from nonresponders across various epithelial tumor types in cohorts treated with ICB. 299 Considering the profound impact of the gut microbiota on the immune system, research investigating the modulation of the gut microbiota via dietary interventions to optimize cancer treatment efficacy has been predominantly centered around immunotherapy. A high-fiber dietary intervention has been associated with significantly prolonged PFS in melanoma patients receiving ICB treatment. 300 Microbiota-derived STING agonists, specifically c-di-AMP, induce the production of type I interferon (IFN-I) in intratumoral monocytes. This activation results in the transformation of mononuclear phagocytes within the TME into immunostimulatory monocytes and DCs. Additionally, it promotes the polarization of macrophages to antitumor macrophages and stimulates crosstalk between NK cells and DCs. A high-fiber diet can trigger this mechanism by enriching the population of Akkermansia muciniphila , which produces c-di-AMP and enhances the therapeutic effect of ICB in melanoma patients. 301 The presence of Akkermansia , a mucin-degrading bacterium, is strongly associated with favorable outcomes in cancer patients. 302 Moreover, inulin, a polysaccharide dietary fiber, can enhance the effectiveness of anti-PD-1 therapy by increasing the abundance of beneficial commensal microbiota genera (e.g., Akkermansia , Lactobacillus and Roseburia ) and SCFAs, further increasing the number of stem-like T-cell factor-1 (Tcf1) + PD-1 + CD8 + T cells. 303 Similarly, oral administration of pectin, another dietary polysaccharide fiber, can largely improve the efficacy of anti-PD-1 mAbs by increasing the number of butyrate-producing bacteria, which is sufficient to promote T-cell infiltration and activation in the TME. 304

Although research into the antitumor or protumor effects of the intratumor microbiome is still in its early stages, recent studies have started to focus on how the intratumor microbiome can influence the effectiveness of immunotherapy. The colonization of Bifidobacterium in the microenvironment, combined with anti-CD47 monoclonal antibody treatment, stimulates the STING signaling pathway and enhances the cross-priming of DCs to upregulate CD8 + T cells. 305 The probiotic Lactobacillus reuteri (Lr) within melanoma promotes the local generation of IFNγ by CD8 + T cells through the release of its tryptophan breakdown metabolite, indole-3-aldehyde (I3A), thus enhancing ICI efficacy. Dietary intake rich in tryptophan boosts the antitumor immunity induced by Lr and ICI, which is dependent on the CD8 + T-cell AhR signaling pathway. 306

Apart from immunotherapy, recent research has also started to investigate how diet, by influencing the gut microbiota, could affect other forms of cancer treatment. By enriching the gut microbiome with queuosine-producing bacteria, HFD can induce chemotherapy resistance in pancreatic cancer through the upregulation of the oxidative stress protector PRDX1. This resistance can be counteracted by SAM, which is typically produced by bacteria in lean diets, highlighting the influence of diet on chemotherapy effectiveness via gut microbiome adjustments. 307 Expanding on the theme of diet’s influence on chemotherapy effectiveness in pancreatic cancer, another study revealed that the microbiota-derived tryptophan metabolite indole-3-acetic acid (3-IAA) is enriched in patients responsive to chemotherapy. Through dietary manipulation of tryptophan, an increase in 3-IAA production enhances chemotherapy efficacy by disrupting cancer cell metabolic fitness via increased reactive oxygen species and reduced autophagy. 308 These findings further emphasize the crucial role of gut microbiota modulation via dietary interventions in cancer treatment outcomes.

Despite the significant progress in this field, the complex relationships among dietary factors, the gut microbiota, and cancer treatment still need to be understood. Each individual’s microbiome is unique, influenced by genetics, diet, environment, and lifestyle, which adds layers of complexity to the task of identifying universally beneficial interventions. Additionally, the development of high-throughput technologies and bioinformatics tools for microbiome analysis will be vital in deciphering these complex interactions. These advancements could enable the identification of biomarkers for microbiome-related treatment responses and the customization of diet-based interventions to enhance the efficacy of cancer therapies. The identification of specific dietary factors and gut microbiota constituents that can enhance the effectiveness of cancer therapies may lead to the development of personalized treatments to improve therapeutic outcomes for cancer patients.

Implications of dietary intervention for other diseases

Dietary interventions may induce, prevent or delay the progression of various diseases in addition to cancer, which also influence human health and longevity. Healthy dietary patterns that are rich in fiber and beneficial nutrients may reduce the risk of disease, while unhealthy dietary patterns may increase the risk of disease and worsen clinical outcomes. 309 Here, we summarize preclinical and human studies revealing the implications and mechanisms of various dietary patterns on other diseases in addition to cancer, including neurodegenerative diseases, autoimmune diseases, CVD, and metabolic disorders.

Neurodegenerative diseases

Several neurodegenerative diseases (NDs), such as epilepsy, Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and amyotrophic lateral sclerosis (ALS), which feature chronic progressive damage to the nervous system, have been proven to be tightly connected with nutrient availability and dietary patterns. 310 The underlying mechanisms of various dietary interventions mainly include altering neurotransmitters, remodeling, interfering with brain energy metabolism and mitochondrial function, and altering inflammation and oxidative stress. The underlying mechanisms also include altering the composition and balance of the gut microbiome, which further influence the process of neurodegeneration via the gut-brain axis (Fig. 4 ).

Impact of different diets on neurodegenerative diseases. The ketogenic diet (KD) can enhance inhibitory neurotransmission and anti-inflammatory effects in epilepsy, influence the gut microbiota, and elevate beneficial metabolites. KD is particularly beneficial for treating pediatric drug-resistant epilepsy with elevated specific Bifidobacteria and TNF. In Alzheimer’s disease (AD) and Parkinson’s disease (PD), KD could counteract decreased β-HB levels, inhibit the NLRP3 inflammasome, reduce pathology, and alleviate symptoms by inhibiting microglial activation. Fasting mimicking diet (FMD) enhances the gut microbiota composition and metabolites, inhibiting neuroinflammation. This results in the attenuated loss of dopaminergic neurons in the substantia nigra in patients with PD. Caloric restriction (CR) may prevent AD by lowering serum tyrosine levels, reversing the exhaustion of tyrosyl-tRNA synthetase (TyrRS), and upregulating the sirtuin pathway, which attenuates the amyloidogenic processing of amyloid-β protein precursor (APP). Dietary restriction can increase brain-derived neurotrophic factor (BDNF) and chaperone heat-shock protein-70 (HSP70) levels in the striatum and cortex, which are relevant to Huntington’s disease (HD). High-fat diet (HFD) can accelerate recognition-memory impairment in an AD mouse model by increasing blood N-acetylneuraminic acid (NANA) levels, leading to systemic immune exhaustion. Conversely, the Mediterranean diet (MD) may protect against memory decline and mediotemporal atrophy by lowering amyloid-β protein and phosphorylated tau levels, reducing AD risk. This figure was created with BioRender.com

KD has been clinically applied for nearly a century as alternative therapy for childhood intractable epilepsy, but there is sufficient evidence that a modified Atkins diet (MAD) is more tolerable and has a greater probability of causing seizure reduction than a classical KD according to a systematic review. 311 , 312 , 313 Increased levels of the inhibitory neurotransmitter GABA can be observed in preclinical KD models and patient cerebrospinal fluid (CSF), dampening neuronal excitability. 314 , 315 , 316 An increase in peroxisome proliferator activated receptor gamma 2 (PPARγ2) and upregulation of hippocampal catalase in KD-fed rats are observed, which may increase anti-inflammatory and antioxidant activity. 317 In addition, a KD may upregulate potassium channels that are sensitive to ATP opening, reducing the electrical excitability of the brain and increasing the seizure threshold. 318 The gut microbiota, which includes Akkermansia , Parabacteroides , and Bifidobacteria , also contributes to the neuroprotective effects of KD on epilepsy. 319 , 320

Epidemiologic evidence indicates that obesity is an independent risk factor for AD, while HFD is closely associated with an increased risk of obesity. 321 Recognition-memory impairment in an AD mouse model (5xFAD) can be accelerated by high-fat obesogenic diet by increasing blood levels of the metabolite N-acetylneuraminic acid (NANA), which results in systemic immune exhaustion. 322 HFD may also enhance neuroinflammation by increasing circulating free fatty acids and cytokines, which may lead to cognitive impairment. 323 Conversely, healthy dietary interventions, including the Mediterranean diet (MD), CR, and KD, may prevent AD progression. 324 , 325 , 326 Adhering to MD may act as a protective factor against memory decline and mediotemporal atrophy, as indicated by decreased levels of amyloid-β protein and phosphorylated tau, reducing the risk of AD. 327 CR may prevent AD by lowering serum tyrosine levels to reverse the exhaustion of tyrosyl-tRNA synthetase (TyrRS) and upregulating the sirtuin pathway, which attenuates the amyloidogenic processing of amyloid-β protein precursor (APP), as confirmed by in vivo and in vitro models. 328 , 329 KD may reverse the decreased β-HB levels in red blood cells and the brain parenchyma of AD patients, hence inhibiting NLRP3 inflammasome activation and reducing AD pathology. 330 In addition, diet can influence AD by modulating the gut microbiome and metabolites. For instance, a Mediterranean-ketogenic diet (MMKD) is associated with improved AD biomarkers in CSF, as indicated by increased Akkermansia muciniphila levels, which modulate GABA levels and gut transit time. 331 , 332

Gut microenvironmental changes may trigger the development of PD through the gut-brain axis, as determined by the presence of α-synuclein and Lewy bodies in the enteric nervous system and the convincing association between PD and gut inflammation. 333 , 334 Research has revealed changes in the gut microbiome in PD patients compared to healthy volunteers, highlighting the potential benefits of dietary interventions in treating PD patients. 335 High serum sodium is associated with cognitive decline, as observed in the aged population. 336 However, a recent study denies the association between HSD and neurodegeneration or α-synuclein accumulation in a PLP-hαSyn model, suggesting that the mechanism of HSD needs further exploration. 337 Adhering to MD is associated with a decreased incidence of PD, the mechanisms of which may include reducing neuroinflammation, similar to AD. 338 , 339 KD ameliorates motor and nonmotor symptoms in PD patients by inhibiting microglial activation 340 . FMD promotes a favorable gut microbiota composition and metabolites and inhibits neuroinflammation, consequently attenuating the loss of dopaminergic neurons in the substantia nigra in a PD model. 341

Other neurodegenerative diseases with lower incidence rates are also relevant to dietary interventions. A clinical trial suggested that increased consumption of dairy products may increase the risk of phenoconversion, resulting in earlier onset of HD. 342 In addition, high antigliadin antibody titers in patients with HD suggest the potential value of applying gluten-free diet in HD patients. 343 A dietary restriction regimen retarded the progression of neuropathological, behavioral, and metabolic abnormalities in an HD model, resulting in an extension of life span by increasing brain-derived neurotrophic factor and chaperone heat-shock protein-70 (HSP70) levels in the striatum and cortex, the mechanisms of which still need further explanation. 344 A cross-sectional baseline analysis revealed that a higher intake of antioxidants and carotenes may result in greater ALS function. 345 Another meta-analysis revealed that a greater intake of ω-3 PUFAs is associated with a reduced risk of ALS. 346 Although weight loss has been identified as a negative prognostic factor, high-calorie fatty acid diet provides a significant survival benefit for patients in the subgroup of fast-progressing ALS patients only. 347

Autoimmune diseases

Different types of autoimmune diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), inflammatory bowel disease (IBD), Hashimoto’s thyroiditis (HT), and multiple sclerosis (MS), can cause distinct clinical features from abnormal activation of the immune system that erroneously attacks healthy host cells and tissues. Impaired gut barrier function, also referred to as a “leaky gut”, which may disrupt the balance between tolerance and immunity to non-self-antigens, is often observed in autoimmune diseases. 348 This finding suggested a close relationship between diet, the gut, and autoimmune diseases. Dietary interventions may influence the susceptibility, progression and treatment response of these autoimmune diseases through various mechanisms, from adjusting inflammation levels and immune cell composition to adjusting the gut microbiome composition (Fig. 5 ).

Impact of different diets on autoimmune diseases. Extravirgin olive oil (EVOO) can reduce joint inflammation and degradation in rheumatoid arthritis (RA) due to its phenolic compounds. However, the protective effects of a high-fiber diet can be reversed by Prevotella copri colonization, which promotes proinflammatory responses. Fish oil supplementation can suppress proinflammatory cytokines and cartilage degradation, improving RA outcomes. Vitamin D can inhibit the proliferation, differentiation, and function of B and T cells, potentially reducing inflammatory cytokine expression in systemic lupus erythematosus (SLE) patients. A diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs) can alleviate gut symptoms in quiescent inflammatory bowel disease (IBD) patients, possibly by regulating the immune response through reducing fecal microbial abundance. However, a high-fat diet (HFD) can exacerbate pre-IBD inflammation by impairing epithelial mitochondrial bioenergetics and triggering microbiota disruptions, especially when combined with antibiotics. High salt diet (HSD) can exacerbate autoimmune conditions such as multiple sclerosis (MS) by promoting the induction of pathogenic Th17 cells. Intermittent fasting (IF) can improve MS by reducing the number of IL-17-producing T cells, increasing the number of Tregs in the gut, and enhancing antioxidative microbial metabolic pathways. However, the Western diet can impair myelin-debris clearance in microglia, hindering lesion recovery after demyelination and potentially contributing to MS induction. This figure was created with BioRender.com

A healthy MD may benefit RA by reducing inflammatory activity and increasing physical function. 349 Phenolic compounds in extravirgin olive oil (EVOO), an essential component of the MD, can decrease joint edema, cell migration, cartilage degradation and bone erosion by reducing the levels of proinflammatory cytokines and prostaglandin E2 in the joint. 350 However, the protective effect of high-fiber diet may be reversed if there exists colonization of Prevotella copri , which leads to the overproduction of organic acids, including fumarate and succinate, during the digestion of complex fibers and the promotion of proinflammatory responses in macrophages, exacerbating arthritis in an RA model. 351 In addition, abundant supplementation of fish oil benefits the clinical outcome of RA by suppressing the production of proinflammatory cytokines and cartilage degradative enzymes. 352 The erythrocyte level of ω-6 PUFAs acts as a biomarker that inverses the risk of RA, and the remission rate of RA increases when ω-3 PUFAs are added to disease-modifying anti-rheumatic drug (DMARD) treatment. 353 , 354

Dysbiosis of the gut microbiome can be observed in SLE patients, including a decreased richness and diversity of the gut microbiota and a reduced proportion of Firmicutes/Bacteroides (F/B); the latter may promote lymphocyte activation and Th17 differentiation from naïve CD4 + lymphocytes. 355 , 356 Blooming of Ruminococcus (blautia) gnavus occurs at times of high disease activity and during lupus nephritis, indicating that it is the driver of often remitting-relapsing SLE. 357 Another analysis showed that Veillonella dispar has a positive association with the activity of SLE. 358 According to a systematic review, nutritional support in the SLE population is focused mainly on interventions involving ω-3 and vitamin D. 359 The anti-inflammatory effect of ω-3 may contribute to its clinical function, similar to that of RA. 360 Vitamin D blocks the proliferation, differentiation and function of B cells and T cells, which may attenuate the expression of inflammatory cytokines in patients with SLE. 361 Inadequate levels of serum vitamin D have been observed in SLE patients, suggesting the importance of supplementing their diet with vitamin D 362 . Dietary patterns other than single nutrients as supplementary treatments for SLE still require further investigation. 363

Ulcerative colitis (UC) and Crohn’s disease (CD) are the two major clinical phenotypes of IBD. Dietary management and microbiota modulation have been clinically recommended for IBD treatment according to clinical guidelines. 364 Obesity is a risk factor for IBD, especially for CD. 365 As a potential trigger of obesity, HFD, together with antibiotics, exacerbates inflammation in pre-IBDs by impairing epithelial mitochondrial bioenergetics and triggering microbiota disruptions in mouse models. 366 However, IBD increases the risk of malnutrition, which triggers inflammatory responses and subsequently leads to poor clinical outcomes. 367 Therefore, dietary interventions and nutritional care should be planned according to the precise nutritional assessment and dietary assessment for IBD patients. 368 Exclusive enteral nutrition (EEN), the first-line therapy in pediatric patients with active CD, can effectively decrease clinical activity and reduce the complications of CD simultaneously, but its benefit in adults still lacks competent evidence. 369 Similarly, CD exclusion diet (CDED) positively correlates with the clinical remission of pediatric patients with active CD. 370 In addition, diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs) can relieve the gut symptoms of patients with quiescent IBD, possibly reducing the fecal abundance of microbes and thereby regulating the immune response of the host. 371

Dietary interventions may also influence the risk and clinical outcome of other autoimmune diseases. A recent study on HT suggested that low intake of animal foods, mainly meat, has a protective effect on thyroid autoimmunity and potentially has a positive influence on redox balance, which further reduces oxidative stress-related disorders. 372 Improvement in HT has also been observed in other dietary interventions, including elimination of gluten or lactose, energy restriction, and consumption of Nigella sativa, suggesting the potential benefit of diet as a complementary treatment for HT. 373 MS is more common in western countries, suggesting diet as a potential risk factor. 374 Western diet triggers impaired myelin-debris clearance in microglia, thereby impairing lesion recovery after demyelination, which may explain its role in MS induction. 375 Moreover, an elevated intake of dietary salt can exacerbate autoimmune conditions by promoting the induction of pathogenic Th17 cells, contributing to MS. 376 Conversely, IF diet ameliorates the clinical course and pathology of MS by reducing the number of IL-17-producing T cells, increasing the number of Tregs in the gut and increasing the richness of gut bacteria, which enhance antioxidative microbial metabolic pathways. 377 Vitamin D supplementation has been shown to lower the incidence and benefit MS patients with sufficient evidence, and a “Coimbra Protocol” referring to daily doses up to 1000 I.U. vitamin D3 per kg body weight is clinically applied to treat patients with MS. 378 , 379

Cardiovascular diseases (CVD)

According to epidemiological studies, obesity and unhealthy diet are risk factors for CVD. Greater dietary fiber intake from cereal, vegetables and fruits is associated with a lower risk of CVD, suggesting that high-fiber diet is a potential protective factor. 380 An experimental model fed with diet lack of prebiotic fiber induces hypertension through inducing deficiency of SCFA production and GPR43/109A signaling, suggesting the underlying mechanisms of dietary fiber. 381 Besides, high-fiber diet and acetate supplementation can lead to changes in the gut microbiota, particularly an increase in Bacteroides acidifaciens , which is protective against the development of CVD. 382 Other healthy dietary patterns, including the Nordic diet, the Dietary Approaches to Stop Hypertension (DASH) diet, the MD, and the vegetarian diet, also have protective effects on CVD risk. 383 High sodium intake is the leading dietary risk factor for CVD. 384 High salt load may induce persistent hepatic steatosis and inflammation by inhibiting SIRT3 expression, thereby contributing to cardiovascular damage. 385 Conversely, a low-sodium diet may dampen the risk of CVD, which is highly recommended by current dietary guidelines. 386 Amino acids play different roles in the progression of CVD. Diet with high-unsaturated fatty acid composition and less saturated fat might be cardioprotective. 387 In contrast, higher intake of BCAAs is associated with increased platelet activity and arterial thrombosis formation; therefore, BCAA levels are associated with the risk of CVD. 388

Therapeutic implications of diet for CVD treatment have also been a focus of recent studies. CR attenuates hypertension, left ventricular remodeling and diastolic dysfunction in DS/obese rats by reducing cardiac oxidative stress and inflammation. 389 In addition, a combination of CR and exercise can improve cardiac mitochondrial dynamics, decrease cardiac apoptosis, and maintain cardiac [Ca 2+ ] i homeostasis in obese insulin-resistant rats. 390 CR also helps to maintain the iron homeostasis of cardiomyocytes. 391 These findings suggest the function of CR in cardiac protection. However, strictly adhering to CR is very difficult for most patients. IF is easier to perform than CR and has similar potential clinical value. 392 FMD, a 5-day fasting dietary pattern, increases cardiac vascularity and function and resistance to cardiotoxins in a high-fat, high-calorie diet (HFCD) mouse model, thereby postponing the process of cardiac aging. 393 Alternate day fasting (ADF) improves cardiovascular marker levels, including reduced fat mass, an improved fat-to-lean ratio, and increased β-HB-hydroxybutyrate levels, suggesting its clinical relevance for CVD intervention. 394 KD has a beneficial effect on the blood lipid profile, the NLRP3 inflammasome, myocardial energy metabolism, and the vascular endothelium, benefiting CVD patients. 395 However, research on healthy individuals has reported that lipid profiles deteriorate in response to a KD, suggesting that its role in preventing CVD in the normal population needs further inquiry (Fig. 6 ). 396

Impact of different diets on cardiovascular diseases. Calorie restriction (CR) can reduce cardiac oxidative stress and inflammation, improve cardiac mitochondrial dynamics and maintain cardiac ion homeostasis, which may be protective against cardiovascular disease (CVD) in obese and/or insulin-resistant models. Fast-mimicking diet (FMD) increases cardiac vascularity and function and resistance to cardiotoxins in a high-fat, high-calorie diet (HFCD) mouse model. Alternate day fasting (ADF) improves cardiovascular markers, for example, reduced fat mass. Ketogenic diet (KD) inhibits the NLRP3 inflammasome and improves the blood lipid profile but may lead to impaired blood lipid profiles in healthy individuals. High-salt diet (HSD) can inhibit SIRT3 expression and induce persistent hepatic steatosis and inflammation, thereby contributing to cardiovascular damage. A diet lacking prebiotic fiber induces hypertension through inducing a deficiency in short-chain fatty acid (SCFA) production and GPR43/109A signaling. High branched-chain amino acid (BCAA) intake is associated with increased platelet activity and arterial thrombosis formation. This figure was created with BioRender.com

Metabolic disorders

Overnutrition is a driving factor for obesity and related metabolic disorders, mainly including type 2 diabetes mellitus (T2DM), metabolic syndrome, nonalcoholic fatty liver disease (NAFLD), and polycystic ovarian syndrome (PCOS). 397 In addition, these metabolic disorders have a complicated internal relation, for instance, T2DM and NAFLD are independent factors for each other, and PCOS is closely related to insulin resistance and T2DM. 398 , 399 These epidemiological characteristics suggest a high correlation between dietary patterns and multiple metabolic disorders (Fig. 7 ). Changes in the gut microbiome may also explain the etiology of metabolic disorders by altering the levels of metabolites, such as SCFAs and succinate. 400

Impact of different diets on metabolic disorders. High-fat diet (HFD) can directly increase caloric intake, induce inflammatory mediators such as JNK and IκB kinase (IKK) to promote hypothalamic inflammation, and contribute to adipose tissue hypoxia and inflammation, which all lead to the development of obesity and/or insulin resistance. Over-intake of fructose can also increase caloric intake and induce obesity by impairing hepatic insulin sensitivity. However, time-restricted feeding (TRF) with equivalent caloric intake from HFD can adjust various signaling pathways and rhythmic creatine-mediated thermogenesis and reverse excessive daytime sleepiness induced by paraventricular thalamic nucleus (PVT) dysfunction, resulting in a protective effect on HFD-induced obesity. High-fiber diet can reduce inflammation and insulin resistance by influencing the gut microbiota and associated molecules, for instance, SCFA-producing bacteria. Every-other-day fasting (EODF) regimen can also shift the gut microbiota composition and stimulate beige fat development within white adipose tissue to inhibit insulin resistance. Ketogenic diet (KD) is clinically beneficial for the glycemic control of type 2 diabetes mellitus (T2DM) and nonalcoholic fatty liver disease (NAFLD). However, in experimental models, KD can decrease sensitivity to peripheral insulin by upregulating insulin receptors. Intermittent fasting (IF) alone or combined with exercise can reduce intrahepatic triglyceride (IHTG) levels and hepatic steatosis in NAFLD patients by downregulating hepatic inflammatory pathways, modifying lipogenic gene expression and inducing autophagy. Calorie restriction (CR) can be effective at reducing weight loss and reversing ovulatory/metabolic dysfunction in polycystic ovarian syndrome (PCOS) patients. This figure was created with BioRender.com

HFD is the standard method to induce obesity in animal models and results from the overconsumption of fat, which directly increases caloric intake. The elevation of inflammatory mediators such as JNK and IκB kinase (IKK) in hypothalamic inflammation may also explain the obesity induced by HFD. 401 Interestingly, a TRF with equivalent caloric intake from HFD has been shown to have a protective effect on HFD-induced obesity and associated complications by adjusting various signaling pathways and causing rhythmic creatine-mediated thermogenesis, which may further improve nutrient utilization and energy expenditure and reverse excessive daytime sleepiness induced by paraventricular thalamic nucleus (PVT) dysfunction. 402 , 403 , 404 Adipose tissue hypoxia and inflammation may lead to adipocyte dysfunction and obesity-induced insulin resistance in HFD-fed models, as indicated by increased infiltration of adipose tissue macrophages (ATMs), activation of the NLRP3 inflammasome and increased levels of proinflammatory cytokines. 405 , 406 , 407 In addition to fat intake, the overintake of fructose may also impair hepatic insulin sensitivity, and several metabolic pathways are independent of increased weight gain and caloric intake. 408 Within this complex interplay of diet, metabolism, and inflammation, IL-17 has been identified as a key player in metabolic dysregulation associated with HFD, where inhibiting IL-17A production or blocking its receptor can attenuate obesity by enhancing adipose tissue browning and energy dissipation. 409 Complementarily, IL-17F promote the expression of TGFβ1 in adipocytes, which fosters sympathetic innervation and suggests a novel therapeutic target for obesity that could stimulate thermogenic activity in fat tissue, thereby improving metabolic health and providing a potential treatment strategy for obesity and its related metabolic disorders. 410

Cohort studies have demonstrated that healthy diets, including the Portfolio diet, DASH diet, and MD, are associated with a decreased risk of T2DM. 411 , 412 , 413 The promotion of SCFA-producing bacteria induced by dietary fibers observed in T2DM patients suggests the potential value of fiber supplementation in clinical practice. 414 In addition, increased fiber consumption is associated with decreased insulin resistance, the mechanism of which mainly includes the gut microbiota and associated molecules. 415 , 416 IF is an effective strategy for controlling weight and increasing insulin sensitivity in patients with diabetes and can also improve cardiometabolic outcomes. 417 , 418 The every-other-day fasting (EODF) regimen selectively stimulates beige fat development within white adipose tissue and shifts the gut microbiota composition in experimental models, explaining the mechanism through which IF ameliorates obesity, insulin resistance, and hepatic steatosis. 419 KD has therapeutic effects on glycemia, lipid control, and weight reduction in T2DM patients. 420 However, KD may contribute to decreased sensitivity to peripheral insulin and impaired glucose tolerance by upregulating insulin receptors, as determined by previous studies, which contradicts clinical findings. 421

NAFLD features hepatic steatosis or adiposity with a potential risk of developing into inflammation, fibrosis, and cancer. MD, as the most recommended dietary pattern for NAFLD, can reduce liver steatosis and improve insulin sensitivity even without weight loss in an insulin-resistant population. 422 Reduced liver fat may be associated with ameliorated inflammation induced by antioxidants, low glycemic response induced by dietary fiber, and improved hepatic lipid metabolism. 423 KD is more clinically meaningful for glycemic control in individuals with T2DM and NAFLD than low-calorie diet or high-carbohydrate, low-fat (HCLF) diet. 424 , 425 Mechanistically, ketone bodies may modulate inflammation and fibrosis in hepatic cells. 426 IF alone or combined with exercise is effective at lowering intrahepatic triglyceride (IHTG) levels and reducing hepatic steatosis in patients with NAFLD, possibly by downregulating hepatic inflammatory pathways, modifying lipogenic gene expression and increasing levels of autophagy. 427 , 428

PCOS features a series of metabolic irregularities, mainly androgen excess and ovarian dysfunction. A meta-analysis showed that women with PCOS have a lower overall diet quality with higher cholesterol, lower magnesium and lower zinc intake. 429 Dietary modification with lower caloric intake to achieve weight loss is recommended as a first-line therapy for managing PCOS, and higher supplementary nutrient intake, including vitamin D, chromium and ω-3, may also benefit patients suffering from PCOS. 430 MD, KD and their combination can all lead to significant improvements in body weight, metabolic function and ovulatory dysfunction in PCOS patients. 431 , 432 , 433 In addition, IF may be beneficial for treating anovulatory PCOS by reducing body fat and improving menstruation, hyperandrogenemia, insulin resistance and chronic inflammation. 434 CR may also improve weight and metabolic disorders in patients with PCOS, alone or in combination with supplementation. 435 However, the exact mechanisms of these dietary interventions remain unclear and need further exploration.

While the potential of dietary interventions to influence systemic diseases of the whole body is supported by various studies, a critical outlook reveals the necessity for more rigorous, long-term clinical trials to validate these findings. It is essential to approach these interventions with caution, considering individual differences and the intricate balance of potential benefits against nutritional deficiencies or other risks.

Conclusions and perspectives

Our review provides compelling evidence that dietary interventions, including calorie restriction, fasting or FMD, KD, protein restriction diet, HSD, HFD, and high-fiber diet, have substantial potential for modulating metabolism, redirecting disease progression, and enhancing therapeutic responses. These findings highlight the pivotal role of diet, an important environmental factor, in influencing tumor metabolism and the course of various diseases, such as cancer, neurodegenerative diseases, autoimmune diseases, CVD, and metabolic disorders.

Despite compelling evidence, the potential impact of dietary interventions on disease treatment, particularly cancer treatment, is not fully understood. 436 The latest American Society of Clinical Oncology (ASCO) guidelines suggest that “there is currently insufficient evidence to recommend for or against dietary interventions such as ketogenic or low-carbohydrate diets, low-fat diets, functional foods, or fasting to improve outcomes related to quality of life (QoL), treatment toxicity, or cancer control”. 437 The intricate relationship between dietary interventions and treatment outcomes can be influenced by numerous factors, such as overall lifestyle habits, health status, specific disease type and its corresponding treatment, degree of dietary alterations, and patient adherence. A comprehensive assessment of these variables is crucial for understanding the precise impact of diet on treatment efficacy. 438 , 439

With the recognition of metabolic reprogramming inherent in disease progression, particularly in malignancies, it is becoming essential to explore the value of implementing dietary interventions and translating the evidence into practice. Future research should focus on unraveling the specific molecular mechanisms involved, which will enable the development of more effective, personalized dietary interventions that serve as adjunct therapies in comprehensive disease management.

Building upon the initial observation, it is crucial to interpret and apply these findings with caution due to potential variations and discrepancies. The efficacy of dietary interventions may vary significantly, for instance, depending on the mouse model used. 440 Each model might have unique metabolic and immune responses that could influence the outcome of dietary interventions. Similarly, the type of cancer cells used to induce tumor formation, whether primary cells derived directly from patient tissues or cultured cell lines, can have profound impacts on the experimental results. 441 Orthotopic or heterotopic transplantation technique is another significant factor that can influence how tumors respond to dietary interventions. Furthermore, the duration of treatment and the specifics of dietary interventions can substantially influence the results, as short-term interventions might not yield the same results as long-term interventions, and different dietary components could have varying effects on tumor growth and progression. 120 Therefore, future research in this field should carefully consider the design of animal models and the specifics of dietary interventions to ensure that the findings are robust and translatable to human cancer treatment.

Additionally, clinical trials with larger sample sizes and longer follow-up periods are needed to further validate the efficacy of these strategies and to identify potential side effects and contraindications. It is important for these trials to be designed to represent diverse population groups, including elderly and obese individuals, as these groups may respond differently to dietary modifications. The safety of dietary interventions is another key consideration. While dietary changes generally cause fewer side effects than pharmacological treatments, potential risks should not be overlooked. For instance, severe dietary restrictions may lead to malnutrition or other health complications, particularly in vulnerable population groups. Therefore, in addition to efficacy, these trials should systematically evaluate the safety of dietary interventions, identifying any potential side effects and contraindications.

In conclusion, dietary interventions hold great promise as a novel approach to disease management. However, to realize their full potential, it is essential to continue rigorous scientific investigations into their mechanisms of action, safety profiles, and efficacy in different patient populations. With further research, dietary interventions could become integral components of personalized medicine, providing a new avenue for the prevention and treatment of a myriad of diseases.

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Acknowledgements

This work was supported by grants from the National Natural Science Foundation of China (82103369) and the China Postdoctoral Science Foundation (2022M710757). The funders had no role in the study design, data collection, analysis, decision to publish, or preparation of the manuscript. The figures were created with Biorender.com.

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Key Laboratory of Breast Cancer in Shanghai, Department of Breast Surgery, Fudan University Shanghai Cancer Center, Shanghai, China

Yu-Ling Xiao, Yue Gong, Ying-Jia Qi, Zhi-Ming Shao & Yi-Zhou Jiang

Department of Oncology, Shanghai Medical College, Fudan University, Shanghai, 200032, China

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Y.-Z.J., Z.-M.S., Y.-L.X., and Y.G. designed and finalized the study. Y.-L.X., Y.G., and Y.-J.Q. wrote and edited the paper and generated the figures. All authors have read and approved the article.

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Xiao, YL., Gong, Y., Qi, YJ. et al. Effects of dietary intervention on human diseases: molecular mechanisms and therapeutic potential. Sig Transduct Target Ther 9 , 59 (2024). https://doi.org/10.1038/s41392-024-01771-x

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Received : 01 August 2023

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DOI : https://doi.org/10.1038/s41392-024-01771-x

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