New research from the University of Sheffield has discovered that switching to a rich diet after eating a restricted diet can decrease life expectancy and have negative effects on health.
It has long been known that restricting food intake can extend lifespan however researchers have now provided new insight into why, as well as how diets could benefit humans in terms of slowing ageing and the onset of age-related disease.
Experts, from the Healthy Lifespan Institute at the University of Sheffield and Brown University in the USA, tested the existing evolutionary theory that dietary restriction – a reduction of particular or total nutrient intake without causing malnutrition – triggers a survival strategy in humans and animals.
The theory suggests that this is because humans and animals invest in maintaining and repairing the body in times of low food availability, to await times when food availability increases again.
However, the new findings have challenged this theory.
Fruit flies (Drosophilia melanogaster) fed a restricted diet who were then returned to a rich diet were more likely to die and laid less eggs compared to flies that spent their whole life on a rich diet.
This demonstrates that rather than waiting for food availability to increase in the future, the flies were essentially waiting to die on a restricted diet.
The researchers suggest that instead of dietary restriction increasing repair and maintenance mechanisms, it could actually be an escape from the damaging effects of a rich diet.
This new interpretation can help us to understand why and how diet can have such profound effects on health.
The findings also suggest that changing diet repeatedly or abruptly could be harmful to health in certain situations.
Ph.D. student Andrew McCracken, from the University of Sheffield’s Department of Animal and Plant Sciences, who led the study said: “Dietary restriction is an unusual paradox which has attracted a great deal of interest within the field of ageing.
Our results have now pointed us towards a more refined explanation of why it occurs, and have the potential to wholly shift the focus of future research.
“Our most surprising finding was that under certain circumstances, restricted diets can also be the origin of particular types of damage to the individual.
This enhanced understanding of the penalties and benefits of certain types of diets, will expedite the quest to identify pharmaceutical interventions which mimic dietary restriction.”
Dr. Mirre Simons, from the University of Sheffield’s Department of Animal and Plant Sciences, said: “The effects of diet on health are huge, but we understand little of the exact mechanisms.
Our work has now uncovered a surprising property of dietary restriction, in that it makes flies ill-prepared for rich diets. This was contrary to our expectations and contrary to current evolutionary theory.
In the biology of ageing field evolutionary biology has been highly influential in guiding interpretation of more mechanistic research. Our work thereby contributes to the broader understanding of dietary restriction and the efforts to translate its benefits to humans.”
The research was funded by the National Environment Research Council (NERC), Wellcome, the American Federation of Aging Research & the National Institute on Aging.
The work forms part of the research of the Healthy Lifespan Institute at the University of Sheffield.
The Institute brings together 120 world-class researchers from a wide range of disciplines with the aim of slowing down the ageing process and tackling the global epidemic of multimorbidity – the presence of two or more chronic conditions – in a bid to help everyone live healthier, independent lives for longer and reduce the cost of care.
The evolutionarily conserved role for nutrient-sensing pathways in health and longevity extends from simple organisms to rodents and nonhuman primates. In eukaryotic model organisms, the growth hormone/insulin-like growth factor-I (GH/IGF-I) pathway and its downstream effectors, target of rapamycin (TOR)/ribosomal protein S6 kinase (S6K) and the adenylate cyclase-protein kinase A (PKA) pathways, are involved in the regulation of metabolism and growth in response to nutrient abundance and thereby promote aging (1).
Specific macronutrients activate these pathways and, notably, protein and amino acids play a central role in the metabolic response to affect growth and reproduction, healthspan, physiology, and longevity (2).
The effect of the GH/IGF-I pathway on longevity was first demonstrated in Caenorhabditis elegans (3, 4). Subsequent studies in multiple model organisms identified that orthologs of genes functioning in the TOR/S6K and RAS/cAMP/PKA signaling cascades also provided evidence for the conserved regulation of aging by progrowth nutrient signaling pathways (5–9). GH deficiency, as well as GH receptor deficiency (GHRD), results in low concentrations of IGF-I and insulin and protects against many aging-associated pathologies, including but not limited to cancer and diabetes, and in lifespan extension in mice (10, 11).
A population of humans in rural Ecuador identified with GHRD (also known as Laron syndrome) display no cases of cancer mortality or diabetes, despite being at a higher risk due to the prevalence of obesity (12). Thus, the identification of protein intake as a major modulator of the GH/IGF-I pathway and subsequent activation of the TOR/S6K and RAS/cAMP/PKA signaling cascades provides a powerful intervention to improve long-term health and lifespan from animal models to humans (Table 1).
The aim of this narrative review is to summarize research on the impact of protein restriction on health and longevity in model organisms and to discuss the ongoing research on the effects of a low-protein fasting-mimicking diet (FMD) on biomarkers of healthy aging in mice and humans.
TABLE 1
Definition of terms
| Term | Definition |
|---|---|
| Lifespan | The period of time between the birth and death of an organism |
| The average length of life of a kind of organism or of a material object especially in a particular environment or under specified circumstances | |
| Healthspan | The length of time lived in reasonably good health |
| Longevity | Generally best considered meaning “typical length of life” and sometimes used as a synonym for “life expectancy” in demography |
| Healthy aging | The process of developing and maintaining the functional ability that enables well-being in older age |
| Quality of life | The degree to which an individual is healthy, comfortable, and able to participate in or enjoy life events. The term quality of life is ambiguous, because it refers both to the experience of an individual and to the living conditions in which individuals find themselves |
Why protein matters: from single cells to mammals
Yeast (Saccharomyces cerevisiae) has been among the first organisms utilized to understand the role of the nutrient-sensing Tor1/serine/threonine-protein kinase in S. cerevisiae (Sch9) and Ras2/cAMP/PKA pathways and their role in lifespan through the activation of progrowth and pro-aging signaling (6). In yeast, the deletion of components of this signaling cascade extends chronological lifespan, reduces age-related genome instability, and promotes multistress resistance—a hallmark of healthy aging (6).
The activation of Tor1/Sch9 by amino acids and Ras2/cAMP/PKA by glucose results in the inhibition of the regulator of IME2 in S. cerevisiae (Rim15), a positive regulator of the multicopy suppressor of SNF1 mutation (Msn2/4) and the Glg1-2 suppressor and transcription factor (Gis1) stress resistance transcription factors (13).
Notably, amino acid limitation reduces aging-dependent DNA damage accumulation and increases lifespan through the inhibition of the Tor1/Sch9 cascade (13, 14). Conversely, the amino acids threonine, valine, and serine all promote cellular sensitization and reduce yeast lifespan through the activation of Tor1/Sch9 (14).
In invertebrate models, such as C. elegans and Drosophila melanogaster, reducing the activity of nutrient-sensing pathways, that is, TOR, through the limitation of amino acids, as well as nitrogen and carbon sources, increases lifespan (1, 15–19).
In the fruit fly D. melanogaster, the extension of lifespan can be achieved without reducing the calorie content by modulating dietary carbohydrates, yeast (as a protein source), and other macronutrients and their ratios, whereas the supplementation of essential amino acids reverses this lifespan extension; the supplementation of nonessential amino acids has minimal impact on lifespan (20, 21).
The protein-to-carbohydrate ratio significantly impacts lifespan: diets high in protein (i.e., yeast extract) and low in carbohydrates (sucrose) have been shown to negatively impact the lifespan of the Queensland fruit fly Bactrocera tryoni and D. melanogaster (22).
Maximal longevity for these flies was observed at a protein:carbohydrate ratio between 10:1 and 20:1 (22). Therefore, protein and/or amino acid intake has major effects on lifespan even in simple model organisms. Next, we discuss how these findings translate to mammals and how protein intake modulates the GH/IGF-I pathway and its downstream effectors.
In rodents, the restriction of protein intake or of specific amino acids, as well as the genetic modulation of the GH/IGF-I pathway, have been shown to increase healthspan, thus emphasizing that protein plays an evolutionarily conserved role in lifespan regulation (1).
Restriction of the amino acid tryptophan delays tumor incidence and onset, protects the liver and kidney against ischemia/reperfusion injuries, and lengthens the healthspan (2, 23–25), whereas methionine restriction decreases adiposity and serum glucose, insulin, and IGF-I concentrations, and the mitochondria-dependent production of reactive oxygen species, thereby inducing less oxidative damage in male Wistar rats, male Fisher 344 rats, and female BALB/cJ × C57BL/6J F1 mice (26–29).
Although insulin sensitivity is improved following the restriction of leucine in male C57BL/6J mice, which in turn is likely correlated with healthspan, the restriction of this specific amino acid has not been shown to extend lifespan (30, 31).
A diet with reduced concentrations of branched-chain amino acids, but isocaloric in total amino acids compared with the control diet, modestly improved glucose tolerance and slowed fat mass gain in healthy male C57BL6 mice (32), but promoted rapid fat mass loss and restored glucose tolerance and insulin sensitivity in obese male C57BL6 mice maintained on a high-fat, high-sugar diet without caloric restriction (CR) (33). Protein restriction with supplementation of nonessential amino acids reduced IGF-I concentrations in male 3xTg-AD mice displaying significant cognitive impairment and Alzheimer disease–like pathology, which resulted in a decrease in tau phosphorylation in the hippocampus, but without affecting the concentrations of β-amyloid, and alleviation of the age-dependent impairment in cognitive performance (34).
Here, it should also be mentioned that the healthspan effects observed with CR can be at least partially explained by the restriction of the amino acids methionine and tryptophan that often goes in hand with CR (29, 35, 36). However, in rodents (male and female) specific amino acid restriction can result in food aversion due to a change in taste, which in turn reduces food intake (37, 38).
Although this can be controlled for by using pair-wise feeding of control animals in studies focused on protein- or amino acid–restricted diets, the possibility remains that food aversion–induced CR is a confounding variable (20). Protein and carbohydrate, rather than fats, are the predominant macronutrients that modulate food consumption to meet biological requirements (39–41). When testing the lifetime impact of dietary macronutrient ratios—differing in content of protein (5–60%), fat (16–75%), carbohydrate (16–75%), and energy (8, 13, or 17 kJ/g of food)—on metabolic health and longevity in male and female C57BL6 mice consuming ad libitum, low-protein/high-carbohydrate diets resulted in the longest lifespans (39, 40).
The mice whose diets included 5–15% protein and 40–60% carbohydrates lived the longest, ≤150 wk compared with 100 wk for those on a diet of ∼50% protein, despite the fact that mice that consumed more protein were leaner (39). The reduction in progrowth signaling of the GH/IGF-I axis in mammals has been extensively studied using genetically engineered mouse models (42).
Male and female mice with a GH-insulin/IGF-I signaling deficiency exhibit dwarfism but also increased insulin sensitivity and the delayed manifestation of fatal malignancies and increased health and lifespan (1, 23, 43). Similarly, the inhibition of mammalian target of rapamycin (mTOR)/S6K signaling results in increased lifespan and the reduction of aging phenotypes (7, 44, 45). In summary, essentially all model organisms used in aging research support a link between dietary protein uptake and aging in that reduced protein intake is associated with a sex-independent increase in lifespan.
Protein intake, health, and longevity in nonhuman primates
As an important first step to demonstrate whether the health benefits observed with CR could translate to humans, the University of Wisconsin (UW) and the National Institute on Aging (NIA) used male and female rhesus monkeys to investigate the impact of a 30% CR on lifespan and healthspan (46, 47). Although the studies resulted in somewhat conflicting data regarding an increase in lifespan (rhesus monkeys in the UW study increased their lifespan whereas there were no effects in the NIA study), both demonstrated healthspan benefits in the CR-fed rhesus monkey cohorts: age-related disease and all-cause mortality were reduced in the CR group (UW) with reduced incidences of cancer in the NIA CR group (46, 47).
Thus, whether CR might translate to life extension in humans remains an unanswered but critical question (1, 23, 43). However, one can extrapolate some important results from these studies based on the analysis of the experimental conditions, in particular the food used to feed the animals. Foremost, significantly lower sucrose concentrations were fed in the NIA study (3.95%) compared with the 28.5% in the UW rhesus monkey diet (47).
Protein intake in the NIA study was based on wheat, corn, soybean, fish, and alfalfa meal, whereas the UW study used lactalbumin obtained from milk whey as the main protein source, suggesting that the more plant-based protein source diet used in the NIA study might have reduced the risk for aging-related mortality factors compared with the animal-based protein sources used in the UW study (47). Arguably, CR extended lifespan in the UW study because these animals were maintained on unhealthy diets; for the animals fed a more plant-based protein diet at the NIA, CR had no significant effects on lifespan.
Thus, in line with the basic animal models described previously, dietary protein content, as well as its source, makes a significant, gender-independent contribution to primate healthspan.
Protein intake, health, and longevity in humans
What really qualifies as a macronutritionally healthy diet that successfully extends healthy aging remains an unanswered critical question of high priority for human nutrition research. The benefits of reduced protein intake on outcomes of health in humans have been assessed mostly based on clinical trials and epidemiological studies.
Clinical studies
Short-term randomized controlled trials, which fail to consider the long-term impact that nutrition really has, often favor the substitution of protein for carbohydrate (i.e., high-protein/low-carbohydrate diets) due to benefits for weight management, blood pressure reduction, and improvements in cardiometabolic biomarkers (such as blood lipid and lipoprotein profiles), and improved glycemic regulation (48–50). Multiple studies have demonstrated that these beneficial health effects largely depend on weight loss, enhanced postprandial satiety, and energy expenditure when exchanging protein for carbohydrate (51).
A meta-analysis of 24 trials, including 1063 individuals and a mean ± SD diet duration of 12.1 ± 9.3 wk, indicated that high-protein diets (protein: 30.5 ± 2.4%; carbohydrate: 41.6 ± 3.5%; fat: 27.8 ± 3.2%) produced more favorable changes in body weight (−0.79 kg; 95% CI: −1.50, −0.08 kg), fat mass (−0.87 kg; 95% CI: −1.26, −0.48 kg), and triglycerides (−0.23 mmol/L; 95% CI: −0.33, −0.12 mmol/L) than a standard-protein, low-fat diet (protein: 17.5 ± 1.5%; carbohydrate: 56.9 ± 3.3%; fat: 25.1 ± 3.1%). Changes in concentrations of fasting plasma glucose; fasting insulin; total, LDL, and HDL cholesterol; and blood pressure were similar across dietary treatments (P ≥ 0.20) (51).
Notably, this meta-analysis included overweight and obese men and women, postmenopausal women, hyperinsulinemic men and women, men and women with metabolic syndrome or with type 2 diabetes, and even a small cohort of men and women with heart failure or polycystic ovary syndrome (women only). Thus, high-protein and low-glycemic diets might improve compliance and maintenance of weight loss in overweight adults (52), yet these diets do not align with the low-protein dietary recommendations that can be considered as generally healthy antiaging diets.
Epidemiological studies
Increasing protein intake by 10% (or 5 g of protein) while decreasing carbohydrate intake by 10% (or 20 g carbohydrates) was correlated with a 5% increase in incidences of cardiovascular disease (CVD) in a Swedish study of 43,396 women with an average follow-up of 15.7 y; interestingly, individuals substituted carbohydrates mostly with animal protein, thereby changing their overall protein:carbohydrate intake ratio (53).
Multiple other large studies suggest a positive correlation between low-protein diets and lower rates of aging-related disease. In the 26-y follow-up of the Nurses’ Health Study (NHS; including 85,168 women) and a 20-y follow-up of the Health Professionals Follow-Up Study (HPFS; including 44,548 men), diets high in animal-based protein and fats and low in carbohydrates were associated with higher mortality (HR: 1.12; 95% CI: 1.01, 1.24) for both men and women (54).
Recent work has indicated that dietary needs change during aging, which should be considered when making recommendations for healthy aging diets. When considering male and female individuals aged ≥50 y in the NHANES dataset, no positive correlation between protein intake and increased mortality was supported (55).
However, stratifying this cohort into 2 age groups (50–65 y and >65 y) allowed assessment of nutritional needs and their impact on health based on age cohorts, but the age cohorts were not further divided by sex, which might allow determination of any possible sex-specific differences at older ages. In individuals aged 50–65 y, high protein intake (≥20% protein-derived consumed calories) was associated with a 74% increase in their RR of all-cause mortality (HR: 1.74; 95% CI: 1.02, 2.97), and they were >4 times as likely to die of cancer (HR: 4.33; 95% CI: 1.96, 9.56), an effect not observed in those older than 65 y (55).
In fact, individuals aged >65 y who consumed a high-protein diet had a 28% reduction in all-cause mortality (HR: 0.72; 95% CI: 0.55, 0.94) and a 60% reduction in cancer mortality (HR: 0.40; 95% CI: 0.23, 0.71) compared with those consuming low-protein diets, which was not affected when controlling for other fat or carbohydrate intake or the protein source (55).
IGF-I concentrations decrease with aging in humans, but in this dataset individuals aged ≥50 y who consumed a high-protein diet also had higher IGF-I concentrations. Elevated circulating IGF-I concentrations are associated with increased risk of developing certain malignancies (56, 57).
Thus, it is possible that the benefits of the high-protein intake in those aged ≥65 y rely on maintaining “healthy” IGF-I concentrations (55), which could help to maintain a healthy weight and preserve muscle mass thereby preventing frailty and fall-associated hospitalization (58).
This concept is supported by results in mice: young mice can easily maintain a healthy weight on a low-protein diet, whereas older mice struggle to maintain weight and become increasingly frail (55). Hence, although the great majority of studies suggest a negative correlation between high-protein diets and aging-related diseases, the fact that some studies do not support these findings could be due to differences in the age observed and trade-offs in the nutritional needs at various ages.
In Ecuador, a small population of subjects exhibits a rare case of symmetrical dwarfism (Laron syndrome) due to GHRD and characterized by very low (≤20 ng/mL) circulating IGF-I concentrations (12). This population enables further extension of our understanding of the role of the GH/IGF-I axis and its implications for human aging. Longitudinal studies with a 22-y follow-up of this population indicate the absence of aging-related pathologies, such as cancer and diabetes, despite a very high prevalence of overweight and obesity (a risk factor for cancer and diabetes) in male and female subjects (12).
Similarly, a different population of 230 subjects with Laron syndrome was protected against cancer (59). Both studies included male and female subjects although the sex distribution is unknown. Male and female GHRD mice show improved age-dependent cognitive performance (60), whereas the effect of GHRD on human cognition remained unexplored until recently.
Using MRI, the brain structure, function, and connectivity of 3 male and 10 female subjects with Laron syndrome and 12 unaffected relatives were compared and revealed that the GHRD group displayed enhanced cognitive performance and greater task-related activation in frontal, parietal, and hippocampal regions, results consistent with effects observed in young adults (61).
Dietary patterns in areas with populations with exceptional longevity, so-called “blue zones,” provide further evidence that nutrition in combination with other healthy lifestyle factors (such as exercise, spirituality, and a sense of “belonging”) can have significant impact on human health (62–65). The long-lived populations of Okinawa, Sardinia, Loma Linda, the Nicoya Peninsula, and Ikaria consume predominantly plant-based, low-protein diets that include a high intake of fruits, vegetables, and nuts (66).
In conclusion, the above findings, together with a plethora of additional studies based on human and nonhuman interventions, support the hypothesis that lower protein intake results in lower activity of the GH/IGF-I axis, thereby protecting against the development and onset of aging-related pathologies and aging itself. These effects seem to be also largely sex-independent.
The Source Matters: Animal- Compared with Plant-Based Protein Intake and Health
Results from animal models
Historically, experiments designed to determine how different protein sources might affect health and lifespan in model organisms of aging are scarce. This could be related to various reasons such as the typical diet consumed by a specific model organism, which limits what can reasonably be fed to an animal.
Another major limitation of the experiments designed to evaluate protein sources is that often only 1 type of animal protein (such as casein) is compared with 1 type of vegetable protein (typically soya); notably this does not reflect the complex composition of most human diets.
Nine-week-old C57BL/6J male mice maintained for 3 wk on either a 20% plant protein–based or a 20% dairy protein–based diet with an equivalent percentage of calories derived from either protein source, indicated that plant protein–based and dairy protein–based diets are indistinguishable with respect to their short-term consequences on weight, body composition, and control of glucose homeostasis as assessed by glucose, insulin, and pyruvate tolerance tests in mice (67).
Male rabbits fed with animal protein, but not soya protein, developed vascular lesions and displayed increased atherosclerosis and plaque formation that was independent of dietary cholesterol and saturated fat (68, 69).
In fact, casein was 5-fold more atherogenic than soya protein over just a 6-mo feeding period (70), in part because casein intake increases cholesterol concentrations whereas soya protein decreases cholesterol concentrations in the serum (71, 72). Additionally, cow milk–derived lactalbumin increased atherosclerosis >2-fold over corn- or wheat-derived protein (73).
Animal protein from 12 different sources elevated cholesterol concentrations compared with 11 kinds of plant-derived protein, thus making it likely that these effects translate to other animal- and plant-based proteins (70).
Although the referenced studies in this section rely largely on data from male mice and rabbits, this does not exclude that the same adverse metabolic health effects of animal-based protein sources likely persist in females. In fact, we commented above on the effects that different dietary composition could have had on the healthspan in male and female rhesus monkeys maintained on a CR diet (47).
Protein source and health outcomes in humans
As outlined above, the amount of protein consumed impacts aging and health. However, as omnivores, humans do not consume only 1 sort of protein, and hence, which protein source we choose will inevitably influence other components of our diet, such as macronutrients, micronutrients, and phytochemicals, all of which individually and in combination can in turn influence health outcomes.
Therefore, we must determine the effects that the source material can have to better understand the long-term health effects of protein intake and optimize dietary recommendations. However, very few studies have considered this, which limits the conclusions that can be drawn regarding protein sources and their relation to all-cause mortality or cause-specific mortality.
For humans, dietary regimens, including dietary composition and diet-associated practices, form the largest group of risk factors for disability and mortality caused by chronic diseases (74). Prospective and randomized clinical trials demonstrate that in humans low-protein diets enhance metabolic health, promote lean physical appearance, lower blood glucose, and decrease the risk of developing diabetes (1, 75). In a cohort of 29,017 postmenopausal women without previous diagnosis of cancer, coronary artery disease (CAD), or diabetes, nutrient density models based on mailed questionnaires were used to estimate risk ratios from a simulated substitution of total and type of dietary protein (76).
For women in the highest intake quintile, CAD mortality decreased by 30% (95% CI: 0.49, 0.99) from an isocaloric substitution of vegetable for animal protein. CAD mortality was associated with red meats (risk ratio: 1.44; 95% CI: 1.06, 1.94) and dairy products (risk ratio: 1.41; 95% CI: 1.07, 1.86) (76).
Although no significant correlation between overall protein intake levels and ischemic heart disease (IHD) (RR: 0.97; 95% CI: 0.75, 1.24) or stroke events (RR: 1.02; 95% CI: 0.84, 1.24) was measurable among 43,960 middle-aged (53 ± 10 y) men during an 18-y follow-up, comparison of protein source groups provided further insight into the effects of animal-based compared with plant-based protein: an inverse correlation between plant-based protein intake and IHD (RR: 0.64; 95% CI: 0.48, 0.86) or stroke incidence (RR: 0.72; 95% CI: 0.57, 0.90) in the top compared with the bottom quintile, as well as a negative correlation between animal-based protein intake and IHD and stroke has been demonstrated (77, 78).
In the NHS and HPFS cohorts, high animal-based protein consumption was associated with higher all-cause mortality (HR: 1.23; 95% CI: 1.11, 1.37), cardiovascular mortality (HR: 1.14; 95% CI: 1.01, 1.29), and cancer mortality (HR: 1.28; 95% CI: 1.02, 1.60). In contrast, vegetable-based protein intake was associated with lower all-cause mortality (HR: 0.80; 95% CI: 0.75, 0.85) and cardiovascular mortality (HR: 0.77; 95% CI: 0.68, 0.87) for both men and women (54).
Analyses of the NHS cohort of 84,136 women aged 30–55 y, with no known cancer, diabetes mellitus, angina, myocardial infarction, stroke, or other cardiovascular disease, showed that higher intakes of red meat (RR: 1.16; 95% CI: 1.09, 1.23), red meat excluding processed meat (RR: 1.19; 95% CI: 1.07, 1.32), and high-fat dairy (RR: 1.03; 95% CI: 1.0, 1.06) were significantly associated with elevated risk of CAD. Vegetable protein was significantly associated with an 18% decreased risk when comparing the lowest and the highest intakes across quintiles (RR across quintiles: 1.00, 0.88, 0.85, 0.80, 0.72).
Higher intakes of poultry (RR: 0.9; 95% CI: 0.75, 1.08), fish (RR: 0.81; 95% CI: 0.66, 1.0), and nuts (RR: 0.78; 95% CI: 0.66, 0.93) were significantly associated with lower risk of CAD (79). Using the NHANES dataset, animal protein was positively associated with all-cause mortality (55). Yet, before drawing any conclusions, noteworthy limitations of these studies have to be considered, which can include relatively small sample sizes, the form of dietary assessments, and the evidently scarce data on animal and plant protein sources.
Further complicating the issue is that consuming plant-based vegetarian or vegan diets can also be associated with consuming less overall dietary protein and reduced levels of the essential amino acid methionine, both of which diet patterns promote health and longevity in rodents (27, 80, 81). Some of these concerns were largely addressed in a study that utilized data from 2 large US cohort studies with >130,000 participants, repeated measures of diet, and ≤32 y of follow-up to compare animal and plant protein and the risk of all-cause and cause-specific mortality (82).
A study population from the NIH American Association of Retired Persons (AARP) Diet and Health Study cohort (83) of half a million people aged 50–71 y at baseline, with a 10-y follow-up, further supports these findings: men and women in the highest compared with the lowest quintile of red meat intakes had elevated risks for overall mortality (men, HR: 1.31; 95% CI: 1.27, 1.35, and women, HR: 1.36; 95% CI: 1.30, 1.43), CVD (men, HR: 1.27; 95% CI: 1.20, 1.35, and women, HR: 1.50; 95% CI: 1.37, 1.65), and cancer mortality (men, HR: 1.22; 95% CI: 1.16, 1.29, and women, HR: 1.20; 95% CI: 1.12, 1.30). In contrast, high intake of white meat and a low-risk meat diet were associated with a small decrease in total and cancer mortality (83). Additional studies further establish a positive correlation between red meat and high-fat dairy consumption and risks for developing age-related diseases, including cancer and diabetes (84, 85).
These findings also suggest that processing of meat products might play a significant role in promoting adverse health outcomes. Indeed, well-done red meat; frequent frying, barbecuing, and broiling of meats; and the processing-induced exposure to bioavailable carcinogens, such as heterocyclic aromatic amines, are positively associated with the development of several cancers (86–88). Similarly to what has been shown for red meat alone, the consumption of processed red meat increases the risks for overall mortality (men, HR: 1.16; 95% CI: 1.12, 1.20, and women, HR: 1.25; 95% CI: 1.20, 1.31), CVD (men, HR: 1.09; 95% CI: 1.03, 1.15, and women, HR: 1.38; 95% CI: 1.26, 1.51), and cancer mortality (men, HR: 1.12; 95% CI: 1.06, 1.19, and women, HR: 1.11; 95% CI: 1.04, 1.19) when compared with the lowest intake quintile (83).
The intestinal microbiome plays an important role in modulating the risk of several chronic diseases such as obesity, type 2 diabetes, CVD, and cancer (89). Long-term animal- or plant-based nutrition influences the structure and activity of the micro-organisms residing in the human gastrointestinal system.
The short-term consumption of diets composed entirely of animal or plant products rapidly (1–2 d) altered microbial community structure in 6 male and 4 female US volunteers aged 21–33 y, whose BMIs ranged from 19 to 32 kg/m2: an animal-based diet increased the abundance of bile-tolerant micro-organisms (Alistipes, Bilophila, and Bacteroides) and decreased the abundance of Firmicutes bacteria that metabolize dietary plant polysaccharides; increases in the abundance and activity of Bilophila wadsworthia support a link between dietary fat, bile acids, and micro-organisms associated with inflammatory bowel disease (90).
Thus, long-term adherence to high-protein diets, particularly in combination with indiscrimination toward protein sources, has been linked to adverse health consequences in men and women. Conversely, plant-based protein consumption is associated with a reduced risk for multiple diseases and overall mortality.
These results complement the recommendations by the American Institute for Cancer Research and the World Cancer Research Fund to reduce red and processed meat intake to decrease cancer incidence. Therefore, a moderate consumption of plant-based protein should be considered to cover the body’s nutritional requirements.
Conclusions
Investigations of the amount of protein intake as well as the protein source in a variety of model organisms, nonhuman primates, and in humans emphasize a significant role for dietary protein in health and longevity (Figure 1).
Although we acknowledge that more research is needed, the existing literature, covering basic preclinical, randomized, and epidemiological experiments with large datasets, supports that protein intake and the resulting activation of nutrient-sensing pathways regulate metabolism, growth, and aging.
All available evidence demonstrates that low protein consumption, specifically based on plant-derived sources, is associated with benefits for healthy aging, at least until the age of 65.
Further research is needed to establish how nutrient requirements change throughout the lifespan, and to identify the potential role of sex-dependent differences in nutrient response and needs, particularly at advanced ages when system integrity becomes increasingly frail.
Today, dietary interventions remain the most applicable and cost-efficient means of preventing and treating a wide variety of aging-related diseases for most humans.
One of these interventions, the FMD, has gained significant attraction over the past year due to its safety and feasibility, easy inclusion into personal lifestyle choices, and most of all its high efficacy in reducing risk factors associated with aging, diabetes, CVD, and cancer. However, clinical studies using FMDs are limited and thus require careful evaluation.
More information: “The hidden costs of dietary restriction: Implications for its evolutionary and mechanistic origins” Science Advances, advances.sciencemag.org/content/6/8/eaay3047
