The Science of Caloric Restriction and Longevity: From Fasting to Fasting-Mimicking Diets

"Eat less, live longer" -- this intuitive hypothesis has been repeatedly tested over more than 90 years of scientific research. Since Clive McCay first reported lifespan extension in rats in 1935, caloric restriction (CR) has been established as the most reproducible intervention in aging research. Lifespan extension has been confirmed in nearly every model organism, from yeast to primates. However, chronic caloric restriction is difficult for humans to sustain, and new approaches to overcome these limitations -- the Fasting-Mimicking Diet (FMD) and intermittent fasting -- have developed rapidly in recent years.

90 Years of Caloric Restriction Research: From McCay to the Present

The origins of caloric restriction research trace back to a groundbreaking paper published by Clive McCay of Cornell University in the Journal of Nutrition in 1935 (McCay et al., 1935). McCay reported that restricting the caloric intake of rats to approximately 60% of ad libitum feeding extended maximum lifespan by roughly 33%. This discovery opened an entirely new research domain at the intersection of nutritional science and gerontology. A crucial point is that essential nutrients such as vitamins and minerals were fully provided -- only calories were restricted. To distinguish this from malnutrition, the intervention is defined as "caloric restriction without malnutrition."

Over the subsequent decades, the lifespan-extending effects of caloric restriction were replicated across numerous model organisms, including yeast (Saccharomyces cerevisiae), nematodes (Caenorhabditis elegans), fruit flies (Drosophila melanogaster), mice, and rats. In particular, the systematic research by Weindruch & Walford (1988) demonstrated that the effects of caloric restriction in mice depend on dosage and timing of initiation, and that maintenance of immune function and reduced cancer incidence are the primary drivers of lifespan extension.

The molecular mechanisms of caloric restriction operate through multiple pathways. First, suppression of the mTOR (mammalian target of rapamycin) pathway. mTOR is a kinase that promotes cell growth and proliferation, activated in nutrient-rich environments. Caloric restriction suppresses mTOR and activates autophagy (the cell's self-cleaning mechanism). Second, activation of AMPK (AMP-activated protein kinase). AMPK is a sensor that detects energy deficiency and promotes mitochondrial biogenesis and fatty acid oxidation. Third, activation of sirtuins (SIRT1-SIRT7). SIRT1 in particular is an NAD+-dependent histone deacetylase that contributes to inflammation suppression, DNA repair promotion, and insulin sensitivity improvement. The integrated activation of these three pathways is considered the molecular basis of caloric restriction's anti-aging effects.

Primate Studies: The 20-Year Wisconsin and NIA Debate

Before extrapolating rodent results to humans, verification in primates was essential. Two independent long-term studies took on this challenge: one at the Wisconsin National Primate Research Center (WNPRC) at the University of Wisconsin-Madison, and another at the National Institute on Aging (NIA). Both studies used rhesus macaques (Macaca mulatta) as subjects and implemented 30% caloric restriction over more than 20 years.

In 2009, the WNPRC research team published their initial results in Science (Colman et al., 2009). The calorie-restricted rhesus monkeys showed significantly lower rates of age-related diseases (diabetes, cancer, cardiovascular disease, brain atrophy) compared to the ad libitum group, with overall mortality reduced by approximately 30%. During the 20-year follow-up period, 37% of the ad libitum group died from age-related causes, compared to just 13% in the caloric restriction group. These results attracted widespread attention as evidence that "caloric restriction works in primates too."

However, in 2012, the NIA research team reported a different conclusion in Nature. In the NIA study, no significant difference in overall mortality was found between the calorie-restricted and control groups. This contradiction sparked intense debate within the aging research community. In 2017, both research teams jointly reanalyzed their data and published unified conclusions in Nature Communications (Mattison et al., 2017). The key factor turned out to be "diet quality." The WNPRC control group had ad libitum access to a purified diet containing 30% sucrose, while the NIA control group consumed a healthier natural diet and was already subjected to mild caloric restriction. In other words, the NIA "control group" may have already been receiving partial benefits of caloric restriction. The unified conclusion was that "caloric restriction extends healthy lifespan in primates, with diet quality and age of onset modulating the magnitude of the effect."

The CALERIE Trial: The First Proof of Caloric Restriction Effects in Humans

The first large-scale RCT to rigorously examine the effects of caloric restriction in humans was the CALERIE (Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy) trial, funded by the U.S. National Institutes of Health (NIH). The Phase II trial was designed as a two-year double-blind RCT targeting 218 healthy non-obese adults aged 21-50 (BMI 22-28), with a prescribed 25% caloric restriction. The actual caloric restriction achieved averaged 11.9% (Ravussin et al., 2015, JAMA Internal Medicine).

The CALERIE trial's major findings were wide-ranging. The caloric restriction group showed decreases in fasting insulin levels, thyroid hormone (T3) levels, and metabolic rate, along with a 20-27% reduction in oxidative stress markers (Redman et al., 2018, Cell Metabolism). Furthermore, significant improvements in cardiovascular disease risk factors (LDL cholesterol, blood pressure, CRP) were observed. Notably, these effects were achieved with only about 12% caloric restriction.

In 2023, Waziry et al. reported in Nature Aging the results of analyzing DNA methylation data from CALERIE trial participants using DunedinPACE (an epigenetic aging velocity index) (Waziry et al., 2023). The caloric restriction group showed a 2-3% deceleration in the rate of biological aging compared to controls. While this effect may appear modest, when translated to mortality risk, it corresponds to a 10-15% reduction -- an impact comparable to smoking cessation interventions at the population level. The CALERIE trial holds historical significance as the first study to demonstrate at the molecular level that caloric restriction can slow the rate of human aging itself.

The Fasting-Mimicking Diet (FMD): Valter Longo's Innovation

While chronic caloric restriction has potent anti-aging effects, long-term adherence is unrealistic for most people. To address this challenge, Professor Valter Longo of the University of Southern California (USC) developed the Fasting-Mimicking Diet (FMD). The FMD is a program in which participants consume a specific low-calorie diet for just 5 days per month (approximately 1,100 kcal on day one, approximately 750 kcal on days two through five) and return to their normal diet for the remaining 25 days. The diet is designed to be high-fat, low-protein, and low-carbohydrate, mimicking the metabolic changes of fasting through food.

Professor Longo's early research demonstrated that FMD extended lifespan in mice, reduced cancer incidence, and improved cognitive function (Brandhorst et al., 2015, Cell Metabolism). At the molecular level, FMD substantially reduces IGF-1 (insulin-like growth factor 1), activates stem cell proliferation, and promotes autophagy. The reduction in IGF-1 is particularly significant, as decreased IGF-1 signaling has been consistently associated with lifespan extension from nematodes to mice.

In 2024, Brandhorst et al. published the latest results of an FMD human clinical trial in Nature Communications (Brandhorst et al., 2024). In this RCT of 100 participants, the group that completed three cycles (three months) of FMD showed an average reduction in biological age of 2.5 years as measured by DNA methylation clocks. Additionally, reductions in liver fat, improvements in insulin resistance, and decreases in CRP (C-reactive protein) were confirmed. Remarkably, these effects were maintained even during the periods when participants returned to their normal diets. The FMD program, commercialized as ProLon, is currently available under the guidance of healthcare professionals and holds promise as a practical approach for achieving anti-aging effects without sustained caloric restriction.

The Science and Limitations of Intermittent Fasting

Alongside FMD, intermittent fasting (IF) has also attracted considerable attention. Representative protocols include time-restricted eating (TRE) using the 16:8 method (16 hours fasting, 8-hour eating window) and the 5:2 method (5 days normal eating, 2 days restricted to approximately 500 kcal). In 2019, Dr. de Cabo of the National Institutes of Health and Professor Mark Mattson of Johns Hopkins University published a comprehensive review on intermittent fasting in the New England Journal of Medicine (NEJM), systematically summarizing the scientific foundations of the field (de Cabo & Mattson, 2019).

According to de Cabo & Mattson (2019), intermittent fasting triggers metabolic switching: when glucose is depleted during fasting, the body shifts to ketone body production, and ketone bodies act as signaling molecules to promote increased BDNF (brain-derived neurotrophic factor) expression, autophagy activation, and adaptive responses to oxidative stress. Multiple RCTs have reported weight loss, improved insulin sensitivity, and reduced blood pressure. Wilkinson et al. (2020) reported in Cell Metabolism that a 12-week intervention of 10-hour TRE in patients with metabolic syndrome significantly improved body weight, blood pressure, and blood lipids.

However, the long-term effects and risks of intermittent fasting require careful evaluation. In 2023, Liu et al. reported in Nature Medicine the results of a large-scale cohort study in China suggesting that extreme time-restricted eating may be associated with elevated cardiovascular event risk in certain populations. Additionally, intermittent fasting carries a risk of muscle mass loss -- an RCT by Lowe et al. (2020) confirmed significant reductions in lean body mass in the 16:8 group. For individuals with a history of eating disorders, fasting patterns may also reinforce restrictive eating behaviors. Furthermore, genetic diversity influences the response to caloric restriction and fasting, with effects potentially varying by FTO genotype and APOE genotype. There is no "silver bullet" dietary intervention; a tailored approach is needed, one that accounts for individual metabolic profiles, muscle mass, age, and medical history.

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