Fasting as a Path to Longevity: The Facts | Lifespan with Dr. David Sinclair - Season 2, Episode 2
In a Nutshell
Calorie restriction and time-restricted eating slow aging by 2-3% and cut mortality risk 10-15% by activating ancient survival pathways—mTOR, AMPK, and sirtuins—that trigger DNA repair, autophagy, and reduced inflammation. These effects have been replicated across yeast, worms, flies, mice, and primates, with rhesus monkey studies showing 30% calorie cuts delayed cancer, diabetes, and heart disease. Modern three-meal-plus-snack patterns clash with human evolution and ancestral feast-famine cycles, driving obesity and accelerated aging; eating less often, while preserving nutrients, is the most potent intervention available until drugs fully mimic these pathways.
These notes were generated by AI and may contain inaccuracies.
Calorie restriction puts the body in survival mode. A study showed that calorie restriction can slow down the rate of aging, translating into a 10-15% lower chance of mortality. Calorie-restricted mice lived about 28% longer. LCA, a bile acid in the gut, could mimic the anti-aging effects of calorie restriction. When researchers changed the microbiome of the mice, calorie restriction no longer worked.
Lifespan discusses cutting-edge science of aging and living healthier. David Sinclair is a scientist and professor working on understanding why we age and discovering ways to slow and reverse the aging process.
The team investigated food marketing and bad nutrition science. Much of the struggle to keep weight down is due to marketing dating back to the early 20th century. A 50,000-year-old Neanderthal toe bone fragment from a cave in Siberia may help explain why some people gain weight easily. The bone was held at a Harvard genetics department faculty meeting. Professor David Reich sequenced DNA from the bone, with his lab located down the hall from Sinclair's at Harvard.
The episode covers not just what to eat, but when to eat. When asked for one suggestion to live a long life, the answer is eat less often. The episode covers caloric restriction, new science about genes and small molecules that mimic caloric restriction, ancestral diets going back millions of years, why modern society and eating patterns are harmful, and how to determine what works for individual longevity.
Caloric restriction and fasting are not the same but overlap. Caloric restriction is defined as overall reducing the amount of calories consumed. Fasting means eating less often, usually within a certain time window. Both produce similar effects including lower blood pressure, lower body weight, better memory, and more energy.
A study showed that calorie restriction can slow the rate of aging by 2-3%. While this may not sound significant, it translates into a 10-15% lower chance of mortality. Calorie restriction is the strongest, most potent, most reproducible way to extend lifespan in animals and probably humans, at least until drugs are developed that mimic this effect more accurately.
A step-by-step roadmap will be provided for implementing fasting or calorie restriction safely and effectively, covering figuring out the what and why, choosing the right strategy, what to monitor, and how to adjust as progress is made.
The majority of people are overweight and at least 30% in the United States are obese. Science shows that obesity increases the rate of aging, making people susceptible to diseases beyond metabolism—pretty much all diseases accelerate when not maintaining a lean body type.
Humans living 100,000 years ago were not eating three meals a day. They ate only when food was available, especially during tough winter periods. This includes African tribes on the savannah, Neanderthals in the Eurasian region, and Denisovans further east.
Neanderthals lived from around 100,000 years ago until about 50,000-40,000 years ago. They were a robust species that ate plants and hunted large animals. Many people north, west, and east of Africa carry Neanderthal genes. Their bodies evolved to respond to famine and survive during times of scarcity.
Early humans from the African savannah also faced hunger much of the time, surviving droughts when food was scarce. Tribes today descended directly from those people contain no Neanderthal DNA. The San people in southern Africa around Botswana live a lifestyle similar to how humans existed tens of thousands of years ago, typically eating only one or maybe two meals a day.
Modern nutrition advice recommends three square meals a day and never being hungry. This differs significantly from ancestral patterns where people ate when food was available.
The thrifty genotype hypothesis was first proposed over 60 years ago by James Neel. Because humans descended from people who survived periods of feast and famine, thrifty genes allow storing a lot of fat to survive scarcity. People who gain weight easily carry these ancestral genes that quickly store energy for as long as possible.
The evolutionary mismatch hypothesis, proposed in 1994, is a broader version of Neel's idea. Gene variants many people carry are not suited for a world with little exercise and excessive eating. These variants lead to chronic diseases including obesity, diabetes, cardiovascular disease, and dementia.
A cave in southern Siberia, Russia, has sheltered humans for over 100,000 years and contains layers of bones and artifacts. David Reich has worked on bone fragments from this cave for over a decade. His postdoc supervisor Svante Pääbo won the Nobel Prize for extracting DNA from fossils. A tiny toe bone fragment from this cave yielded a complete Neanderthal genome published in a 2014 Nature paper. The genome is 99.7% the same as modern humans, but variants explain differences in metabolism and food response today.
The TCF7L2 gene variant is carried by many Europeans. This variant predisposes people to obesity and diabetes. TCF7L2 regulates Raptor, the regulatory associated protein of mTOR complex 1, which regulates energy input especially amino acids. Downregulating mTOR is beneficial for longevity, but the variant causes weight gain, obesity, and type 2 diabetes. TCF7L2 also controls the THADA gene, turned on in fat, thyroid, and muscle. THADA is involved in non-shivering thermogenesis, keeping the body warm without shivering. Brown fat produces heat rather than energy, which was advantageous historically but not helpful today with air conditioning and abundant food.
Ketone IQ is a drink with active ingredient R-Beta-hydroxybutyrate precursor called R13 butanediol. The liver converts it into beta-hydroxybutyrate, the main ketone fuel for brain and muscle. A 2012 study showed consuming an exogenous ketone raised blood beta-hydroxybutyrate to levels seen during long fasting or strict ketogenic diet. A 2018 study found that as cognition declined, the brain became less able to use glucose, but ketone uptake remained preserved. A 2022 study gave soccer players ketones and found improved reaction time, cognition, and reduced fatigue. A 30% off discount is available at ketone.com/lifespan using code lifespan.
In ancient Greece, Egypt, and India, fasting and eating less were regarded as beneficial for health. As far back as 1500 BC in Vedic India, upvas was a term for ritual and medical fasting to allow healing and purification. In the 5th-6th century BC during Hippocrates' time, people noticed eating less when sick, leading to the idea that fasting was good during illness. A thousand years ago, eating two meals a day was normal, with dinner around midday and a smaller meal called supper. Eating more was considered excessive or gluttonous.
Luigi Cornaro lived a life of excess in his first half, eating and drinking too much and developing diseases of old age quickly including gout and likely type 2 diabetes. In his early 40s, he switched to caloric restriction of about 350 grams of food per day. His health turned around. His book Discorsi della vita sobria (Discourses on the Sober Life) documents how he thrived after eating much less. He lived at least to 82, possibly to 102, far exceeding average lifespan in the early 1500s.
Throughout history, being a little hungry was normal and considered advantageous for health, character building, and religious discipline.
Two hundred years ago with the industrial revolution and factory work, eating patterns shifted. People ate breakfast before work, typically porridge or cereal with tea or coffee. Lunch became a midday meal, and dinner occurred after work, creating the three-meal pattern. Food remained fresh with polyphenols and vitamins. The early 20th century refrigerator allowed buying and storing large amounts of food, including canned goods that destroyed nutrients.
Kellogg's marketing played a key role in promoting three meals daily. Around 1917, Lena Cooper wrote in Good Health magazine that breakfast is the most important meal of the day because it gets the day started, with no evidence or science cited. This became a powerful slogan for Kellogg's. A 2010 Super Bowl Snickers ad with Betty White featured the slogan you're not you when you're hungry.
A single flawed 1964 paper from Czechoslovakia by Pavol Fabry claimed eating five meals a day produced less obesity than three or fewer meals. The study was not randomized, did not control for physical activity, socioeconomic status, or total calorie counts. It became the basis for nutritionists and advertising claiming people should eat more, all the time, with snacks. From the 1970s onward, nutritionists promoted eating from morning with small meals throughout the day to prevent obesity. Many nutrition studies have been sponsored by food companies, blurring lines between independent advice and marketing.
Before diving into the science of fasting, there's a concerning trend in how children are being trained to eat in ways that speed up aging and reduce lifespan. In the 1970s through mid-1980s, it was normal for kids to play outside without supervision and not eat anything after school until dinner. Children stayed thin and exercised regularly. However, the 1980s and 1990s brought helicopter parenting where parents wouldn't let children feel hungry, fueling the rise of brands like Go-Gurt, Snack Packs, and Lunchables marketed as essential for concentration. The result has been childhood obesity rates going through the roof, a trend that continues today.
Scientific literature shows mixed results on meal frequency. Some studies indicate little difference in outcomes as long as total daily calories remain the same, with the exception that eating late at night and sleeping with a full stomach is suboptimal. There's conflicting evidence on hunger levels—some studies show six meals daily reduce hunger while others show the opposite. Despite these inconsistencies, time-restricted eating consistently proves healthier for metabolism and long-term health. It increases insulin sensitivity, helping the body remove sugar from blood, increases fat oxidation, and can help prevent hunger.
A calorie (lowercase c) was coined in 1824 by Nicholas Clement in Paris as the energy needed to raise one gram of water by one degree Celsius. Today's capital C calorie represents 1,000 of these units—the energy to raise one kilogram of water by 1°C. Labels in the United States typically show kilocalories represented by a large C.
Though fasting and calorie restriction have been practiced for thousands of years, formal scientific study began in the early 20th century. In 1915, physicians Frederick Allen and Elliot Joslyn (namesake of Boston's Joslin Diabetes Center) developed the "starvation diet" to treat diabetics. By limiting food and adding exercise, they could literally cure patients of diabetes. This approach was abandoned around 1922 when insulin was discovered.
Osborne and Mendel conducted early studies on diet and lifespan using rats, finding that female rats eating less or given fewer amino acids developed more slowly, remained smaller, but had longer lifespans.
The first formal studies on caloric restriction's effect on longevity in rats were conducted in 1935 by Clive McCay and colleagues at Cornell University, who are regarded as founders of this research field. McCay's book of memoirs sits near and dear to Dr. Sinclair, normally kept on his home bookshelf. The book contains insights about 1930s research, including the quote: "The animal work involves much art as well as science. Each animal should be carefully studied and minor adjustments made to suit the needs of the animal. Always handle animals with great gentleness and try to make pets of them." This contrasts with modern lab practices where making pets of mice is discouraged.
McCay's team showed that restricting calories in rats by 30-40% led to mean lifespan extension between 33% and 50%—still close to the world record for lifespan extension in a mammal. This represented a paradigm shift, transforming calorie restriction and fasting from scientific curiosity to an entire research field studied ever since.
In the 1980s and 1990s, Richard Weindruch and Roy Walford conducted experiments showing that calorically restricting mice extended lifespan 20-30% while making them much healthier.
Roy Walford participated in Biosphere 2, an enclosed greenhouse environment where participants attempted to recapitulate Earth. They were locked inside for nearly a year wearing Star Trek-like suits. When they couldn't grow enough food, they inadvertently underwent caloric restriction. This became one of the first informal studies of the diet's effects on humans, showing biochemical changes similar to those observed in rats.
The CALERIE study (Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy) was conducted across three sites: Tufts University, Louisiana State, and Washington University. The goal was 25% calorie reduction over 2 years, but participants achieved only 12% reduction on average—about 280 fewer calories daily. Dr. Sinclair visited Eric Ravussin at Louisiana State during the study.
A 2023 paper in Nature Aging by Waziri et al. used the DunedinPACE clock (developed in Dunedin, New Zealand) to measure aging pace. The calorie-restricted group eating 12% fewer calories showed a 2-3% decreased pace of aging, corresponding to a 10-15% decrease in all-cause mortality risk.
A 2018 study by Belsky et al. used "Klemera-Doubal" (KD) years to measure aging. Controls aged at 0.71 KD years per year while the calorie-restricted group aged at only 0.11 KD years per year—0.6 fewer biological years per chronological year.
Most people won't die from infections or accidents but from aging-related diseases, making it important to track health markers over time. Dr. Sinclair recommends the Whoop band, developed partly from Harvard research, which provides insights into sleep, recovery, and body strain. A 2022 study comparing six wearables against clinical-grade ECG and sleep devices showed Whoop had strong accuracy for heart rate and heart rate variability during sleep.
After wearing Whoop for two months with lifestyle changes including healthier diet and more exercise, Dr. Sinclair saw his resting heart rate drop to 45 and heart rate variability rise to 90, placing him in the top few percent for his age. He recommends building a health record to reference when older, aiming to maintain 40s or 50s biomarkers at age 90.
Whoop is available with a free 5.0 smart band and month membership at join.whoop.com/lifespan or using code "lifespan."
The calorie-restricted group lost about 16 pounds (7 kilograms) on average and reached a new weight set point. Lipid improvements included: LDL cholesterol decreased by 10 mg/dL, triglycerides decreased by 24 mg/dL, and HDL cholesterol increased by 4 mg/dL. Additional benefits were lower blood pressure, improved physical endurance, better cognition, increased sex drive, and better sleep—all from 12% calorie restriction.
The calorie-restricted group received nutrition coaching and education, with 90% maintaining adequate nutrient levels including omega-3s, vitamins B, C, E, K, calcium, and others. The ad libitum group without coaching did not improve their diet. This demonstrates that calorie restriction requires maintaining adequate nutrition and monitoring for deficiencies.
Studies with calorie restriction ranging from 15% to 47% largely confirm the CALERIE findings of improved health and slower aging indicated by molecular and epigenetic clocks.
Okinawans have practiced calorie restriction for over a century, eating less than mainland Japanese. Their practice of "hara hachi bun me" (eat until 80% full) results in lower average consumption. They eat a highly nutritious, mostly plant-based diet with fish as the primary meat source, combined with significant physical activity working fields and strong social connections. Okinawans live longer than mainland Japanese and Americans.
In the 1990s and 2000s, Brad Wilcox and Craig Wilcox compared Okinawans who remained in Okinawa versus those who moved to Hawaii. Those adopting American lifestyles lost their longevity advantage. The researchers wrote "The Okinawa Diet" book in the early 2000s. Dr. Sinclair followed this diet for three years—primarily fish and green leafy vegetables—achieving excellent cholesterol levels, leanness, and feeling great before stopping when he had children.
Calorie restriction with optimal nutrition (CRON) emphasizes that nutrition quality matters when restricting calories. CRONies are people practicing long-term calorie restriction. Paul and Meredith McGlothin visited Dr. Sinclair's lab in the 2000s after a decade of calorie restriction, which they continue today. Brian Delaney was president of the Calorie Restriction Society International at that time.
The McGlothins appeared energetic, not tired. Paul was New York State chess champion at the time, demonstrating cognitive function remained intact. A 60 Minutes episode covered their story. Their blood work resembled calorie-restricted primates with low inflammation and low atherosclerosis risk—some of the first human data on calorie restriction before the formal CALERIE study.
Since McCay's rat studies, calorie restriction has extended lifespan and improved health across organisms including worms, flies, spiders, Daphnia (water fleas), dogs, rodents, and monkeys. In Dr. Sinclair's lab, yeast with less glucose lived longer and appeared healthier under the microscope.
A 2019 Cell Metabolism study led by Rafael de Cabo at NIH compared three mouse groups: ad libitum eating (control), 30% calorie restriction, and once-daily feeding with the same total calories as controls. The 30% calorie-restricted group lived 28% longer. The once-daily feeding group saw an 11% lifespan increase, demonstrating that both quantity and timing of food intake matter.
French researchers studied 30% calorie restriction in grey mouse lemurs, finding mortality from age-related diseases dropped 60%. Mean survival was 6.4 years in controls versus 9.6 years in the calorie-restricted group.
The crown jewel of calorie restriction research involves decades-long studies in rhesus monkeys at the National Institute on Aging (NIA, started 1987) and University of Wisconsin (started 1989). Rafael de Cabo runs the NIA study; Rosalyn Anderson, a former postdoc from Dr. Sinclair's lab, runs the Wisconsin study.
Both studies aimed for 30% calorie restriction and showed impressive health improvements including decreases in diabetes, cancer, and heart disease.
The 2009 Science paper from Wisconsin (initially 76 monkeys) reported 80% of calorie-restricted monkeys still alive versus 50% of controls. The paper titled "Caloric restriction delays disease onset and mortality in rhesus monkeys" showed dramatic visual differences: control monkeys had shoddy hair and skin inflammation while calorie-restricted monkeys had healthy coats. Disease incidence was dramatically lower in calorie-restricted monkeys across cancer, cardiovascular disease, and glucose metabolism issues.
An NIA study published in Nature a few years later found no significant impact of calorie restriction on survival, creating confusion. Both studies had similar designs and agreed on health improvements, but disagreed on lifespan extension. The teams, led by Rafael de Cabo and Rosalyn Anderson, collaborated on a 2017 Nature Communications paper analyzing data from nearly 200 monkeys across both studies, confirming that calorie restriction delays aging in nonhuman primates, though the extent of lifespan extension remains debated.
Though researchers disagreed about the extent of lifespan extension in the primate studies, the difference in outcomes between the two sites remained puzzling. Ros and Ruffer conducted follow-up experiments in mice to test whether dietary composition explained the discrepancy. One group received a diet equivalent to the Wisconsin monkey study and another received the NIA equivalent. The results showed that diet composition did not determine outcomes. What mattered far more was when and how many calories the animals consumed.
Rufford shared an anecdote from the NIA study: researchers noticed that calorie-restricted monkeys were gaining weight unexpectedly. Investigation revealed a technician who felt sympathy for the thinner animals and was providing extra snacks. She was promptly fired, though researchers hoped the incident had minimal impact on study results.
Studies in mice, primates, and many other species demonstrate that calorie restriction without malnutrition extends lifespan. A critical clarification is that benefits are not simply due to weight loss. Losing weight alone does not extend lifespan in animals, and this explanation fails to account for why yeast, spiders, and water fleas also live longer under calorie restriction.
A 2005 paper from Sinclair's lab stated: "Recent data from yeast, worms, and flies in mammals support the idea that calorie restriction is not simply a passive effect, but an active, highly conserved stress response that evolved early in life's history to increase an organism's chance of surviving adversity." This was termed the hormesis hypothesis of calorie restriction. Calorie restriction activates survival circuits that have existed for millions or possibly billions of years. The mechanism is not about thinness but about resilience—what does not kill you makes you live longer.
Calorie restriction shifts the balance between growth signaling and maintenance. When nutrients are abundant, cells receive signals to grow and build. When calories or essential nutrients such as amino acids are limited, growth signals quiet and survival mode activates. This includes repair processes, cleanup of misfolded proteins, and stress resistance mechanisms that protect against disease and aging.
Three main longevity pathways respond to calorie restriction:
- mTOR, which senses amino acids
- AMP kinase, the target of metformin
- Sirtuins, activated by NAD
Calorie restriction adjusts all three pathways to turn on DNA repair, reduce inflammation, preserve health, prevent disease, and potentially extend lifespan.
Early 2000s research in yeast demonstrated that overexpressing sirtuin genes extended lifespan. Deleting the SIR2 gene eliminated the lifespan extension normally produced by calorie restriction. This established that benefits of reduced calorie intake act through a genetic pathway rather than simply slowing metabolism. At the time, most researchers attributed benefits to reduced free radicals and less DNA damage. These experiments showed a gene was responsible for defending cells against adversity.
Sirtuins have since been shown to mediate calorie restriction effects not only in yeast but also in mice and human cells.
Calorie restriction reduces insulin and IGF-1 (insulin-like growth factor 1), which regulate mTOR. Cynthia Kenyon's landmark paper demonstrated that deleting a single gene could double worm lifespan. The gene, DAF-2, has an equivalent in humans that controls the insulin and IGF-1 receptor pathway. Low IGF-1 levels in blood tests are viewed positively in this context.
Calorie restriction produces multiple measurable benefits:
- Improved DNA repair
- Reduced inflammation
- Decreased oxidative stress
- Increased autophagy (recycling of old proteins)
- More youthful gene expression patterns and reversal of the epigenetic clock
- Mitochondrial multiplication and regeneration
- Increased diversity of the gut microbiome, which influences inflammation and nutrient absorption
Many people only track total body weight, which provides limited information. Withings produces devices including watches, a sleep monitor pad, and advanced bathroom scales. Sinclair has used Withings devices for over 13 years. The Withings smart scale recorded his resting heart rate at 59 beats per minute, dropping to approximately 49 during sleep.
Visceral fat surrounds organs and correlates with cardiovascular disease, diabetes, and mortality risk. The Withings Body Scan scale estimates muscle mass, fat mass, and visceral fat percentage. Sinclair's readings showed 11.3% arm fat, 14% torso fat, and 2.7% visceral fat. Over recent months through diet and exercise changes, he lost over 12 pounds while preserving muscle mass.
The Withings scale includes an FDA-cleared ECG that analyzes heart rhythm and can detect atrial fibrillation. Listeners can access devices at withings.com/lifespan with code "lifespan" for 10% off.
In 2003, Sinclair's lab discovered plant molecules that activate sirtuins, including resveratrol from red wine. These are termed STACs (sirtuin-activating compounds). Resveratrol binds to SIRT1 and causes a conformational change that activates the enzyme. This led to the hypothesis that an endogenous molecule produced during adversity such as calorie restriction might activate sirtuins. NAD was identified but was not the sought-after activator.
A 2024 Nature study identified lithocholic acid (LCA), a gut-derived bile acid, as capable of mimicking calorie restriction's anti-aging effects. LCA is produced when gut bacteria break down a liver-derived precursor. The study demonstrated that LCA enters cells and activates SIRT1 through the same mechanism as resveratrol—by inducing a conformational change in the enzyme.
LCA alone, when administered to mice, replicated multiple calorie restriction benefits including muscle regeneration, improved grip strength, endurance, and glucose handling. Changing the mouse microbiome eliminated the benefits of calorie restriction, indicating the microbiome mediates these effects. The study suggests bile acids, in addition to short-chain fatty acids, contribute to health benefits during reduced calorie intake.
Researchers caution against consuming bile acids to elevate LCA. High levels are associated with potential liver toxicity and increased cancer risk in animal models, particularly alongside DNA-damaging agents. Human clinical trials are needed to establish safety and efficacy.
TUDCA is a bile acid produced by the liver and used in traditional Chinese medicine, notably in bear bile. It functions as a chemical chaperone that assists cells in managing stress within the endoplasmic reticulum. This helps prevent cell death relevant to age-related diseases. TUDCA appears to reduce inflammation and oxidative stress. Animal research suggests it improves insulin sensitivity, regulates glucose metabolism, and offers neuroprotective effects relevant to Alzheimer's and Parkinson's disease. Human data remain limited. Individuals considering TUDCA supplementation should review PubMed literature and consult a physician.
After approximately age 30, most people gain several pounds annually. This gradual increase associates with accelerated aging and reduced lifespan. Calorie restriction and fasting can counteract this trajectory. Sinclair emphasizes that the goal is to present scientific evidence rather than prescribe specific protocols, allowing individuals to make informed decisions.
Some researchers argue calorie restriction does not truly slow aging but merely reduces obesity effects. Control animals in many studies are fed ad libitum and overconsume. However, the NIA monkey study showed benefits even when control monkeys ate modestly and remained thin.
Concerns about muscle loss are common. The CALERIE trial, the first large randomized human study of calorie restriction, found participants lost some lean mass but improved muscle quality. Additional human studies demonstrate enhanced mitochondrial function, reduced oxidative stress, and better insulin sensitivity in skeletal muscle. Resistance training remains necessary to maintain strength during calorie restriction.
Calorie restriction does not uniformly extend lifespan. In house flies, it shortened lifespan in one study. Rafa de Cabo at NIH found that while calorie restriction improved health across many mouse strains and both sexes, lifespan extension was inconsistent. Steve Austad studied genetically diverse mice descended from wild ancestors with approximately 0.3% genetic variation—closer to human variation of 0.1% than typical inbred lab mice at near 0%. In these mice, calorie restriction lowered cancer risk but did not extend lifespan.
A 2009 study by Jim Nelson examined over 40 inbred mouse strains. Some strains gained hundreds of extra days under calorie restriction while others lived shorter lives under identical protocols. Genetic background significantly influences outcomes, with implications for human applicability given diverse genetics, lifestyles, diets, and microbiomes.
Despite these caveats, calorie restriction remains among the most powerful available interventions for extending healthspan and potentially lifespan. Sinclair notes the difficulty of sustained calorie restriction in practice. The next episode addresses whether similar longevity benefits can be achieved without strict calorie restriction. Human evolution occurred in environments where food was less consistently available than today. Modern constant access to food and frequent eating contrast with ancestral patterns. Eating less and less frequently activates ancient repair and survival pathways beyond weight loss effects.
The following episode will provide practical guidance on implementing dietary strategies safely, addressing concerns about muscle loss, hormones, sex drive, and mood during fasting or calorie restriction.
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