Eating less, living longer: fasting, mitophagy and cellular cleanup

The Short Version

When you go without food for a while, your cells do more than burn fat: they switch into a repair mode that recycles old and damaged parts, including worn-out mitochondria [1][3][4]. Harvard psychiatrist Chris Palmer describes this as autophagy and its mitochondria-specific form, mitophagy, and calls calorie restriction "the oldest, truest, best proven way to prevent aging in a wide variety of animal species." [1] Harvard geneticist David Sinclair describes the same period without food switching on longevity genes called sirtuins, turning down mTOR and, after two or three days, triggering a deeper protein cleanup called chaperone-mediated autophagy [3][4]. They agree on the biology, but differ on how far to push it: Sinclair fasts most of each day and for two days about once a month, while Palmer stresses that fasting can only be sustained so long and turns to diets that mimic it, with medical supervision for anyone who is seriously ill [1][4]. Mark Hyman, a functional medicine doctor, adds a caution that no single eating pattern fits everyone [2].

Table of contents

What They Said

Chris Palmer: fasting as a medical tool that became a diet

Palmer comes to fasting from psychiatry and the history of epilepsy treatment [1]. His starting point is that going without food is not the same as starving. "Most people think going without food is bad, and they equate it with starvation," he says. "But in fact, when we go without food, it causes tremendous shifts in metabolism, both brain and body metabolism. And it puts the body into a mode of autophagy and conservation of resources and all sorts of things that are beneficial to human health." [1] That, he argues, is why fasting has been used therapeutically "in almost every culture and almost every religion for millennia," even though it was long dismissed as "religious folklore." [1]

The medical turning point came when fasting was tested on seizures. The observation goes back to Hippocrates, and "in 1921, one physician used intermittent fasting on a child with seizures and found that, oh, lo and behold, this religious folklore stuff has something to it." [1] The catch was obvious: "The problem with fasting is that you can only fast for so long before you starve to death, and that's not a very effective treatment." [1] When people ate normally again, "their seizures usually come right back, oftentimes with a vengeance." [1] So Dr. Russell Wilder at the Mayo Clinic built the ketogenic diet "with one and only one purpose": to "mimic the fasting state" long term [1]. It worked: in Palmer's account, 50% of patients became seizure-free and another 35% had at least a 50% reduction in seizures [1]. Decades later, among patients who don't respond to drugs, about one-third become seizure-free, one-third get at least a 50% reduction, and one-third see no benefit [1].

Palmer is emphatic that the ketogenic diet "wasn't developed as a weight loss diet. It wasn't developed as the diet that all human beings should follow." [1] Its purpose was to reproduce what fasting does to the brain [1].

For Palmer, the most important thing fasting and its imitators do is renew mitochondria. The ketogenic diet affects neurotransmitters, calcium channels, gene expression, brain inflammation, the gut microbiome and insulin signalling, but "the real magic is that this diet stimulates two processes that relate to mitochondria." [1] One is mitophagy, "getting rid of old and defective mitochondria and replacing them with new ones." The other is mitochondrial biogenesis: after months or years on the diet, "many of their cells in their bodies and brains will have more mitochondria, and those mitochondria will be healthier." [1] He believes that is why the diet works for epilepsy and chronic mental disorders alike [1].

He explains how the cleanup works. "Mitophagy is a subset of autophagy," with some unique regulators of its own, and autophagy as a whole is switched on "by fasting states or fasting mimicking states." [1] "When your body senses that you don't have enough food, it actually hunkers down and starts to recycle dead old parts." [1] Those parts are carried to lysosomes, degraded, and the products are used "for either energy or to build new things." [1] Autophagy "is always occurring at a low level, but you can really hyper-stimulate the process through fasting, calorie restriction, fasting mimicking diets, other things." [1] What strikes him most is its precision: "the body doesn't just destroy the healthiest tissue along with the old dead stuff it has these processes that identify the old and effective parts first, and they go first, and that's what's beautiful about the whole thing. And that's why fasting is so important." [1]

Palmer then ties mitophagy to aging. "It has long been known for decades that mitochondrial dysfunction is associated with everything that ails us, essentially," he says, and he cites a David Sinclair paper proposing that mitochondria "are actually the unifying link of everything that we know about aging." [1] If "mitochondrial dysfunction, is possibly the unifying cause of aging and all of the aging-related disorders," then mitophagy is the body's way of addressing it: "how can we get rid of old ones or defective ones and replace them with new ones?" [1] His answer is diet: "I think the most powerful signal and tool that we have right now is, in fact, related to diet. It's calorie restriction." [1] He adds a limit in the same breath: "Fasting and intermittent fasting, and again, you can only do those things for so long, and then fasting, mimicking diets can also stimulate this process of mitophagy." [1]

He applies the same idea to mood. For ordinary people who feel burned out but aren't on psychiatric medication, he says calorie restriction, ketogenic diets and carbohydrate restriction induce metabolic changes that can benefit the brain regardless of what the person ate before, and he would suggest trying carbohydrate restriction for a while [1]. One reader with chronic anxiety, a vegetarian, cut out some high-carb foods without going ketogenic and within three weeks said, "I can't believe how much better I feel." [1] For people with serious illness, though, Palmer wants medical supervision "because meds may need to be adjusted." [1] "Weight loss, everybody wings it," he says, but a serious disorder needs "a real shot at this diet." [1]

David Sinclair: hunger turns on the defense genes

Sinclair arrives at the same place from the biology of aging. Calorie restriction as a longevity tool, he says, was first noticed early in the 20th century, ignored, and rediscovered in the 1930s, when Clive McCay "put cellulose in the food of rats so they couldn't get as many calories" and "those rats lived 30% longer." [4] Interest returned in the 2000s when his lab and others found longevity genes [4]. "We showed in 2005 in a science paper that if you have low levels of insulin and another molecule called insulin-like growth factor, those low levels turn on the longevity genes," the sirtuins, of which there are seven [4]. The opposite also holds: "by having high levels of insulin all day, being fed means your longevity genes are not switched on." [4]

Constant eating, in his view, actively speeds aging. "Your clock is ticking faster by always being fed," he says [4]. He suspects a second problem too: always having food around means "it's not allowing the cell to have periods of rest and reestablish the epigenome." [4] And there are simpler gains: low glucose makes muscles and brain more sensitive to insulin so they "suck the glucose out of your bloodstream," which "will ward off type 2 diabetes." [4]

Is it hunger itself, or the absence of food, that matters? Studies at the NIH simulating hunger while still providing calories suggest "there's a small component that's due to hunger. But most of it actually is because you've got these periods of not being fed. And then the body turns on these defensive genes." [4] He describes a Rafael de Cabo experiment with over 10,000 mice given different combinations of fat, carbohydrate and protein, in which mice fed within one hour a day lived dramatically longer than mice eating freely, regardless of diet [4]. His conclusion: "it's not as important what you eat. It's when you eat during the day." [4]

Sinclair lists three pathways that eating less activates: mTOR, which senses amino acids and suppresses cell growth when it is turned off; AMPK, which senses energy levels; and the sirtuins, repair proteins that protect against disease [3]. On Huberman Lab he explains that fasting raises sirtuin activity and lowers mTOR, and that together this turns on the body's defenses: clearing old proteins, improving insulin sensitivity and repairing cells [4]. Sirtuins also need NAD, which declines with age, with obesity and with never being hungry [4].

Longer fasts reach a deeper level. "There's a natural cleansing that happens when you're hungry," macroautophagy, he says, but his friend Ana Maria Cuervo at Albert Einstein College of Medicine discovered a "deep cleanse" that "kicks in day two, day three, which really gets rid of the deep proteins." [4] That is chaperone-mediated autophagy, and "she just put out a big paper that said, if you trigger this process in an old mouse, it lives 35% longer." [4] On the School of Greatness he describes it as a deeper level of cellular cleaning that removes toxic proteins, linked to disease prevention and potentially age reversal [3].

Sinclair describes his own routine in detail. "My typical day, I would only eat within a two-hour window," he says, usually one mostly vegetable-based dinner [4]. He exercises fasted: "now my body's so used to it, I don't feel like I need food after exercising." [4] He fasts longer only occasionally: "I find it quite difficult to go more than 24 hours. But when I do it, maybe it's once a month, I'll go for two days." [4] "If I could go longer, I would," he adds, but his workload means "I need to eat at least once a day." [4] He even credits the diet for no longer getting dental plaque, because "I don't have those sugars in my mouth that the bacteria feed on." [4]

His central principle is pulsing. "I pulse things so that I get periods of fasting, and then I eat, then I take a supplement, then I fast, then I exercise," he says, "because you can't just expect to take something constantly and do something constantly for it to work." [4] The goal "is to get the cells to be perceiving adversity," because otherwise "Our cells respond, they go, hey, everything's cool, no problem. And they become relaxed and they're in turn on their defenses and we age rapidly." [4] "People who exercise and eat less have a slower ticking clock. It's a fact." [4]

He is practical about starting. "Definitely try to skip a meal a day, that's the best thing," ideally breakfast or dinner so the fast joins the sleep period [4]. The first two to three weeks are hard, he warns [4]. His advice is to build up gradually rather than going cold turkey [4]. He adds that people adapt to the feeling of not eating, much as they adapt to cold water [4].

On what to eat when you do eat, he is more moderate than his line about timing suggests. He cautions that some sweet fruits such as grapes may spike blood sugar, prefers polyphenol-rich fruits like blueberries and blackberries, and supports the Mediterranean diet, noting that olive oil has been shown to activate anti-aging pathways similar to those triggered by fasting [3].

Mark Hyman: quality and personalization over any single rule

Mark Hyman's conversation is not about fasting, and that absence is itself informative [2]. His emphasis is on food quality: whole, unprocessed foods, low sugar and refined carbohydrates, lots of colorful vegetables, nuts, seeds and healthy fats, and regeneratively raised meat and fish if they suit you [2]. He counts food as 80% of his longevity approach, with exercise, sleep, meditation and connection as the other foundations, and advanced tools as add-ons [2]. In his seven-system model of functional medicine, the energy system of the mitochondria is one of the systems that nutrients determine [2].

His main contribution here is a warning against dogma. Each body has different needs: a diabetic might thrive on keto, a vegan might feel depleted after a few years, and a stressed-out vegetarian may need more protein and less sugar [2]. He encourages lab tests and paying attention to how you feel after eating: “Listen to your body—not the dogma.” [2]

Where They Agree, and Where They Don't

The consensus

Periods without food trigger repair. Palmer and Sinclair describe the same core event from two directions: when food is absent, the body shifts into recycling and defense [1][4]. Palmer frames it as autophagy and mitophagy clearing old and defective parts first [1]; Sinclair as sirtuins switching on, mTOR switching off, and old proteins being cleared [3][4].

Calorie restriction has the strongest record. Palmer calls it the best-proven anti-aging intervention across animal species [1]; Sinclair traces it back to McCay's rats living 30% longer and to de Cabo's mice [4].

The benefit is not about weight. Palmer stresses that the ketogenic diet was built to mimic fasting's effect on the brain, not to slim anyone down [1]. Sinclair argues that the timing of eating matters more than its content, and that most of the effect comes from periods without food rather than fewer calories or the feeling of hunger [4].

Low insulin is the common signal. Palmer's diets lower glucose and insulin and improve insulin signalling [1]. Sinclair's sirtuins switch on when insulin and insulin-like growth factor are low [4].

The differences

How far to take it. Sinclair eats within about a two-hour window most days and fasts for two days about once a month, and says he would go longer if his schedule allowed [4]. Palmer's framing is more cautious: fasting "can be a good short-term intervention," but "it's not a good long-term treatment," since "you can only fast for so long." [1] His solution is a diet that mimics fasting rather than more fasting [1]. The difference reflects their aims. Sinclair is trying to slow aging in a healthy person and uses pulses of hardship on purpose [4]. Palmer treats patients with epilepsy and serious mental illness, who need an intervention they can sustain for months or years [1].

What to eat. Palmer's lever is carbohydrate restriction, down to ketosis for serious illness [1]. Sinclair says timing matters more than content, although he still favors plants, polyphenol-rich fruit and the Mediterranean diet [3][4]. Hyman declines to pick a pattern for everyone [2].

Who should do it. Sinclair shares what he does rather than making recommendations [4]. Palmer separates everyday burnout, where people can experiment, from serious illness, where medical supervision is essential [1]. Hyman argues that responses vary by person and should be checked with lab tests [2].

Why It Works

Autophagy and mitophagy

Autophagy is the cell's recycling system. It always runs at a low level, but fasting, calorie restriction and fasting-mimicking diets strongly stimulate it [1]. The cell tags old and defective parts, carries them to lysosomes, breaks them down and reuses the pieces for energy or new structures [1]. Mitophagy is the version that targets mitochondria, with its own regulators, and mitochondria themselves take part in running autophagy [1]. Because mitochondria help make neurotransmitters and steroid hormones and help regulate stress and inflammation, renewing them could explain effects on mood and on aging at the same time [1].

Mitochondrial biogenesis

The second half of the process is building new mitochondria. Palmer says people on the ketogenic diet for months or years end up with more, healthier mitochondria in the cells of their bodies and brains [1].

Sirtuins, mTOR and AMPK

In Sinclair's model, the absence of food is sensed by three systems: sirtuins respond to low insulin, mTOR to low amino acids, and AMPK to low energy [3][4]. Raising sirtuins and lowering mTOR together turns on defenses including protein cleanup, better insulin sensitivity and cell repair [4]. Constant feeding keeps the longevity genes off and lets the epigenome degrade faster [4].

Chaperone-mediated autophagy

After two or three days without food, a deeper form of cleanup begins, removing proteins that ordinary autophagy does not reach [3][4]. Triggering it in old mice extended their lives by 35% in Cuervo's work, as Sinclair reports it [4].

The limits of the evidence

Palmer is clear that randomized trials of ketogenic diets for mental disorders don't exist yet: "If you're looking for randomized controlled trials, documenting efficacy in large numbers of patients with these disorders, we don't have them." [1] Much of the aging evidence Sinclair cites comes from rats and mice [4]. The epilepsy evidence, in contrast, is a century deep [1].

Where It Applies, and Where It Doesn't

Daily time-restricted eating is the accessible version. Skipping a meal to extend the overnight fast is Sinclair's first recommendation, and he describes it as hard for two to three weeks and then easier [4].

Longer fasts are occasional. Even Sinclair finds more than 24 hours difficult and does two days about once a month [4]. Palmer points out that fasting cannot continue indefinitely [1].

Fasting-mimicking diets fill the gap. For people who need the effect long term, as in epilepsy or serious mental illness, Palmer uses the ketogenic diet to reproduce the fasted state without starving [1].

It does not work for everyone. A third of drug-resistant epilepsy patients see no benefit from the ketogenic diet [1], and some of Palmer's patients could not or would not follow it [1]. Hyman warns that the same diet can help one person and deplete another [2].

Medications change the picture. Palmer says anyone with serious mental illness or heavy medication use needs a medical professional involved, since doses may need adjusting, and that stopping psychiatric medication without supervision is dangerous [1].

If you have diabetes, a history of eating disorders, are pregnant, or take medication, check with a doctor before fasting or starting a ketogenic diet.

What To Do With It

1. Extend your overnight fast. Skip breakfast or dinner so your fast joins your sleep, as Sinclair recommends [4].
2. Build up in stages. Ease into fasting gradually rather than going cold turkey; expect the first two to three weeks to be hard [4].
3. Keep drinks simple while fasting. Sinclair drinks water, tea and coffee during the day [4].
4. Try an occasional longer fast only if you are healthy. Sinclair does two days about once a month to reach chaperone-mediated autophagy, and finds more than 24 hours hard [4].
5. Pulse rather than hold. Alternate fasting and eating, and hard and easy days, so your cells keep perceiving adversity [4].
6. Lower insulin through what you eat, too. Cut sweets and highly processed foods, and consider carbohydrate restriction for a few weeks if you feel burned out, as Palmer suggests [1].
7. When you eat, eat well. Favor vegetables, polyphenol-rich berries and olive oil, which Sinclair says activates pathways similar to fasting [3], and avoid ultra-processed food, as Hyman urges [2].
8. Personalize and measure. Watch how you feel and get lab work to see how your body responds, rather than following any diet as dogma [2].
9. Use supervision for medical goals. If you want to use fasting or a ketogenic diet to treat a condition, work with a clinician and never change psychiatric medication on your own [1].

Sources

1. Dr. Chris Palmer on Diet, Metabolism, Mitochondria, and Mental Health (Huberman Lab Essentials) · Huberman Lab
2. How Food Heals or Harms Your Body, Aging & Mental Health with Dr. Mark Hyman · The School of Greatness
3. How to Stop Aging (Can Humans Become Immortal?) with Dr. David Sinclair · The School of Greatness
4. Dr. David Sinclair: The Biology of Slowing & Reversing Aging · Huberman Lab