Intermittent fasting is any eating pattern that alternates periods of eating with planned periods of little or no food, from a daily 16:8 eating window to two low-calorie days a week on the 5:2 plan. Autophagy, literally "self-eating", is the process cells use to break down and recycle their own worn-out parts, and fasting switches it on strongly in yeast and animals. Whether a given fasting schedule measurably raises autophagy in people is still open; human trials mostly measure weight and metabolic markers.
That gap matters because the two ideas are usually sold as one. Social media turned a Nobel Prize for yeast genetics into a promise that the body "starts cleaning itself" at hour 16 of a fast. The real story is more modest: a genuine discovery about how cells sense nutrients, a growing set of randomised trials in people, and confident timing claims nobody has measured. This guide separates the three, and sits within the biohacking topic hub.
What is intermittent fasting?
Intermittent fasting is an umbrella term for schedules that restrict when people eat rather than, or as well as, what they eat. Four patterns cover most of the research.
Time-restricted eating confines food to a fixed daily window. The best-known version is 16:8, eating within eight hours and fasting for sixteen; 14:10 is a common gentler start.
Alternate-day fasting alternates normal days with fast days of no food or a small meal.
The 5:2 diet means eating normally five days a week and cutting to roughly 500 to 600 calories on two non-consecutive days. It was popularised by the British doctor and broadcaster Michael Mosley through the BBC series Horizon and The Fast Diet, written with Mimi Spencer and published in early 2013.
Periodic prolonged fasts last from about 24 hours to several days, done occasionally. These carry the most autophagy claims and the most risk. The earlier overview of time-restricted eating covers the day-to-day mechanics of a 16:8 window.
Fasting is older than the science
Fasting is one of the oldest religious disciplines, long predating any metabolic explanation. Muslims fast from dawn to sunset through the month of Ramadan. Observant Jews abstain from food and drink for roughly 25 hours on Yom Kippur. Many Christian traditions keep Lent, the forty days before Easter, as a season of fasting and abstinence. Hindu, Buddhist and Jain traditions have their own fasts. These practices were framed around discipline and attention to the spirit, not cellular biology, but they show that people can sustain demanding fasting patterns when the practice carries meaning beyond weight.
What is autophagy?
Autophagy is the cell's internal recycling system: damaged proteins and worn-out organelles are wrapped in a membrane sac called an autophagosome, delivered to the lysosome and broken down into reusable raw materials. It runs all the time and speeds up under stress, including nutrient scarcity.
The word was coined in 1963 by the Belgian biochemist Christian de Duve, discoverer of the lysosome and a 1974 Nobel laureate. For three decades autophagy was hard to study because nobody knew which genes controlled it.
That changed with Yoshinori Ohsumi, a Japanese cell biologist working with baker's yeast. In 1992 his group used mutant yeast that could not digest the contents of their vacuole to show that starvation made autophagic structures accumulate where they could be seen under the microscope, and in 1993 they reported a screen that identified the first set of genes essential for the process, about 15 of them. He went on to show how their proteins build the autophagosome and that similar machinery operates in human cells. He received the 2016 Nobel Prize in Physiology or Medicine "for his discoveries of mechanisms for autophagy", according to the Nobel Assembly at Karolinska Institutet.
The prize was for the mechanism, not for fasting; these were not human fasting studies.
How fasting and autophagy are linked: mTOR and AMPK
Cells decide whether to build or recycle by reading their nutrient and energy status, and two signalling hubs do most of that reading. mTOR (in yeast simply TOR) is active when nutrients and growth signals such as insulin are plentiful, and in that state it holds autophagy down; the Nobel committee's scientific background notes that when starvation inactivates TOR, autophagy is activated. AMPK is switched on when cellular energy runs low and pushes the cell toward recycling.
Fasting, in principle, moves both dials toward autophagy, but the direct evidence comes largely from model organisms. A 2004 study by Noboru Mizushima, Ohsumi and colleagues in Molecular Biology of the Cell used mice engineered so that autophagosomes glow green, and found that food deprivation induced autophagy in most tissues, to different degrees in different organs. The same paper noted that some tissues ran autophagy actively without starvation: it is not a simple on-off switch.
Does autophagy start at hour 16?
No such threshold has been established in humans. The claim that autophagy "kicks in" at 16, 18 or 24 hours is extrapolated from animal and cell work, and rodents' faster metabolism makes a day without food a far larger stress for them than for a person.
The underlying problem is measurement. There is no validated routine test for autophagy in a living person's organs. Researchers infer it from markers in white blood cells or muscle biopsies, but these are indirect, capture one tissue at one moment, and do not always agree. Autophagy is also a flux, a rate of material moving through the system, which a snapshot cannot fully capture. The accurate statement is narrower than the slogan: fasting is a well-documented trigger in yeast and animals, longer fasts plausibly raise it in some human tissues, and the timing, size and health meaning of that rise have not been pinned down in people.
What the human trials actually show
The strongest human evidence comes from randomised trials, which mostly measured weight, body composition and metabolic markers rather than autophagy.
The metabolic-switching review. On 26 December 2019, Rafael de Cabo and Mark Mattson published "Effects of Intermittent Fasting on Health, Aging, and Disease", a review article in the New England Journal of Medicine. Their central idea is metabolic switching: after enough hours without food the body shifts from glucose toward fatty acids and ketones, and repeatedly flipping between fasted and fed states engages cellular stress-response pathways. Much of their evidence came from animals and short human studies, and long-term human data were limited.
The TREAT trial. Lowe and colleagues published the TREAT randomised trial in JAMA Internal Medicine in 2020. It assigned 116 adults with overweight or obesity to a 16:8 window (noon to 8 p.m.) or three structured meals a day for 12 weeks. The fasting group lost about 0.94 kg, but the between-group difference was not significant, and an in-person subgroup showed greater loss of lean mass with time-restricted eating. The authors concluded it was not more effective for weight loss than eating throughout the day.
The 12-month calorie-restriction trial. Liu and colleagues reported a larger, longer test in the New England Journal of Medicine in April 2022. They randomised 139 adults with obesity to either calorie restriction alone or the same calorie target eaten only between 8 a.m. and 4 p.m. Both groups were told to eat 1,500 to 1,800 kcal a day for men and 1,200 to 1,500 for women. After 12 months the time-restricted group had lost 8.0 kg and the calorie-restriction group 6.3 kg; the 1.8 kg difference was not statistically significant, and body fat, blood pressure and metabolic markers followed the same pattern.
The consistent message is that fasting schedules perform broadly like other ways of eating less. For people who find a window easier than calorie counting, that is a practical advantage, not evidence of a separate metabolic effect.
The 8-hour window and the 91% headline
In March 2024 an abstract presented at an American Heart Association scientific meeting made headlines. Led by Victor Wenze Zhong of Shanghai Jiao Tong University School of Medicine, the analysis used US National Health and Nutrition Examination Survey data from 2003 to 2018 on about 20,000 adults, and reported that people whose food intake fell within an eight-hour window had a 91% higher risk of cardiovascular death than those eating across 12 to 16 hours.
The AHA's own release supplied the main caveats: it was a preliminary abstract, not peer reviewed at the time, and eating windows were estimated from two days of self-reported diet. Independent researchers widely criticised it for confounding, since people who eat in a narrow window may differ in health, work patterns or income, and observational data cannot show cause and effect. It does not prove that time-restricted eating harms the heart; it does show how thin long-term human data still are.
Who should not fast without medical supervision
Fasting is not appropriate for everyone, and several groups should not try it without a clinician's direct involvement:
- people who are pregnant or breastfeeding;
- children and teenagers;
- anyone with a current or past eating disorder;
- people with type 1 diabetes, or anyone taking insulin or sulfonylureas, because of the risk of low blood sugar;
- people on blood-pressure medication or other drugs whose dosing depends on food intake;
- people who are underweight, and frail older adults.
For healthy adults who try it, the cautious approach is to start with a 12- or 14-hour window, drink water while fasting, eat enough protein in the eating window to protect lean mass (a concern raised by TREAT), and never skip or retime prescribed medication. Dizziness, faintness or preoccupation with food are signals to stop. Anyone with a health condition or taking regular medication should consult a clinician before changing their eating pattern.
Billy Carson's reading
Billy Carson has not published a video devoted specifically to fasting, so this section describes how his broader biohacking approach frames it rather than a stated position on any schedule. In the site's guide to biohacking for beginners, a defined eating window is one of five foundational habits, alongside sleep, exercise, heat and cold exposure, and breath or meditation. Carson treats those habits as maintenance for the body as an instrument of awareness: the aim is energy and clarity for inner work, not a number on a wearable.
The same body-awareness thread runs through his coverage of grounding and earthing, which he presents as one part of a nervous-system reset alongside breathwork and meditation. Read in that light, a fasting window fits his framework as a practice of attention and self-discipline, closer to the contemplative fasts of the religious traditions than to a metabolic hack. That is a framing of his lifestyle approach, not a medical claim.
Watch, read and go deeper
For the lifestyle side of this subject, start with Awaken Zone on 4BK TV. On the 4biddenknowledge Podcast, Carson and Carolyn Zaermeyer discuss daily routines for vitality and ageing well in the episode on looking younger naturally. Readers interested in the body's daily clock, which governs both meal timing and the pineal gland's night-time melatonin release, can continue with the pineal gland explainer. Every evidence-focused health guide is collected in the biohacking topic hub.
Frequently asked questions
Does intermittent fasting cause autophagy?
Fasting is a strong trigger of autophagy in yeast and animal studies, where cutting off nutrients lowers mTOR activity. In humans, longer fasts plausibly raise it in some tissues, but there is no validated routine way to measure it, so the effect of any given schedule is not known.
How many hours of fasting does it take to start autophagy?
There is no established hour. Claims of 16, 18 or 24 hours are extrapolated from rodent and cell research, and autophagy runs continuously at a baseline level rather than starting from zero.
Is 16:8 or 5:2 better for weight loss?
Few trials compare them head to head, and both tend to perform about as well as conventional calorie reduction. The 12-month trial in the New England Journal of Medicine in 2022 found no significant difference between calorie restriction with or without an eight-hour window.
What did Yoshinori Ohsumi win the Nobel Prize for?
Ohsumi received the 2016 Nobel Prize in Physiology or Medicine for discovering the mechanisms of autophagy. Working in baker's yeast in the early 1990s, he identified the genes essential for the process. The prize recognised basic cell biology, not any fasting protocol.
Who should avoid intermittent fasting?
People who are pregnant or breastfeeding, children and teenagers, anyone with a history of eating disorders, people with type 1 diabetes or taking insulin, sulfonylureas or blood-pressure medication, and underweight or frail older adults should not fast without medical supervision. Consult a clinician first.
Sources and further reading
- Nobel Assembly at Karolinska Institutet, press release and scientific background for the 2016 Nobel Prize in Physiology or Medicine (Yoshinori Ohsumi).
- Mizushima N., Yamamoto A., Matsui M., Yoshimori T., Ohsumi Y., "In vivo analysis of autophagy in response to nutrient starvation using transgenic mice expressing a fluorescent autophagosome marker", Molecular Biology of the Cell, 2004.
- de Cabo R., Mattson M. P., "Effects of Intermittent Fasting on Health, Aging, and Disease", New England Journal of Medicine 381 (2019): 2541–2551.
- Lowe et al., "Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial", JAMA Internal Medicine 180 (2020).
- Liu et al., "Calorie Restriction with or without Time-Restricted Eating in Weight Loss", New England Journal of Medicine 386 (2022): 1495–1504.
- American Heart Association newsroom release on the Zhong et al. abstract, March 2024.
- Michael Mosley and Mimi Spencer, The Fast Diet (Short Books, 2013).
These statements have not been evaluated by the Food and Drug Administration. This content is for information only and is not medical advice.
