24-Hour Fast
Eat-Stop-Eat · One Full Day
Evidence grade 2 of 5, Thin. Read how the Atlas grades evidence.
2 human trials, about 40 studies, ~1,000 participants across studies. The written rationale for this grade is still being prepared.
- FDA
- Not a regulated intervention
- WADA
- Not prohibited
Reference only. Not medical advice.
What can go wrong
Common short-term effects on fast days are headache, fatigue, irritability, and hunger. People on glucose-lowering medication, especially insulin or sulfonylureas, face real hypoglycemia risk and must work with a clinician to adjust dose and timing before fasting; full-day fasting is not appropriate for type 1 diabetes. It is not advised in pregnancy or lactation, in children, or in frail older adults given lean-mass and safety-data concerns. Anyone with a current or past eating disorder, or at risk for disordered eating, should not adopt fasting and should be directed to a qualified healthcare provider or registered dietitian; clinicians are advised to screen for disordered eating before recommending it. This entry does not provide fasting schedules, calorie targets, or how-to instructions. If you are considering a full-day fast and have any medical condition, take medication, or have any history of disordered eating, talk to a licensed clinician first.
A full 24-hour fast, usually dinner-to-dinner once or twice weekly, popularized by Brad Pilon's 2007 Eat Stop Eat. A single day is long enough to substantially deplete liver glycogen and shift fuel use toward fat and ketones, and short enough to avoid the electrolyte and refeeding complexity of multi-day fasts, which makes it a common bridge from daily time-restricted eating into longer fasts. The honest caveat is that the dedicated controlled evidence for this exact protocol is thin: the one randomized trial built around it missed its primary cholesterol endpoint and showed only non-significant weight change, while improving insulin resistance. Most of the metabolic benefits attributed to '24-hour fasting' are extrapolated from 5:2 and alternate-day designs rather than measured on the single full-day fast itself.
Dinner to dinner. One or two full-day fasts a week, the entry point to extended fasting.
What people take it for
These are the reasons people give for taking it. They are not effects it has been shown to have.
What people get wrong
Often saidA weekly 24-hour fast resets your metabolism
ActuallyIt substantially depletes liver glycogen and shifts fuel toward fat and ketones, but resting metabolic rate is largely preserved at 24 hours, not reset. The effect is real but commonly overstated.
Often saidThe full-day fast has strong RCT backing
ActuallyIts dedicated controlled evidence is thin. The one randomized trial designed around it (WONDERFUL) missed its primary LDL-C endpoint and showed non-significant weight change; a 2025 review confirms no trial has tested the branded Eat Stop Eat protocol. Most cited benefits are extrapolated from 5:2 and alternate-day designs.
Often saidFasting itself, not the calorie cut, drives the benefits
ActuallyA randomized trial isolating fasting from energy restriction in lean adults found that alternate-day fasting was less effective at reducing fat than matched daily calorie restriction and added no fasting-specific metabolic benefit. The weight and metabolic gains are associated mostly with the energy deficit, not the fast per se. This is disputed for other outcomes.
Often saidA 24-hour fast floods you with growth hormone for everyone equally
ActuallyA crossover RCT did find growth hormone rose during a 24-hour fast, independent of weight loss, but the increase was far larger in people who started with low baseline growth hormone and modest in those who started high. It is a pilot-scale mechanism signal, not a demonstrated clinical outcome.
Who profits, and who pushes back
Hypoglycemia-risk clinicians
Full-day fasting can be reasonable for some healthy adults, but anyone on insulin or sulfonylureas needs medication adjustment and monitoring first, and it is not appropriate for type 1 diabetes.
Eating-disorder clinicians
Deliberate fasting can trigger or worsen disordered eating; providers should screen patients first, and people with any eating-disorder history should not adopt fasting regimens.
Evidence reviewers
Most direct evidence for whole-day fasting is low or very low quality, and the branded weekly full-day fast has never been tested in a dedicated trial, so claims should be calibrated accordingly.
Brad Pilon
Sells this. Has a commercial interest in a product sold as this compound, as of July 2026.
Popularized the weekly 24-hour fast; sells the book and related programs.
Dr. Jason Fung
Uses 24-hour and longer therapeutic fasts clinically for type 2 diabetes. Primary revenue clinical, not supplements.
Where the research comes from
| Place | Share of published research |
|---|---|
| USA | 46% |
| Other | 36% |
| Canada | 18% |
Research published from: USA, Canada, Global.
How we got here
Day-long fasting is ancient and predominantly non-Western in its deepest roots. Hindu observers have kept the twice-monthly Ekadashi fast for Vishnu for well over a millennium; Jewish tradition holds the roughly 25-hour Yom Kippur fast plus older customs of fasting on Mondays and Thursdays; Muslims fast dawn-to-sunset through Ramadan and on optional Shawwal and Monday/Thursday days; Eastern Orthodox Christians fast on Wednesdays and Fridays, a practice they inherited from Jewish custom. The modern secular version of the weekly full-day fast was crystallized by Brad Pilon, who studied applied human nutrition at the University of Guelph in Ontario and whose graduate work on the metabolic effects of short-term (12 to 72 hour) fasting in humans became the basis for his 2007 book Eat Stop Eat. Pilon, a former supplement-industry research-and-development manager, reframed the full-day fast as a simple weight tool rather than an ordeal. It remains the standard on-ramp from daily TRE into extended fasting, even though, as reviewers note, the branded protocol has never been tested head-to-head in a trial of its own.
- pre-1000 CETwice-monthly Ekadashi day-fasts established in Hindu practice; day-long communal fasts already old across many peoples
- antiquityJewish ~25-hour Yom Kippur fast and Monday/Thursday fast customs; later inherited Wednesday/Friday fasting in Eastern Orthodox Christianity
- 7th century CEIslamic dawn-to-sunset fasting (Ramadan, optional Shawwal and Monday/Thursday days) formalized
- 2007Brad Pilon publishes Eat Stop Eat, popularizing the once-or-twice-weekly 24-hour fast for a modern audience
- 2010sThe full-day fast is widely adopted within intermittent fasting as the bridge between daily TRE and multi-day fasts
- 2021WONDERFUL trial reports that low-frequency 24-hour water-only fasting did not lower LDL-C but did reduce HOMA-IR and metabolic-syndrome score
- 2024Umbrella reviews and a network meta-analysis map IF benefits but rate most direct full-day-fast evidence low or very low quality
- 2025Crossover RCT (published in Frontiers in Endocrinology) reports a 24-hour water-only fast raises growth hormone independent of weight loss
At a glance
- Duration
- 24 hours
- Frequency
- 1-2x per week typical
- Autophagy
- Moderate
- Glycogen
- Substantially depleted
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about 24-Hour Fast. A short entry is not a safe entry.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
Sources
Horne BD et al., WONDERFUL randomized controlled trial
Secondary analysis of WONDERFUL (RDW effect modifier)
Templeman I, Betts JA et al., RCT isolating fasting from energy restriction
24-hour water-only fasting crossover RCT, growth hormone secondary analysis
Network meta-analysis of intermittent fasting types vs continuous energy restriction
Intermittent fasting and health outcomes: umbrella review of meta-analyses of RCTs
Eat Stop Eat evidence-based review (Brad Pilon protocol)
Pilon background and short-term fasting research (University of Guelph)
NIDDK (NIH) clinician guidance on intermittent fasting and type 2 diabetes
Intermittent fasting: consider the risks of disordered eating for your patient
Religious and global fasting traditions (Ekadashi, Yom Kippur, Ramadan/Shawwal, Orthodox)
Religious fasting overview (Wikipedia, corroborated by Review of Religions)