72-Hour Fast
Three-Day Water Fast · Deep Autophagy Tier
Evidence grade 2 of 5, Thin. Read how the Atlas grades evidence.
4 human trials, about 70 studies, ~1,200 participants across studies. The written rationale for this grade is still being prepared.
- FDA
- Medical supervision advised
- WADA
- Not prohibited
Reference only. Not medical advice.
What can go wrong
Multi-day water fasting carries real, documented risk: electrolyte disturbance, hypotension, fainting, arrhythmia, and refeeding syndrome when food is reintroduced after a prolonged fast, with potentially dangerous phosphate, potassium and magnesium shifts. A 2025 supervised study recorded serious events including hypokalemia and arrhythmias in some participants. It is especially hazardous for people with diabetes on glucose-lowering medication, cardiovascular or kidney disease, who are pregnant or breastfeeding, who are underweight, or who have a history of an eating disorder. This is general education, not medical advice and not a protocol. Anyone considering a multi-day fast should talk with a qualified clinician first, and supervision matters more at 72 hours than at shorter durations.
A three-day water-only fast, often promoted for an 'immune reset.' Human acute physiology is real: growth hormone rises several-fold (protein-sparing), insulin and IGF-1 fall, and ketones and autophagy signaling increase. The widely repeated regeneration-of-the-immune-system claim, however, traces to Cheng and Longo 2014, where hematopoietic-stem-cell renewal was shown in mice; the human portion was a small Phase I trial in cancer patients on chemotherapy that the authors described as preliminary. Most randomized human data in this space are small chemotherapy-tolerability pilots, not healthy-population immune trials. A 2025 supervised study even reported an acute inflammatory and platelet-activation signal during prolonged fasting. Multi-day fasting also carries real refeeding and electrolyte risk, so a clinician's involvement matters here more than at shorter durations.
At 72 hours, growth hormone surges 300%, autophagy peaks, and the immune system begins to reset.
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 said72 hours regenerates your immune system, in people
ActuallyThe stem-cell regeneration finding behind this claim was shown in mice (Cheng and Longo 2014). The human part of that paper was a small Phase I safety trial in cancer patients fasting with chemotherapy, which the authors called preliminary and said needed Phase II confirmation. In humans the immune-reset claim is preliminary, not established.
Often saidLonger is always better for autophagy
ActuallyAutophagy signaling rises with fasting duration, but human autophagy data are sparse and indirect, and risk rises in parallel. A 2025 supervised study even found an acute inflammatory and platelet-activation signal during prolonged fasting, so 'more is more' is not supported.
Often saidYou can break a three-day fast with a normal meal
ActuallyReintroducing food after a prolonged fast can trigger refeeding syndrome, with dangerous shifts in phosphate, potassium and magnesium. Clinical guidance treats the refeeding window as the higher-risk period and calls for medical monitoring in at-risk people.
Often saidThe growth-hormone surge is about 300 percent
ActuallyIn Hartman 1992 the 24-hour growth-hormone production rate rose roughly 5-fold (about 500 percent) during a two-day fast in healthy men, so the commonly quoted 300 percent figure understates the measured human response.
Who profits, and who pushes back
Refeeding-syndrome clinical literature
Rapid reintroduction of food after a prolonged fast can cause life-threatening electrolyte and fluid shifts, including hypophosphatemia; the refeeding window needs monitoring in at-risk people.
Dr. Peter Attia
He classifies fasting evidence as fuzzy, stopped multi-day fasting after losing roughly 20 pounds of lean mass, and now favors shorter 20-to-24-hour fasts.
Cheng and Longo 2014 authors
They described the human results as encouraging preliminary findings that would need to be expanded and confirmed in the ongoing Phase II randomized phase.
Mol Metab 2025 supervised fasting study
Prolonged fasting triggered an acute inflammatory response (CRP rose about 129 percent) and increased platelet-activation markers, with serious events in some participants.
Dr. Valter Longo
Sells this. Has a commercial interest in a product sold as this compound, as of July 2026.
Demonstrated stem-cell and immune effects of prolonged fasting. Co-founded L-Nutra/ProLon, a commercial interest in the prolonged-fasting space.
Dr. Pradip Jamnadas
Advocates supervised extended fasting for metabolic disease. Practicing cardiologist; clinical framing.
Extended-fasting community
Large self-reported base; strong norms around electrolytes and graduated refeeding.
Where the research comes from
| Place | Share of published research |
|---|---|
| USA | 44% |
| Other | 32% |
| Germany | 24% |
Research published from: USA, Germany, Global.
How we got here
Multi-day abstention from food has deep roots well outside modern wellness. It is recorded in religious practice across Islamic Ramadan, the Jewish Yom Kippur fast, Christian Lenten and monastic fasting, and Buddhist and Hindu disciplines where monastics and lay practitioners abstain for spiritual focus and self-discipline. Indigenous and pre-modern healing traditions also used periods of food withdrawal. Therapeutic fasting was formalized in the West in the 19th and early 20th centuries by figures such as Herbert Shelton in the United States. The modern 72-hour interest crystallized around two strands of laboratory work: human endocrinology documenting a dramatic fasting growth-hormone surge (Hartman 1992) and the Cheng and Longo 2014 finding that prolonged-fasting cycles reduced IGF-1 and PKA signaling to trigger hematopoietic-stem-cell renewal in mice, with only a preliminary human safety arm. Those results, amplified by university press releases, moved the three-day fast from fringe practice into longevity conversation, even as rigorous controlled human evidence stayed thin.
- pre-modernMulti-day religious fasting established across Islamic, Jewish, Christian, Buddhist and Hindu traditions
- early 1900sHerbert Shelton and contemporaries popularize structured therapeutic fasting in the United States
- 1965-1966Angus Barbieri completes a medically supervised 382-day fast at Maryfield Hospital, Dundee; later documented by Stewart and Fleming (Postgraduate Medical Journal, 1973)
- 1992Hartman and colleagues document a roughly 5-fold rise in growth-hormone production during a two-day fast in 9 normal men (J Clin Endocrinol Metab)
- 2014Cheng and Longo show prolonged-fasting cycles drive hematopoietic-stem-cell regeneration in mice, with a preliminary Phase I human chemotherapy arm (Cell Stem Cell)
- 2017Longo, founder of L-Nutra/ProLon, says he will stop taking consulting fees and donate his company shares to charity amid conflict-of-interest scrutiny
- 2025A supervised prolonged water-only fasting and refeeding study reports an acute inflammatory and platelet-activation signal (Molecular Metabolism)
At a glance
- Duration
- 72 hours
- HGH at 72h
- ~300-400% baseline
- Autophagy
- Very high
- Supervision
- Strongly advised
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about 72-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
Cheng CW, Longo VD, et al. Prolonged Fasting Reduces IGF-1/PKA to Promote Hematopoietic-Stem-Cell-Based Regeneration and Reverse Immunosuppression
Hartman ML, et al. Augmented growth hormone secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men (about 5-fold rise, n=9)
Prolonged fasting promotes systemic inflammation and platelet activation in humans: a medically supervised water-only fasting and refeeding study (CRP +129%, n=20, 2025)
de Groot S, et al. Short-term fasting and chemotherapy tolerance, randomized pilot (n=13)
Systematic review and trial record (Springer / clinical literature)
Bauersfeld SP, et al. Effects of short-term fasting on quality of life and chemotherapy tolerance, randomized cross-over pilot (n=34)
Stewart WK, Fleming LW. Features of a successful therapeutic fast of 382 days' duration (Angus Barbieri case)
Postgraduate Medical Journal, 1973 (Wikipedia summary with citation)
Refeeding Syndrome clinical reference (electrolyte and phosphate risk, NICE-based thresholds)
Human skeletal-muscle autophagy and mTOR signaling during 72-hour fasting (about 30% LC3B-II rise, n=8)
Peter Attia on fasting evidence as 'fuzzy' and lean-mass loss from prolonged fasting
Valter Longo conflict of interest: stopped consulting fees and donated L-Nutra/ProLon shares to charity (2017)
USC press release framing the 2014 fasting-immune-regeneration finding (context for the popular claim)