Jason FungMD · nephrologist, Scarborough Health Network, Toronto (CPSO #70179, independent practice since 29 May 2001)
Yes, outside pregnancy, childhood and being underweight
What it rests onGilliland's 1950s and 1960s case series of 46 patients, 14 male and 32 female, on a 14-day water, tea and coffee fast in a metabolic ward, with 44 of 46 completing. Historical and religious fasting practice. His own uncontrolled clinical experience.
Source: thefastingmethod.com ↗
Stacy T. SimsPhD · exercise physiologist and nutrition scientist
No, not if you train
What it rests onSex differences in the hypothalamic response to low energy availability and kisspeptin signalling; impaired glucose tolerance reported in women but not men under fasting protocols; the interaction of exercise stress with restricted intake on cortisol and abdominal fat.
Satchidananda PandaPhD · circadian biologist, Salk Institute (affiliation as of 27 Jul 2026; resignation effective 13 Aug 2026)
Take the longer window if you train
What it rests onHis own trials and the circadian mechanism, plus explicit concern about energy availability in active people.
Dr. Daniel PompaD.C. (Life University, 1995) · not a physician, no active licence · health educator and programme founder
No, but they should still fast
What it rests onClinical observation across his coaching practice, plus his wife's three-month adaptation as the worked example. He offers no trial data and says so
Source: drpompa.com ↗
Sims and Panda are talking about women who train hard, where the risk is energy availability. Pompa is talking about symptomatic women with hormonal problems who are looking for relief, and his worked example is his wife's three-month adaptation. Fung is talking about people with obesity and type 2 diabetes, where his stated starting point is someone who has not missed a meal in twenty-five years. Those are close to opposite populations, and the same protocol carries very different risk in each.
What makes this a real disagreement rather than a scoping artefact is that Fung does not accept the carve-out in principle: he calls the concern a myth designed to discourage fasting, rather than a boundary condition on his own advice.
All four exclude pregnancy, and none of them recommends fasting for children. Fung, Sims and Panda would all stop the intervention if menstrual cycles were disrupted, though only Fung states it as an explicit rule and only Sims treats it as predictable rather than exceptional, and all three treat being underweight as a contraindication.
Sims, Panda and Pompa hold that the standard male protocol is the wrong default for women, and Pompa and Sims both put sleep and recovery ahead of window length. What is left in dispute is whether the fix is modification or abstention, and whether there is anything to fix at all.
A sex-stratified randomised trial of a multi-day-per-week fasting protocol with menstrual, bone and performance endpoints, in trained and untrained women separately, and powered for sex differences rather than adjusted for them. Nothing of that shape exists.
Fung's evidence is clinical impression and historical practice, Pompa's is his own coaching practice and he says so, and Sims's is mechanistic plus observational. Fung's own stopping rule is the falsifiable part: if amenorrhoea appeared at a materially higher rate on his protocol than on matched caloric restriction, his position would have to move.
You do, and your call is dated, kept, and yours to change. Follow the people you already trust and what they publish turns into a short daily practice.
Free to sign up and use. We do not ask for a card.