Fasting on a plant based diet: what the doctors say
Fasting on a plant based diet: what McDougall, Barnard, Goldhamer, Kahleova and other doctors say about water fasts and eating windows, with sources.
Updated . For education only, not medical advice.
Our view
The word fasting covers two quite different things among these doctors: long fasts on water alone, done in a clinic, and a daily stretch of hours without food12. McDougall drew the same line in 2023, keeping the therapeutic fasting of his friend Alan Goldhamer apart from intermittent fasting3.
On long water fasts the doctors who speak about them agree that they belong under medical care: Neal Barnard called them dangerous without supervision, and Goldhamer described exams, lab tests and checks twice a day at his TrueNorth Health Center45. That center is where Goldhamer offers these fasts, and the blood pressure series he cites comes from his own patients, described without a comparison group16.
We give most weight to the clinicians who judge fasting against the diet itself, and John McDougall is the clearest case: he does not fast, sent only patients who had failed his elimination diet to a water fast, and saw few of them gain anything extra from it78. On daily eating windows the evidence the doctors cite is thinner and mixed, from a trial of two meals a day in diabetes to Doug Lisle’s doubt that skipping dinner adds years, while the most striking fasting results, from Valter Longo’s lab, were in mice91011.
Where they agree
Long fasts on water alone are not a home project for any doctor here who discusses them: Barnard said they must be done at a place that knows how, and McDougall referred his patients to TrueNorth rather than advising them to fast alone48. Goldhamer explained that the monitoring is what tells his staff when to stop a fast before it causes trouble5.
Goldhamer and Barnard agree that the fast itself is only part of the process: Barnard said food has to return slowly and with care once it ends, and Goldhamer said that after a week of fasting people go back to healthy eating412. Goldhamer also pointed out that the commercial five day ProLon kit says little about what to eat on the other days of the month13.
They also accept that some form of fasting can help: Barnard called its benefits real and said most of us eat more than we need, and McDougall noted that fasting is part of nearly every religion143. Chef AJ, describing water fasting at TrueNorth, stressed how strict it is, with nothing but water while it lasts15.
Where they differ
The doctors differ on who gains from a long fast, which daily pattern is worth trying, how strong the evidence is, and how to read the results of centers and products tied to fasting8141013.
Who gains from a long water fast
Barnard said in 2021 that the results that impressed him most came from long, closely supervised water fasts, especially in autoimmune conditions16. At TrueNorth, Goldhamer said in 2025, these fasts run from 2 to 40 days, usually one to three weeks, and some patients need more than one1. McDougall was cooler in 2016: he called the water fast the ultimate elimination diet, yet in his experience few of the arthritis patients he sent gained an added benefit8.
Daily windows and meal timing
For healthy people Goldhamer described in 2025 a 12 hour fast each day, and in 2022 he suggested an eight hour eating window for anyone aiming to shed weight or hold a lower weight217. Hana Kahleova favored two meals, breakfast and lunch, five to six hours apart, while granting in 2017 that such long overnight fasts do not suit everyone every day18. Peter Rogers went the other way in 2021, eating only dinner on workdays, mainly to save time19. Chef AJ, by contrast, said in 2017 that she would not start someone with a lot of weight to lose on intermittent fasting, keeping it for people close to their goal20. Barnard doubted 5:2 plans, which in his view cut food sharply rather than stop it, and which many people cannot keep up14.
How strong the evidence is
Lisle was the sharpest skeptic: in 2023 he called the evidence that a longer daily fast or skipping dinner extends life sketchy, and pointed out that humans have mostly eaten their largest meal at the end of the day2110. Kahleova cited a 2017 analysis of about 50,000 Adventists in which those who gained least weight fasted longer overnight, and a trial in which two meals beat six of equal calories for weight and liver fat229. Goldhamer cited a study from his center, published in BMC Complementary and Alternative Medicine, in which each symptom of each patient was scored daily, and which found one serious event, a fall in blood sodium, and no deaths2324.
Centers and products
Two of the voices here are linked to fasting services or products: Goldhamer’s TrueNorth Health Center runs medically supervised water fasts, and according to Goldhamer in 2025, Longo’s group produces ProLon, a powder kit taken for five days instead of meals, priced at about 250 dollars113. Goldhamer added that the kit is pricey but at least keeps people off processed junk during those days13.
What each expert says
JMJohn McDougall
McDougall does not fast; he said in 2017 that he twice went three days without food as an experiment and found it too much pain7. In 2016 he described sending some arthritis patients whom his elimination diet had failed to help on to TrueNorth for a water fast, with little added benefit in his experience8. In 2023 he called his knowledge of intermittent fasting superficial and suggested that going without food might leave people more conscious of it3.
Barnard said in 2021 that fasting has real benefits but that 5:2 plans are a sharp cut in food rather than a true fast14. He was most impressed by long, supervised water fasts at places such as TrueNorth, and warned that they are dangerous done alone and must end with a gradual return to food164.
Goldhamer’s TrueNorth Health Center supervises water fasts of 2 to 40 days and, he said in 2025, has run about 25,000 of them over 40 years15. For healthy people he described a daily 12 hour fast, which he and many staff follow along with a week of fasting once a year212. He reported that all 174 consecutive high blood pressure patients in a series from his clinic lowered their pressure enough to stop medication6.
Rogers has eaten one meal a day on workdays, dinner only, which he described in 2021 as a way to save time19. In 2022 he said he had lost 34 pounds in about six months this way, and that he now has breakfast too on days with time to spare, to fuel his workouts2526.
Chef AJ said in 2017 that she came to intermittent fasting by accident, by eating within a narrow window27. She sees it as a tool for people near their goal weight, not a starting point, and described water fasting at TrueNorth as nothing but water, not even lemon2015.
In a 2017 PCRM release on about 50,000 Adventists, Kahleova reported that those who gained least weight ate a large breakfast, skipped snacks and had longer overnight fasts, up to 18 hours22. In 2025 she described her earlier trial in type 2 diabetes in which two meals a day led to more weight loss, less liver fat and better insulin sensitivity than six meals of the same calories9.
Lisle argued in 2023 that the hour of a meal matters little to nature, and that for most of history people had their biggest meal at night21. He saw no good evidence that skipping dinner or stretching the daily fast makes people live longer10.
In the documentary Science of Fasting, Longo described an experiment in which mice with cancer went without food for 48 hours ahead of chemotherapy at extreme doses and all survived, while the normally fed ones died; these were animal results, not human ones2811. He explained that fasting pushes normal cells into a protective standby mode that cancer cells fail to enter2930. His group also makes ProLon, a commercial five day fasting kit, according to Goldhamer13.
We offer no fasting plan of our own, and we do not suggest any fast, short or long, to readers of this page. The doctors here who describe long water fasts place them under daily medical monitoring at a clinic, never as something to try alone45. Before any fast, talk to your own doctor, above all if you take medication or live with a health condition. This page is education, not medical advice.
Each small number links to its reference below: the video at the exact second, or the page where it was said.