Should you fast for longer than a day?

Eric Charles WestmanMD, MHS · internist and Associate Professor of Medicine, Duke University

No. Eat some protein every day. Prolonged fasting is not established and he will not endorse it until there are trials.

What it rests onHis stated reason is an absence of evidence rather than a mechanism: 'Prolonged fasting is not thought to be a generally healthy thing. There are some people who think it might be healthy and there are theoretical reasons, but until we get clinical trials and studies of a large number of people (which would be equivalent to the approval of a drug), I would still recommend having some protein every day.' Where someone does try it he wants it supervised and measured, specifically to catch muscle loss. He separately rejects the fasting-window framing itself in wes-eat-when-hungry: 'My teaching doesn't say, Power through 18 hours, if you're hungry, just don't eat, because you're intermittent fasting, no!' He is inside the low-carbohydrate camp and declines its most popular practice.

Jason FungMD · nephrologist, Scarborough Health Network, Toronto (CPSO #70179, independent practice since 29 May 2001)

Yes, on a schedule, with medical supervision and a stop condition. 24, 36 and 42-hour regimens are his published protocols.

What it rests onHe publishes named regimens: a 24-hour fast that still contains one meal a day, a 36-hour fast from dinner to breakfast two mornings later, and a 42-hour fast that also skips the next morning's meal. His argument is that as fasts lengthen the benefits accrue faster and the complication risk rises with them. Read with its conditionals, which are his own and are emphatic: 'I DO NOT RECOMMEND ANYBODY WHO IS TAKING ANY MEDICATION TO TRY LONGER FASTS WITHOUT CLEARING IT WITH THEIR DOCTOR' (fun-medsupervise), glucose-lowering drugs must be reduced on fasting days and only a prescriber can say by how much (fun-reducemeds), stop the moment you feel unwell (fun-stopifunwell), and on eating days eat to satiation rather than also cutting calories (fun-eattosatiation). He is not telling people to push through; he is telling supervised people to fast on a schedule. His evidence includes a published case series at the 42-hour length; he sells fasting programs.