Fasting protocols

Sixteen hours fasting, an eight-hour eating window, every day. The most common entry point and the one most people mean by "intermittent fasting". The largest randomised trial of it found no weight advantage over three ordinary meals a day.

A single meal in roughly a one-hour window, fasting the other 23. Simple to describe and hard to meet nutritional needs on. Unusually for a protocol this widely searched, it is supported by almost no protocol-specific trial evidence.

Two days a week at roughly 500 to 600 calories, five days of ordinary eating. The two restricted days are not true fasts. No trial that compared it directly against eating a little less every day found a significant weight difference either way, and the one that asked about hunger found more of it on this schedule.

A fast day, then an ordinary day, repeating. Most trials allow about a quarter of normal intake on the fast day rather than nothing. It is the only fasting schedule a 2025 network meta-analysis found beat steady calorie restriction, and it is also the schedule people quit most often in the longest trial run on it.

Dinner to dinner, once or twice a week. Long enough to draw liver glycogen down substantially and shift toward fat oxidation. Over a year, the closest well-studied relative of this schedule performed no better than simply eating less every day.

Two full days without food. Electrolytes stop being optional here, and the risk that matters is not hunger. It is drinking large volumes of plain water while the body is actively shedding sodium.

Three days, usually undertaken for autophagy rather than weight loss. The autophagy case is weaker than almost every page promoting this protocol implies, and refeeding becomes a genuine clinical consideration rather than a formality.

Water only, no calories from any source, for an extended period. The strictest common protocol, the most searched, and the one where the gap between what is claimed and what has been measured is widest. Extended water-only fasting is studied under medical supervision, and the published cardiometabolic improvements largely regress after refeeding.