The Complete Guide to Fasting

The Complete Guide to Fasting is the practical companion to Jason Fung's argument that deliberate periods without food are a legitimate therapeutic tool rather than a hardship to be endured. Written with Jimmy Moore, it sets out to answer the operational questions: how long, how often, what is permitted, what to expect, and how to stop.

The rationale offered is hormonal. Fung argues that fasting lowers circulating insulin, which in turn permits access to stored fat, and that intermittent fasting differs from continuous calorie restriction in preserving metabolic rate and lean mass. The book then walks through protocols of increasing length, from 16-hour and 20-hour daily windows to 24-hour, 36-hour and extended fasts, with guidance on fluids, electrolytes, coffee and tea, breaking a fast, and managing hunger, which he describes as cyclical rather than steadily increasing.

Fasting is not appropriate for everyone, and this is where the book requires supplementation from a clinician. Anyone taking insulin or a sulfonylurea faces a real risk of hypoglycaemia and needs medication adjusted under supervision. Fasting is not appropriate during pregnancy or breastfeeding, for children and adolescents, for people who are underweight, or for anyone with a history of disordered eating, where restriction can reactivate the pattern. Several other medications and conditions also warrant review. The book acknowledges some of this, but its enthusiasm can carry a reader past it. Discuss any fasting protocol with your own physician before beginning.

On the evidence, intermittent fasting produces weight loss in trials, and it works. Whether it outperforms equivalent continuous calorie restriction is less clear, and head-to-head comparisons have generally found similar results. The insulin-centred explanation Fung offers is contested. The practical value of the book lies in its execution guidance rather than its theory.