The Obesity Code

The Obesity Code argues that obesity is a hormonal disorder rather than a failure of arithmetic or willpower, and that the standard advice to eat less and move more fails so consistently because it misidentifies the problem. Jason Fung's central claim is that insulin, not calorie surplus alone, is the principal regulator of body fat.

The model runs as follows. Chronically elevated insulin, driven by refined carbohydrates and frequent eating, promotes fat storage and blocks its release. Over time insulin resistance develops, insulin levels climb further, and the body defends a higher weight set point. Conventional calorie restriction lowers metabolic rate without lowering insulin, which is why weight returns. The treatment that follows is to reduce refined carbohydrates and sugar, reduce eating frequency, and use intermittent fasting to lower insulin directly.

It is for readers who have lost and regained weight repeatedly and want a physiological explanation, and for clinicians interested in a hormonally framed account of weight regulation.

On the evidence, the book is right about something important and overconfident about the mechanism. Body weight is hormonally regulated and defended, metabolic adaptation after weight loss is real and well documented, and telling patients to try harder is poor medicine. But the carbohydrate-insulin model as Fung presents it has been tested directly in controlled feeding studies, and the results have largely not supported it. Isocaloric diets differing in carbohydrate content have produced broadly similar fat loss. Obesity researchers remain divided, with most of the field treating insulin as one factor among several rather than the master switch. The dietary pattern the book recommends is reasonable for many people. Fasting protocols carry real contraindications, including for anyone on insulin or a sulfonylurea, anyone pregnant, and anyone with a history of disordered eating, and should be discussed with a physician first.