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What it argues
The Obesity Code is Jason Fung's argument that obesity is a hormonal disease, not a behavioral one, and that the treatment follows from that understanding. Fung is a Canadian nephrologist who treats kidney disease — often downstream of obesity and diabetes — and he came to dietary intervention through watching his patients' conditions worsen despite conventional medical advice. His central claim is that insulin is the master regulator of fat storage, and that chronically elevated insulin is the root cause of weight gain that resists conventional calorie restriction.
The book begins by dismantling the calorie balance model. Fung argues that treating obesity as simple energy imbalance — eat less, exercise more — misunderstands how the body regulates weight. When calories are restricted, the body reduces its metabolic rate to compensate; when more energy is burned, hunger increases to compensate. The result is that calorie restriction produces temporary weight loss followed by rebound, a pattern well-documented in the research literature and in the lived experience of most people who have dieted.
What it gets right
- 1.
Obesity is a hormonal disease, not a caloric disease: chronically elevated insulin drives fat storage regardless of how many calories are consumed.
- 2.
Calorie restriction fails long-term because the body compensates by reducing metabolic rate and increasing hunger — the set-point defended by hormonal regulation cannot be overcome by willpower alone.
- 3.
Insulin is elevated by refined carbohydrates, sugar, frequent eating, and processed foods; it is reduced by low-carbohydrate eating, whole foods, and fasting.
What it covers
Who wrote it
Jason Fung is a Canadian nephrologist and the co-founder of the Intensive Dietary Management Program in Toronto, where he treats obesity and type 2 diabetes primarily through dietary intervention. He received his medical degree from the University of Toronto and completed his fellowship at UCLA. His other books include The Complete Guide to Fasting, The Diabetes Code, The Cancer Code, and Life in the Fasting Lane. Fung is widely credited with popularizing the intermittent fasting movement in clinical and popular contexts, though his methods remain outside mainstream medical guidelines.
