The Obesity Code

The Hormonal Theory of Obesity

Jason Fung is a Canadian nephrologist who saw diabetes and obesity in his clinic daily and became convinced that the dominant medical model — calories in, calories out — was not only insufficient but actively misleading. The Obesity Code is his comprehensive argument for a hormonal model of obesity that places insulin at the centre.

The book’s central proposition: obesity is not caused by eating too many calories; it is caused by chronic elevation of insulin, which instructs fat cells to store rather than release energy. The implication is that reducing calories while maintaining high insulin levels (as in low-fat, high-carbohydrate diets) will not produce sustainable weight loss — the body will simply reduce metabolic rate to compensate.

The Insulin Model

Fung’s argument rests on several pillars. First, insulin directly controls fat storage: elevated insulin levels prevent fat cells from releasing fatty acids for energy. Second, different foods produce dramatically different insulin responses even at equivalent calorie counts — refined carbohydrates and sugar produce far higher insulin spikes than protein or fat. Third, frequent eating maintains chronically elevated insulin levels; fasting reduces them.

This explains several observations that the caloric model cannot: why two people eating identical calories lose different amounts of weight, why low-fat diets consistently underperform relative to expectations, and why the same people who lose weight through caloric restriction typically regain it within three to five years.

The Role of Insulin Resistance

Fung extends the model to insulin resistance: the condition in which cells require increasingly high insulin levels to accomplish the same glucose uptake. This is the foundational condition behind type 2 diabetes, and Fung argues it is also behind the most intractable forms of obesity. The development of insulin resistance is a gradual process driven by chronic high-insulin exposure — typically from a diet high in refined carbohydrates, sugar, and frequent eating.

Intermittent Fasting as the Solution

The book’s prescriptions centre on intermittent fasting: periods of complete food abstinence that allow insulin levels to drop low enough for fat burning to occur. Fung provides clinical evidence from his own practice — including reversal of type 2 diabetes — for the effectiveness of extended fasting protocols.

Jason Fung and the Fasting Movement

Jason Fung’s authority on this subject is grounded in his day job. As a nephrologist — a kidney specialist — in Toronto, he spent years treating patients whose kidney failure was a downstream consequence of type 2 diabetes, and he became frustrated that conventional treatment managed blood sugar with ever-larger doses of insulin while doing nothing to address the underlying disease. That clinical frustration is the origin of his entire body of work. The Obesity Code, published in 2016, was his first book and his breakthrough, and he followed it with The Diabetes Code, The Complete Guide to Fasting (co-written with the fasting advocate Jimmy Moore), and The Cancer Code, each extending the same metabolic framework into a new domain. Fung also founded the Intensive Dietary Management program and built a large online following, becoming one of the most visible figures in the popular revival of therapeutic fasting. His success is part of a broader cultural shift, alongside writers like Gary Taubes, away from the low-fat orthodoxy that dominated nutritional advice for decades and toward a focus on carbohydrate quality and meal timing.

How to Read the Argument Responsibly

The fairest way to approach The Obesity Code is to separate its strong claims from its overreaches. The book is at its most persuasive when it documents the limits of the simple calorie model: the well-known failure of long-term caloric restriction, the metabolic adaptation that slows the body’s energy expenditure during dieting, and the different hormonal responses that different foods provoke. These are real phenomena supported by research. Where readers should remain cautious is Fung’s tendency to treat insulin as very nearly the sole cause of obesity, when the science increasingly points to a multifactorial picture involving genetics, the gut microbiome, sleep, stress, the brain’s appetite regulation, and the food environment. His polemical tone toward mainstream medicine, while rhetorically effective, can overstate the certainty of his conclusions. Anyone considering extended fasting — particularly people with diabetes, eating-disorder history, or who are pregnant — should treat the book as a starting point for a conversation with a physician rather than a self-administered prescription.

Who Should Read It

The Obesity Code is most valuable for readers who have struggled with weight or metabolic syndrome and found the standard “eat less, move more” advice both confusing and ineffective. It reframes the problem in a way that many find clarifying and even liberating, and the practical chapters on what to eat and when are accessible to a general reader without scientific training. It pairs well with Peter Attia’s Outlive and Michael Greger’s How Not to Die as part of a broader self-education in metabolic health, with the understanding that those books sometimes reach different conclusions. Read critically and discussed with a doctor, it is a genuinely useful corrective to decades of oversimplified dietary advice.

One reason the book resonated so widely is that it offers an explanation that fits many readers’ lived experience. People who have dieted repeatedly, lost weight, and regained it tend to feel that the calorie model blames them for a failure of willpower; Fung’s hormonal account reframes that same history as a predictable physiological response rather than a personal failing. Whether or not insulin deserves quite as much of the credit as he assigns it, that shift in framing has genuine psychological value, and it is part of why The Obesity Code became a word-of-mouth success rather than just another diet book.

Final Verdict

The Obesity Code presents a compelling hormonal model of obesity that explains failures of the conventional caloric approach. The model may be somewhat oversimplified, but the clinical implications — particularly the case for low-carbohydrate eating and intermittent fasting — are well-supported.

Our rating: 4.6/5 — An important challenge to conventional dietary wisdom. The insulin framework is clinically valuable even if not the complete answer.