Jason Fung is a kidney specialist who noticed something that most obesity researchers seemed to be ignoring: the patients he treated for kidney disease who also had type 2 diabetes were almost uniformly told to eat less and exercise more, and almost uniformly failing to maintain any improvement. He began researching insulin's role in fat storage and ended up writing a book that challenged the dominant paradigm in obesity medicine — and launched the intermittent fasting movement in the process.
The Insulin Hypothesis
The “calories in, calories out” model of obesity holds that body weight is determined by the balance between energy consumed and energy expended. It is compelling in its simplicity and deeply flawed in its practical consequences: it implies that obesity is a failure of willpower, and it predicts that caloric restriction should produce sustained weight loss — which it does not for most people.
Fung’s alternative is the insulin hypothesis: body fat is regulated primarily by insulin, not by caloric balance. When insulin is chronically elevated — as it is in diets high in refined carbohydrates and frequently eaten foods — the body is in a fat-storing mode regardless of caloric intake. Reducing insulin through dietary composition (low refined carbohydrates) and meal timing (intermittent fasting, longer overnight fasts) allows the body to access fat stores and reach a lower set point.
Intermittent Fasting
Fung’s treatment of intermittent fasting — which he presents not as a diet but as a pattern of eating — is the book’s most widely cited contribution. The evidence he cites for fasting’s effects on insulin sensitivity, autophagy, and metabolic health is substantial, and his clinical experience with fasting as an intervention for type 2 diabetes reversal is genuinely impressive.
The practical protocols he describes — daily fasting windows, 24-hour fasts, longer therapeutic fasts for diabetes reversal — are presented with enough clinical grounding to be taken seriously while being accessible to readers without medical backgrounds.
Obesity is not a caloric problem. It is a hormonal problem. Treating it with caloric restriction is like treating heart failure by telling people to try harder.
— Jason Fung, The Obesity Code
6 Key Ideas From This Book
Insulin signals cells to absorb glucose and signals fat cells to store energy rather than release it. When insulin is chronically elevated — as it is after frequent meals of refined carbohydrates — the body cannot access its own fat stores effectively, regardless of how little food is consumed.
Chronic high insulin produces insulin resistance, which requires even higher insulin to achieve the same effect, which produces more insulin resistance — a vicious cycle. This insulin resistance is the metabolic background against which type 2 diabetes, cardiovascular disease, and many cancers develop.
A period of not eating — even a twelve-hour overnight fast — allows insulin to fall to its baseline level, enabling the body to access fat stores for energy. Caloric restriction without fasting keeps insulin elevated and fat stores locked, which is why caloric restriction typically fails to produce sustained fat loss.
Fung's clinical work with therapeutic fasting has documented reversal of type 2 diabetes — not management, but actual reversal to non-diabetic glucose regulation — in patients who were told the condition was irreversible. This challenges the standard of care and has subsequently been validated by several clinical trials.
Chronic stress elevates cortisol, which elevates insulin and promotes fat storage — particularly visceral fat. Sleep deprivation has the same effect. Fung's treatment of the stress-insulin-obesity connection is one of the more nuanced aspects of the book, extending the insulin hypothesis beyond diet to lifestyle.
The advice to eat "small, frequent meals throughout the day" to "boost metabolism" is, Fung argues, insulin physiology in reverse. Every meal spikes insulin; frequent meals keep insulin chronically elevated; chronic insulin elevation prevents fat mobilisation. The metabolic advantages of eating less frequently are more robust than the advantages of caloric restriction.
Any Weaknesses?
Fung’s insulin hypothesis is well-supported but not the complete story — other factors including gut microbiome, inflammation, and genetic variation contribute to obesity in ways the book doesn’t address. The book presents an important and underweighted perspective, not the final word.
His critique of the caloric model sometimes overstates its case — calories do matter; the issue is that they don’t fully explain obesity. Some readers may come away thinking calories are irrelevant, which goes beyond what the evidence supports.
Anyone who has tried caloric restriction and failed, and wants to understand the metabolic mechanisms that explain why — and what alternative approach might actually work for their physiology.
Outlive by Peter Attia for a broader metabolic health framework, and The Complete Guide to Fasting also by Fung for the practical protocols his theory implies.
Endocrinologists and diabetologists who want a clinician's perspective on the insulin-obesity connection — Fung's clinical experience with fasting for diabetes reversal is a significant contribution regardless of one's view of the broader obesity theory.
The dietary implications of this book conflict with standard nutritional advice (eat less, move more, choose low-fat foods). If you have chronic health conditions, discuss any dietary changes with your healthcare provider before implementing Fung's protocols.
Frequently Asked Questions
Is The Obesity Code worth reading?
A genuinely important book that corrects a fundamental error in how obesity is understood and treated. Fung's insulin hypothesis is not the complete story, but it is a crucial chapter that mainstream medicine has underweighted — with serious consequences for how patients are treated. Read it and take it seriously.
Who should read The Obesity Code?
Anyone who has tried caloric restriction and failed, and wants to understand the metabolic mechanisms that explain why — and what alternative approach might actually work for their physiology.
What is The Obesity Code about in one sentence?
Jason Fung is a kidney specialist who noticed something that most obesity researchers seemed to be ignoring: the patients he treated for kidney disease who also had type 2 diabetes were almost uniformly told to eat less and exercise more, and almost uniformly failing to maintain any improvement.
The Verdict
A genuinely important book that corrects a fundamental error in how obesity is understood and treated. Fung's insulin hypothesis is not the complete story, but it is a crucial chapter that mainstream medicine has underweighted — with serious consequences for how patients are treated. Read it and take it seriously.
→ Find on Amazon