The Obesity Code summary

by Jason Fung · 2016 · 7 key ideas · 18-minute read

A kidney specialist argues that obesity is driven mainly by the hormone insulin rather than by calories alone, and that changing what and when we eat, including fasting, is the way out.

Why read it

For anyone frustrated by failed calorie-counting diets who wants a different way to think about weight. It popularised intermittent fasting, but its central theory is contested by many obesity researchers, so read it alongside other views.

The Obesity Code in brief

Fung begins by arguing that the standard advice, eat less and move more, has failed. Most people who lose weight by cutting calories regain it. He concludes that the problem isn't willpower but a flawed theory of what causes obesity.

His alternative is a hormonal theory. The body, he argues, regulates its weight around a set point, much like a thermostat, and the hormone insulin plays the central role in setting it. When insulin is persistently high, the body stores fat and defends a higher weight, adjusting hunger and metabolism to resist change.

He then explains what raises insulin: refined carbohydrates and especially sugar, frequent eating and snacking, and factors such as stress and poor sleep. Over time, persistently high insulin leads to insulin resistance, which in turn drives insulin even higher in a vicious cycle.

The solution addresses both what and when we eat. Fung recommends cutting added sugars and refined grains, moderating protein, eating more natural fats and fibre, and above all, giving the body regular periods without food through intermittent fasting so that insulin levels can fall.

The book is persuasive and practical, and it helped make intermittent fasting mainstream. But its insulin-centred explanation of obesity is disputed, and controlled studies have not shown that the insulin theory explains weight gain better than overall energy intake.

The 7 key ideas

1. Why calorie counting fails

The body fights back against hunger.

2. The body's thermostat

Change the thermostat, not just the window.

3. Insulin drives fat storage

High insulin, storage mode; low insulin, burning mode.

4. The insulin resistance trap

Constant exposure breeds resistance.

5. When you eat matters

Not just what you eat, but how often.

6. What to eat: five steps

Less sugar, less white flour, more real food.

7. Fasting as a tool

Give insulin a break.

Key takeaways

• Cut added sugars and refined grains first.
• Reduce constant snacking and allow longer gaps between meals.
• Eat whole foods with plenty of fibre.
• If you try fasting, start gently and talk to a doctor if you take medication.
• Don't blame willpower alone for failed diets; hunger and metabolism push back.
• Treat the insulin theory as one view in an ongoing debate, not settled science.

Our take

The Obesity Code offers a sympathetic message, that failed diets aren't simply a matter of willpower, and practical advice that overlaps with mainstream guidance: cut sugar and refined carbohydrates, stop constant snacking, eat whole foods. It also helped popularise intermittent fasting, which suits many people. But its central claim, that insulin rather than energy balance is the main cause of obesity, is disputed by many obesity researchers, and controlled studies have not confirmed it. Fung tends to present this contested theory as established and to downplay calories. Use the practical tips, and check fasting plans with a doctor, especially if you take medication.

About Jason Fung

Jason Fung is a Canadian physician who specialises in kidney disease (nephrology) and has worked in Toronto. He has become a well-known advocate of intermittent fasting for weight loss and type 2 diabetes.

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