The Obesity Code


Unlocking the Secrets of Weight Loss

Why you’ve never been able to lose weight, and how that can change now.

Everything you believe about how to lose weight is wrong. Weight gain and obesity are driven by hormones—in everyone—and only by understanding the effects of insulin and insulin resistance can we achieve lasting weight loss.

In this highly readable and provocative book, Dr. Jason Fung sets out an original, robust theory of obesity that provides startling insights into proper nutrition. In addition to his five basic steps, a set of lifelong habits that will improve your health and control your insulin levels,

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Why you’ve never been able to lose weight, and how that can change now.

Everything you believe about how to lose weight is wrong. Weight gain and obesity are driven by hormones—in everyone—and only by understanding the effects of insulin and insulin resistance can we achieve lasting weight loss.

In this highly readable and provocative book, Dr. Jason Fung sets out an original, robust theory of obesity that provides startling insights into proper nutrition. In addition to his five basic steps, a set of lifelong habits that will improve your health and control your insulin levels, Dr. Fung explains how to use intermittent fasting to break the cycle of insulin resistance and reach a healthy weight—for good.

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  • Greystone Books
  • Paperback
  • March 2016
  • 328 Pages
  • 9781771641258

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About Jason Fung

Jason FungDr. Jason Fung is recognized as one of the world’s leading experts on fasting for weight loss and diabetes reversal. His work has been featured in outlets such as the Atlantic, the New York Post, Forbes, the Daily Mail, and Fox News. He is the author of several books, including The Complete Guide to Fasting (co-authored with Jimmy Moore) and the bestselling The Obesity Code and The Diabetes Code.

Dr. Fung lives in Toronto, ON, where he also founded the Intensive Dietary Management Clinic that provides a unique treatment focus for obesity and Type 2 diabetes.

Author Website


“Fung zeroes in on why insulin resistance has become so prevalent and offers specific outside-the-box solutions that have emerged as the key to maximizing health.”Jimmy Moore, author, Keto Clarity and Cholesterol Clarity

“Not only full of insights but also surprisingly funny. Read it to understand why the world became fat, how to reverse the epidemic—and how to stay thin yourself.”Andreas Eenfeldt, MD, Founder of 

“A fantastic book that exposes some of the world’s most pervasive myths about obesity and weight management. A must read for anyone interested in the science of diet.”Kris Gunnars, nutrition researcher 

Discussion Questions

1. Have you or someone you know experienced chronic obesity, Type 2 diabetes, high cholesterol, or any other dietary diseases? If so, what were these individual challenges and their impacts on daily life?

2. Did you or they ever try any prescribed treatments (not including Dr. Fung’s central advice) for these dietary diseases—whether medically prescribed insulin, other medications, surgeries, or other dieting approaches? If so, what were the results? And were they temporary or lasting?

3. Why does Dr. Fung call the low-fat, calorie reduction model of weight loss a “cruel hoax”—and have you or friend ever experienced the vicious cycle of weight loss and gain he describes from following it?

4. According to Dr. Fung, regulating the body’s insulin by controlling primarily “when” you eat is the best way to combat obesity. Explain how he says this works.

5. According to Dr. Fung, the “stress hormone” cortisol also plays a part in obesity. Given the stress connection to obesity, what should a good weight-loss plan include or not include?

6. Dr. Fung offers many vivid explanations and analogies to clarify complex biological processes. Do you find these generally easy to understand? Which are your favorite or resonated with you the most?

7. At times, Dr. Fung is critical of the medical establishment, government health agencies, and the “weight loss industry.” Mostly, he feels they are too collectively focused on treating the symptoms, rather than the causes, of obesity or on selling expensive products to combat it. What do you think of his criticisms?

8. Have you or someone you know ever attempted a diet plan that included intermittent fasting? If so, what was your or their basic approach? What was the result? And was it temporary or lasting?

9. In the book, Dr. Fung addresses many of the most persistent myths about fasting. What are some of the main myths, and how does Dr. Fung respond? Do you find his arguments convincing?

10. The final chapter begins with a quote by Marie Antoinette: “There is nothing new, except what has been forgotten.” How does this relate to intermittent fasting? Are there any other ways of thinking about fasting that help make it seem less “radical” as a nutritional approach?


From the Introduction

The art of medicine is quite peculiar. Once in a while, medical treatments become established that don’t really work. Through sheer inertia, these treatments get handed down from one generation of doctors to the next and survive for a surprisingly long time, despite their lack of effectiveness. Consider the medicinal use of leeches (bleeding) or, say, routine tonsillectomy.

Unfortunately, the treatment of obesity is also one such example. Obesity is defined in terms of a person’s body mass index, calculated as a person’s weight in kilograms divided by the square of their height in meters. A body mass index greater than 30 is defined as obese. For more than thirty years, doctors have recommended a low-fat, caloriereduced diet as the treatment of choice for obesity. Yet the obesity epidemic accelerates. From 1985 to 2011, the prevalence of obesity in Canada tripled, from 6 percent to 18 percent.1 This phenomenon is not unique to North America, but involves most of the nations of the world.

Virtually every person who has used caloric reduction for weight loss has failed. And, really, who hasn’t tried it? By every objective measure, this treatment is completely and utterly ineffective. Yet it remains the treatment of choice, defended vigorously by nutritional authorities. As a nephrologist, I specialize in kidney disease, the most common cause of which is type 2 diabetes with its associated obesity. I’ve often watched patients start insulin treatment for their diabetes, knowing that most will gain weight. Patients are rightly concerned. “Doctor,” they say, “you’ve always told me to lose weight. But the insulin you gave me makes me gain so much weight. How is this helpful?” For a long time, I didn’t have a good answer for them.

That nagging unease grew. Like many doctors, I believed that weight gain was a caloric imbalance—eating too much and moving too little. But if that were so, why did the medication I prescribed—insulin—cause such relentless weight gain?

Everybody, health professionals and patients alike, understood that the root cause of type 2 diabetes lay in weight gain. There were rare cases of highly motivated patients who had lost significant amounts of weight. Their type 2 diabetes would also reverse course. Logically, since weight was the underlying problem, it deserved significant attention. Still, it seemed that the health profession was not even the least bit interested in treating it. I was guilty as charged. Despite having worked for more than twenty years in medicine, I found that my own nutritional knowledge was rudimentary, at best.

Treatment of this terrible disease—obesity—was left to large corporations like Weight Watchers, as well as various hucksters and charlatans mostly interested in peddling the latest weight-loss “miracle.” Doctors were not even remotely interested in nutrition. Instead, the medical profession seemed obsessed with finding and prescribing the next new drug:

•You have type 2 diabetes? Here, let me give you a pill.
•You have high blood pressure? Here, let me give you a pill.
•You have high cholesterol? Here, let me give you a pill.
•You have kidney disease? Here, let me give you a pill.

But all along, we needed to treat obesity. We were trying to treat the problems caused by obesity rather than obesity itself. In trying to understand the underlying cause of obesity, I eventually established the Intensive Dietary Management Clinic in Toronto, Canada. The conventional view of obesity as a caloric imbalance did not make sense. Caloric reduction had been prescribed for the last fifty years with startling ineffectiveness.


Excerpted from an author Q&A on by Dr. Frank Lipman. Read the full interview.

Dr. Lipman: If it is not calories, then what is the ultimate cause of obesity?

Dr. Fung: It turns out that the problem with obesity is a poor distribution of energy, not the amount. Food energy is being diverted into fat storage instead of being used up. Insulin is the major hormonal regulator of this process.

Excessive insulin causes obesity. We can easily see this when patients are prescribed insulin for various medical reasons. Weight gain is an inevitable side effect. So obesity is a hormonal, rather than a caloric disorder. Once we understand that insulin is too high, then we can understand that weight loss depends upon reducing the high insulin levels.

Reducing refined carbohydrates is a well-known and successful strategy for reducing insulin. This is the basis of such diets as the Atkins. Most of us know that reducing sugar and white flour and starchy carbohydrates is a great way to lose weight. It is often remarkable that physicians who work with thousands of obese patients almost universally use diets low in refined carbohydrates. By contrast, academic physicians who do mostly research and do not work with patients instead counsel calorie counting. Those physicians who work with many patients understand the futility of calorie counting and have seen the benefits of reducing refined carbohydrates.

But this is not the entire picture. Insulin is the major driver of weight gain, but there are factors other than carbohydrates that increase insulin. The major player here is insulin resistance.

Dr. Lipman: Can you explain insulin resistance and what causes it?

Dr. Fung: The major job of insulin is to push glucose into cells. When glucose stays outside of the cell, it is said to be insulin resistant. To overcome this resistance, the body increases insulin levels, which, of course, may lead to obesity. So insulin resistance is a major cause of increased insulin levels, but what causes this resistance in the first place?

Insulin itself causes insulin resistance. If insulin levels are too high for too long, the body develops resistance as a protective mechanism. As an analogy, consider what happens when you listen to music that is much too loud. You start to lose hearing, as your body develops resistance to this loud noise by tuning it out. In the same way, your body protects itself from too much insulin for too long by developing insulin resistance.

This is a classic vicious cycle. Too much insulin causes resistance. Too much resistance causes higher insulin levels. And the cycle goes round and round, all the while stimulating weight gain. This explains why obesity is so time dependent. Those who have been obese for a long time have a much harder time losing weight.

So losing weight depends upon decreasing foods that stimulate insulin, but also on breaking the insulin resistance cycle. Since resistance depends upon both high levels and persistence, the answer is to leave your body long periods of time with low insulin. In other words, let your body rest from the high insulin. Just as in the example with the loud music, if you leave yourself some periods of silence, this will break the resistance cycle.

Dr. Lipman: So, how do you give your body a rest from the insulin?

Dr. Fung: The best way is to have periods of time where you are not eating. In a word—fasting. This can be for 16 hours, 20 hours, 24 hours, or even longer. Giving your body a period of low insulin breaks the resistance and results in weight loss.

Understanding the fundamental, underlying cause of obesity results in simple, successful strategies for weight loss. There are two important questions in obesity. What to eat? We all pretty much agree here. Reduce processed foods. Reduce white flour and sugars. Eat lots of vegetables. However, we ignore the second, crucial question: When to eat? We don’t need to eat more frequently to lose weight, we need to eat less frequently. Eliminating snacks is a simple way to reduce the frequency of eating. Our grandmothers already knew the truth. No snacking.