Q and A With Dr. Jason Fung About His Book The Obesity Code…Unlocking The Secrets Of Weight Loss


Dr. Lipman: You say in your book that obesity is not from eating too many calories or expending too few. Can you explain?

Dr. Fung: Obesity is often considered a problem of excessive calories. This caloric obsession has been indoctrinated into all of us since we were children. Too many calories in, too few calories out, or some combination is what we believe leads to weight gain and obesity. If it were indeed true that excess calories leads to weight gain, then the solution is simple: Reduce calories eaten. This has formed the standard dietary advice of the last 50 years. And it has failed spectacularly. Obesity rates have skyrocketed upward despite continual exhortations to cut calories. So, the proof is in the pudding. This advice does not work.

The other major strategy has been to increase exercise. Total calorie expenditure is not simply exercise, but includes basal metabolism. However, basal metabolism is not under conscious control, so it is assumed to be stable. In fact, the basal metabolic rate may increase or decrease up to 40% depending upon many factors, but one major one is caloric intake. But here once again, this advice has failed us.

Exercise comprises a very small proportion of our daily calorie expenditure. Let us assume basal metabolism of 2,000 calories per day. Walking for 45 minutes might burn 100–150 calories. If you’ve ever watched the calorie counter on your treadmill, you’ve probably already noticed how few calories are actually burned. This means that 95% of caloric expenditure is not related to exercise.

This is not to say that exercise is not important. There are many health benefits. However, weight loss is not one of them. Diet comprises 95% of the solution, with only 5% being exercise. So worrying about that 5% while largely ignoring the 95% is not a winning strategy.

If excess calories were indeed the cause of obesity, then it should also be easy to induce obesity. Experimental overfeeding studies have been performed since the 1960s, all with the same result. Deliberate overfeeding did not cause long-term weight gain. As soon as the force-feeding problem was finished, participants would spontaneously stop eating until they returned to their original weight.

So, it is clear that giving extra calories does not cause weight gain, and reducing calories does not cause weight loss. This was emphatically demonstrated in the Women’s Health Study, a huge, ambitious study of almost 50,000 women. They reduced their daily calorie intake by more than 300 calories per day. Over seven years of the study, they did not even lose a single pound!

The problem is that calorie excess is not the ultimate cause of weight gain. If you overeat, you will spontaneously reduce your intake and increase energy expenditure until you lose the weight. The opposite is true too. If you try to simply reduce your calories, then you will become hungry, and your basal metabolism will fall until you stop losing weight and start to regain it. This has been known for over 30 years and is the reason why a simple calorie reduction strategy is doomed to fail.

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.

Obesity 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.

Dr. Lipman: What about breakfast?

Dr. Fung: Breakfast is the most controversial meal. There are many people who consider this the most important meal of the day. The word itself contains the clue to understanding breakfast. This is the meal that breaks your fast. This implies that fasting must be an integral part of every single day. So, yes, breakfast is important, but it does not mean that you need to eat it as soon as you wake up.

The French, a notoriously skinny people, have another word for breakfast—petit déjeuner, which means “little lunch.” So, many French people routinely skip the meal and just drink coffee and break their fast at lunchtime. And guess what? There is nothing wrong with that. The breakfast meal can be taken at 12:00 instead of 8:00.

The key is that we are all in a hurry in the morning. So we grab a muffin. We eat juice and toast. We eat a doughnut. These are all highly refined carbohydrates. We are also shortening the time that our bodies are burning fat. During fasting (when we do not eat), our bodies use some of the stored food energy from the day before to power our body. If you don’t eat breakfast, you simply give your body more time to burn off the fat. And there’s nothing wrong with that.

If you have more weight to lose, you can simply extend the fasting period. Instead of 12 hours, you can extend this to 16, or even 24 or 36 hours. This is the process called intermittent fasting. A common regimen would be to use a 24-hour fast (from dinner to dinner, for example) twice a week. Dr. Michael Mosley’s 5:2 diet is similar to this schedule. Fasting represents an option for weight loss that most people have simply not considered. Yet it is one of the oldest dietary interventions available. It is simple, free, and effective. Far from causing harm to your health, it has enormous benefits.