Dr. Jason Fung, 42, a kidney specialist and founder of Scarborough’s Intensive Dietary Managementprogram, says obesity — and even those last 10 pounds — isn’t caused by overeating, but by excessive insulin.

He discovered the real culprit of getting fat, he says, after realizing patients in his nephrology practice would get better with fewer medical interventions if they lost weight.

Since most of his patients were Type 2 diabetics — a disease associated with too much insulin — he made the link.

And the leap.

According to his new book The Obesity Code, the best, fastest, most economical and effective way to control this hormone before — or, after — it gets out of control is by fasting.

Or, at least, skipping breakfast.

So, insulin, not calories, is the real cause of obesity?

Obesity is a hormonal disease. Insulin, a hormone, tells you how much to eat and how much to burn. The body behaves as if the weight is set on a thermostat. Insulin acts to increase that thermostat. So, obesity is not about caloric imbalance. That idea is plainly wrong. That’s why cutting calories doesn’t work.

How does insulin cause obesity?

Excessive insulin causes obesity.

When you eat, insulin goes up and your body stores energy. When you don’t eat, insulin goes down and your body takes stored energy and uses it. Storing energy (sugar and fat) is the function of insulin.

When you have excessive insulin — higher than normal amounts and for much longer times than normal — your body becomes resistant to the effects of this hormone and produces even more insulin. It’s a vicious cycle — and it doesn’t matter what you do; exercise, diet. Too much insulin and you get fat.

So, how are we getting too much insulin?

Snacking.

We have an “eat all the time” mentality. In 1977, we were eating white bread and jam — which raises insulin levels — but we weren’t snacking all the time. And we weren’t obese.

In the last several decades, the number of times we eat in a day has gotten substantially higher. We’ve gone from three meals a day: breakfast, lunch and dinner, to six: breakfast, snack, lunch, snack, dinner, snack. We are stimulating insulin all the time, keeping it constantly high.

How do we bring our insulin down?

First, avoid foods that excessively stimulate insulin. Like, sugar and refined grains. That, we (as a society) have accepted.

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But we also need to think about meal timing.We need periods of time when we aren’t eating, so insulin can go down, leaving our bodies in energy burning mode. If we leave more time between meals — and, therefore, burning energy — we will lose weight.

So, fasting.

Yes. By fasting, we get our insulin levels low and we get rid of the resistance. Fasting, by dramatically lowering insulin, acts to lower the body’s set weight.

But it sounds hard.

It can be difficult if it’s something you’re not used to doing. In that way, it’s different than any other diet. It’s the anti diet. But, once you get started — people are surprised. They’re not hungry. They have energy. And, it’s free. Fasting saves you money. You don’t have to shop, plan meals. It really simplifies your life. It’s also practical and entirely flexible: you can bring it on and turn it off whenever you want. You can fast for only 14 hours a day, right after dinner until, say, lunch.

So, skip breakfast?

Yes. Skipping breakfast is a mini fast, giving yourself the time from dinner until noontime. That’s about 13 or 14 hours of fasting. If you want to do a 24 hour fast — which is even better — don’t eat after dinner and just work through lunch, staying hydrated by drinking water, coffee, tea or chicken broth. By the time you get going you don’t even notice you haven’t eaten and then you’re on your way home for a nice dinner. I do it all the time.

Is it healthy? What about being so hungry that you overeat afterwards?

Yes. It is actually very healthy (children, pregnant or breastfeeding women shouldn’t fast). And you will eat more at that post-fast meal, but not enough to compensate for what you haven’t eaten.

Your book and your credentials seem to give fasting a legitimacy it hasn’t enjoyed in the past. Are you getting much criticism for this seemingly radical diet plan?

I take a lot of heat for sure. When I started to suggest fasting, everyone was aghast. But now, I’ve got about 1,000 patients fasting and they and their doctors can see that they are so much healthier and happier. I’ve gotten a lot of patients losing weight and off insulin.

Family doctors are referring their patients to my diet clinic and, in my local area, I get almost no resistance at all anymore. But in the wider world, it definitely is an uphill battle. There’s a lot of resistance.