Fasting is one of the oldest dietary interventions in the world, and modern science confirms it can have a profoundly beneficial influence on your health. Dr. Jason Fung, a nephrologist (kidney specialist) with a practice in Canada, has written an important landmark book on this topic.

“The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting,” co-authored with Jimmy Moore, details how to implement fasting and overcome some of the most common challenges that might arise, including persistent fears and myths associated with extended water fasting.

For the first decade of his practice, Fung was — like most doctors — conventionally oriented. As a kidney specialist, many of his patients had type 2 diabetes as the primary cause of their kidney failure.

Fasting Helps Reverse Diabetes — And Related Health Conditions

It became clear to him that the conventional treatment of type 2 diabetes was seriously flawed.

Despite patients’ best efforts to manage their diabetes, taking their insulin and following the recommended diet and so on, they would still end up with complications such as kidney disease, requiring dialysis, or they’d need amputations, or they’d go blind.

“As a doctor, we got trained to give medications, but obviously it wasn’t working,” he says. “The answer is actually pretty obvious. Because if diabetes, type 2 predominantly, is what’s causing the kidney disease, you’re not going to be able to do anything about the kidney disease until you get rid of the diabetes.

That was kind of where I started. Then I thought, ‘Everybody says type 2 diabetes is this chronic kind of progressive disease … It only goes forward, one way.’ But actually when you think about it, type 2 diabetes isn’t like that at all …

[I]f you want to get rid of the type 2 diabetes, you have to get rid of the obesity … That’s how you’re going to help people get better. I started thinking about what causes weight gain … It’s certainly not calories.

That’s our big mistake. That’s why we’ve been unsuccessful at creating weight loss, because we’ve got the wrong kind of target …

It’s really about the hormonal balance and predominantly about insulin. We have to reduce insulin. Low-carbohydrate diets are a way to lower insulin … In some people, that’ll reverse their diabetes …

I started using low-carbohydrate diets and it didn’t really work. The problem was that it was a little complicated for people …

I had to make it simpler … I thought, ‘Why not fasting?’ … It’s been used for thousands of years … I started looking at some of the science … There are actually huge benefits that we weren’t recognizing.

Part of it was also we’ve always been trained, ‘You have to eat. You have to eat.’ But in fact, that’s not true. If you think about it, in the old days … there would be lots of days where people didn’t eat …

That’s really what [body] fat is [for]. It’s really simply stored fuel; stored food energy. We’re using it [when we fast]. That’s it. That’s all that happens. There’s no serious side effects or consequences to fasting.

If there was, we would have known about it several thousand years ago. But there wasn’t. That’s where I started from.”