“You’re starving yourself?”

That used to be a common reaction to fasting diets, but the regimen started gaining widespread acceptance in recent years—from Silicon Valley biohackers to Beyoncé—in large part because of Canadian doctor Jason Fung. According to a survey by the International Food Information Council Foundation (paywall), intermittent fasting is now the most popular diet, ahead of Paleo, Whole30, and keto.

Fung, a nephrologist who authored the books The Obesity Code and The Complete Guide to Fasting, is one of the most vocal advocates of intermittent fasting, having seen its effects in his type 2 diabetes patients. But there is a big difference between starvation and fasting, he notes. “Fasting is voluntary and controlled,” whereas starving is not, he says. “It’s the difference between running for fun and running because a lion is chasing after you.”

On certain Reddit communities, such as r/keto and r/intermittentfasting, Fung is regarded as a cult hero of sorts, often cited to provide scientific legitimacy for a diet that can seem outlandish to the unfamiliar. After all, intermittent fasting purports to tout weight-loss benefits without forcing people to change what they eat (though Fung says people really should try to cut out sugar and processed foods) or adding exercise to their routines. The key, instead, lies largely in when one eats.

“We never talk about the timing of the meal,” Fung tells Quartz. One major change he’s seen to the North American diet over the decades is the rise of snacking. People went from eating three meals a day to eating at all waking hours.

“You go back to the 1950s,” he says. “They’re eating white bread, they’re eating white pasta. No one’s eating whole wheat pasta… But they’re fine. There’s not much obesity.” This, he says, suggests it’s not the nutritional content that’s led to the modern rise in obesity, but rather the change in the frequency of eating.

Somehow over time, the idea that people should eat many small meals throughout the day to “boost” their metabolism became mainstream, a direct result of snack-food companies’ advertising, he notes. Snack makers also had a strategy to legitimize their claims. “The insidious part is the food industry sponsors a lot of dietician food conferences, and they were able to teach dieticians you should eat six times a day to lose weight,” says Fung.

“It sounds really stupid because it was really stupid.”

The benefits of fasting

When you stop to think about it, it’s remarkable how little we know about nutritional science in 2018, as evidenced by ongoing debates over whether coconut oil is “pure poison” and new fad diets that seem to pop up every season.

Intensive Dietary Management Jason Fung

Fung doesn’t claim to have all the answers, though he has his opinions. (Coconut oil and egg yolks are fine by his book.) But when it comes to the virtues of intermittent fasting, he says he espouses the results he’s seen in his patients. He himself skips breakfast regularly and doesn’t feel bad for foregoing lunch a few times a week.

Which brings us to the strange question of how a kidney specialist became Reddit’s favorite diet guru.

As a doctor working with diabetes patients, Fung says he saw a clear relationship between insulin and weight gain. As he wrote in The Obesity Code:

I can make you fat. Actually, I can make anybody fat. How? By prescribing insulin. It won’t matter that you have willpower, or that you exercise. It won’t matter what you choose to eat. You will get fat. It’s simply a matter of enough insulin and enough time.

It was this observation that led him to intermittent fasting, as a way to quickly lower insulin in the body. Prolonged periods of low insulin force the body to turn to stored sugar as a fuel source, and when that’s been depleted, to turn to fat.

Insulin, a hormone made in the pancreas, helps the body use sugar for energy, but insulin resistance and prediabetes can occur when the body doesn’t use normal amounts of insulin properly. Diabetes is a leading cause of kidney failure.

Fung says he’s prescribed intermittent-fasting diets, which restrict eating to a fixed schedule, to thousands of patients at his company, Intensive Dietary Management, where he serves as cofounder and medical director. Variations on intermittent fasting include alternative-day fasting, in which people eat normally one day and under 500 calories the next; 18:6, referring to fasting for 18 hours a day and eating within a six-hour window; or one meal a day, or OMAD for short.

Occasional longer fasts, which should be done under the supervision of a doctor if one is diabetic, for example, would see even more pronounced effects, which in addition to weight loss can include mental clarity and a detox process called autophagy.

“I have a bit of an advantage because I tell people to fast all the time—if you do surgery, a colonoscopy, blood work, you had to fast,” he says. “I thought about it from a physiological standpoint and read the studies. Well, there’s no reason why people can’t do this. It’s been done for so many thousands of years.”

Knowing what he does about insulin’s effect on weight gain, he says exercise is a very inefficient way to slim down. “There are a lot of health benefits, but they’re two totally separate issues,” he says. “If I had to guess, diet is 95% of the battle, and exercise is 5% of the battle. The problem is we over-emphasize exercise. If you had a test and 95% of it was on math and 5% on English, you’re not going to study both quite the same.”

Tips for fasting

While studies have shown that fasting is effective in helping people lose weight, many fasters fail because they’re hard to adhere to.

It can take some time to acclimate to a new eating schedule, and Fung recommends keeping busy to keep the mind off food.

Technically, a true fast is water and nothing else. But he says it’s OK to bend the rules to help make it through longer stretches without food. Coffee and tea can help suppress appetite. Black coffee is ideal, but “a little cream is no problem.” The same goes with bone broth.

“It depends on your goal,” he says. “If you’re trying to lose weight, you can eat 500 calories [as people do on alternate-day fasts] and lose weight. You can do coffee with a little cream.”

But people will have to strictly adhere to water if they wish to attain the other benefits of fasting. The protein present in bone broth or cream, for example, will turn off autophagy, the process in which cells degrade and recycle themselves. (Japanese scientist Yoshinori Ohsumi’s discovery of autophagy won him the Nobel prize in physiology or medicine in 2016.)

For Fung, fasting isn’t just a way to trim off some weight, but a lifestyle that “gives me time back in my day.” Even for people who don’t regularly fast, Fung thinks it’s beneficial to do so once in a while.