Here’s The Skinny On Intermittent Fasting

“Everybody fasts,” points out Carolyn Williams, a registered dietician. “It’s just that the amount of time we spend between meals has gotten a lot shorter.” It has become the norm to wake up at 6 a.m. and chug a sweetened coffee drink, then keep grazing, nibbling and snacking until we finally brush our teeth after 10 p.m. For a while, nutrition experts actually recommended switching from three meals a day to smaller, more frequent “mini meals” on the erroneous belief that doing so would keep our metabolic furnace burning.

Turns out, there was never any evidence to support this. “The whole idea is sort of ridiculous,” says Jason Fung, M.D., author of The Obesity Code. “A small amount of food doesn’t make you feel more full, it makes you more hungry. You’re stimulating your appetite and then stopping. That’s what appetizers do.”

In reality, he says, human bodies are designed to go for extended periods without food. That’s actually the whole purpose of body fat. When we eat excess calories, they’re stored as fat — a process with which we’re all too familiar. But when we stop eating, even for as little as 12 hours, our bodies still need calories to fuel basic metabolic processes like breathing and cell repair. So, they pull those calories from our fat reserves. “The problem,” Fung says, “is that when you constantly have a fresh supply of calories to burn, your body never gets to fat-burning mode or ketosis.”

Around-the-clock eating habits can wreak havoc on blood sugar. In healthy people, eating triggers your pancreas to secrete insulin, a hormone that balances blood sugar. When we never take a break from food, insulin is chronically high, and over time that can lead to something called insulin resistance, where the hormone’s usual effects are blunted and blood sugar remains high. This state is often a precursor to developing type 2 diabetes. While the disease has no cure, Fung has been able to help hundreds of people reverse those effects through fasting, to the point of no longer needing medication to control their diabetes. (Obviously, if you’re dealing with a medical issue, you should always consult your physician before making any major dietary changes.)

While research on fasting in human subjects is still limited, a 2019 review published in the New England Journal of Medicine showed that the practice holds a lot of promise for warding off other chronic diseases as well. Researchers believe that the switch from burning glucose to fat for fuel not only helps with weight loss, but also may cause cells to beef up their natural defenses against the things that cause aging and disease, such as cellular inflammation and oxidative stress.

“A lot of the changes we see with fasting are similar to what happens when you exercise,” says Mark Mattson, coauthor of the study and a professor of neuroscience at Johns Hopkins University School of Medicine. And, significantly, those changes continue even after a fast is over. Mattson’s research has linked fasting to improved brain function and longer lifespan.

Fasting 101

Even if you buy into all those benefits, the idea of simply not eating can sound a little extreme. Who wants to live longer when you can never have brunch again? But fasting isn’t the same as starving yourself, says Williams. There’s that intermittent part, which means you have a period of fasting, followed by a period of eating, sometimes referred to as “windows.” The idea is to work your way toward an eating window that’s as short as possible.

One of the most natural ways to do this is literally while you sleep. If you stop eating after dinner, you’ll have fasted close to 12 hours by the time your alarm clock rings at 6 a.m. Most of us don’t tumble out of bed ravenous, either, and many intermittent-fasting regimens permit coffee, as long as you skip the sugar (most plans allow a little milk, cream or even butter for you Bulletproof fans, since these are mostly fat and won’t trigger an insulin response).

“Hormones — especially insulin — are, for the most part, what drives hunger,” Williams says. “What we think of as hunger is low blood sugar. When I used to eat five or six times a day, I felt captive to my blood sugar. When I fast, I have to remind myself to stop and eat.”

Experts recommend starting off with a small fasting window and gradually widening it. The 16:8 plan, which follows a 16-hour fast followed by an eight-hour eating window, is popular, but you’ll see benefits with fasts of as little as 12 hours. Other plans restrict fasts to a few days a week. When you do eat, you can enjoy whatever you want, although Williams says that fasting tends to make you more mindful of what foods fuel your body best. Carb-heavy dinners tend to spike blood sugar, so you may wake up hungrier than if you eat a balance of protein, healthy fats and whole grains.

While she was initially a skeptic, Williams says she now tries to fast most days.

“You can get the majority of nutrients and calories you need in an eight-hour window, and it has made it easier for me to maintain my weight,” she says. “It’s not restrictive, and it feels like a long-term way to eat.”

You are likely to see results pretty quickly, too. One review of 40 studies on intermittent fasting found that the average loss was between seven and 11 pounds in a 10-week span.