Intermittent fasting is one of the more popular (and argument-provoking) diet trends of recent years—it was even the most-searched diet term on Google last year. It’s not hard to guess what it entails: abstaining from food, or at least most food, for specific intervals. It's been taken up by everyone from Silicon Valley biohackers to TikTok models. Diets based on it have been touted for a huge list of benefits: It might be good for weight loss. It might work to balance out one’s blood sugar. Some even follow it for “mental clarity,” the idea being that by not eating a big lunch, you’ll avoid the grogginess that hits a couple hours later.
But what can anyone legitimately expect to gain from taking on an intermittent fasting diet? Those benefits are all related: “The number one reason people do it is for weight loss, but when you lose weight, that helps lower blood pressure, which helps to prevent heart disease and diabetes,” says Krista Varady, nutrition professor at the University of Illinois, Chicago.
As GQ has explored before, paying attention to when you eat is just as important as what you’re eating, and intermittent fasting is one of the simplest ways to start experimenting with a diet.
When it comes to the specifics of intermittent fasting, there are generally three different approaches to take. Some people opt for alternate-day fasting: Eat whatever you want on off days, but limit calories to around 500 on fasting days. Some try the 5:2 diet, which involves fasting during the week and making the weekend your off days.
But taking entire days off is on the extreme end of things. According to Varady, the most popular kind of intermittent fasting is time-restricted eating. In the most popular form, eating is limited to an eight-hour window each day. Depending on your schedule, you can put the window wherever you want, although Varady says making it from noon to 8 p.m. is generally easiest for most people.
“The minute you take dinner out, people get upset about it, because then they can’t socialize with friends or eat with family,” she says.
The beauty of the diet is that it doesn't specify what you should be eating, only when. Pick a schedule that works for you and there isn't much more to it than that. Many people find that easier than cutting out certain foods or counting calories. And while this isn't explicitly the goal, Varady's research indicates that people end up cutting out anywhere from 300 to 500 calories each day without counting them. She also finds that about 80 percent of people who try a time-restricted eating fast say they plan on continuing it.
Typical Western diets are heavy on simple carbohydrates. (Think of the sandwiches or slices of pizza you might eat for lunch or dinner.) These food items provide the glucose the body uses as fuel. But spiking glucose is accompanied by a rise in insulin, which the body uses to process blood sugars. That’s often what causes a sluggish feeling as the day goes on.
According to proponents of the diet, fasting for 16 hours evens these spikes out and recalibrates the body. A longer fast leads to lower blood pressure, and several studies show that time-restricted eating is associated with better cardiovascular health because of what the body ends up burning for fuel. In the absence of glucose, the body switches to burning fat, and as the body releases more fat into the bloodstream, the level of insulin drops accordingly. This, in turn, provides a needed break for cells to rest. In effect, fasting tricks the body into thinking it’s time to conserve its resources.
“We think the benefits come from lowering insulin levels,” says Courtney Peterson, an associate professor of nutrition science at the University of Alabama at Birmingham. “You’re no longer burning through glucose, which lowers your insulin level and gives cells time to repair.”
One of the more recent and well-publicized studies of time-restricted eating, published in JAMA Internal Medicine this fall, throws some cold water on the idea that time-restricted eating produces benefits—at least for anyone looking to lose weight. A 12-week randomized trial found that overweight adults who ate all their meals between noon and 8 p.m. lost roughly the same amount of weight as a control group, and that most of the weight lost was lean muscle mass instead of fat.
“I had personal success with time-restricted eating, and we went into the study with a bias thinking it would work,” says Ethan Weiss, a cardiologist at the University of California, San Francisco, who helped design and conduct the study. “But the time-restricted eating group lost a clinically meaningless amount of weight. They lost, on average, less than a kilogram.”
Study participants were told when to eat, but not told to restrict calories. Even so, the study results indicated to Weiss that time-restricted eating isn’t an effective way to lose weight.
Yet the ultimate take on time-restricted eating is still an open question, mainly because so few studies exist that examine its effects. Varady, for instance, has found the opposite effect: Her research has shown that participants who followed a time-restricted eating diet for 12 weeks lost weight without losing muscle mass. She’s now planning a yearlong study of time-restricted eating.
And while Weiss has since stopped his daily fasts, he admits the jury is still out on whether a fasting diet could be beneficial for some.
“Do I think there’s a possibility that either this version of time-restricted eating or some other form of intermittent fasting could be used for weight loss or health? Absolutely, and I plan on studying it,” he says.
It's a simple, powerful way to mind what you're eating—but not a silver bullet for a better diet.
