The Fad Diet to End All Fad Diets

There isn’t much evidence that intermittent fasting leads to lasting weight loss. Why is it still so popular?

In 2012, the BBC aired a documentary that pushed diet culture to a new extreme. For Eat, Fast, and Live Longer, the British journalist Michael Mosley experimented with eating normally for five days each week and then dramatically less for two, usually having only breakfast. After five weeks, he’d lost more than 14 pounds, and his cholesterol and blood-sugar levels had significantly improved. The documentary, and the international best-selling book that followed, set the stage for the next great fad diet: intermittent fasting.

Intermittent fasting has become far more than just a fad, like the Atkins and grapefruit diets before it. The diet remains popular more than a decade later: By one count, 12 percent of Americans practiced it last year. Intermittent fasting has piqued the interest of Silicon Valley bros, college kids, and older people alike, and for reasons that go beyond weight loss: The diet is used to help control blood sugar and is held up as a productivity hack because of its purported effects on cognitive performance, energy levels, and mood.

But it still isn’t clear whether intermittent fasting leads to lasting weight loss, let alone any of the other supposed benefits. What sets apart intermittent fasting from other diets is not the evidence, but its grueling nature—requiring people to forgo eating for many hours. Fasting “seems so extreme that it’s got to work,” Janet Chrzan, a nutritional anthropologist at the University of Pennsylvania and a co-author of Anxious Eaters: Why We Fall for Fad Diets, told me. Perhaps the regime persists not in spite of its difficulty, but because of it.

[Read: What billionaires’ fasting diets mean for the rest of us]

Intermittent fasting comes in lots of different forms, which vary in their intensity. The “5:2” version popularized by Mosley involves eating normally for five days a week and consuming only about 600 calories for two. Another popular regime called “16/8” restricts eating to an eight-hour window each day. One of the most extreme is a form of alternate-day fasting that entails full abstinence every other day. Regardless of its specific flavor, intermittent fasting has some clear upsides compared with other fad diets, such as Atkins, Keto, and Whole 30. Rather than a byzantine set of instructions—eat these foods; avoid those—it comes with few rules, and sometimes just one: Don’t eat at this time. Diets can be expensive, yet intermittent fasting costs nothing and requires no special foods or supplements.

Conventional fad diets are hard because constantly making healthy food choices to lose weight is “almost impossible,” Evan Forman, a psychology professor at Drexel University who specializes in health behavior, told me. That’s why intermittent fasting, which removes the pressure to make decisions about what to eat, can “actually be reasonably successful,” he said. Indeed, some studies show that intermittent fasting can lead to weight loss after several months, with comparable results to a calorie-counting diet.

But lots of diets lead to short-term success; people tend to gain back the weight they lost. Studies on intermittent fasting tend to last only a few months. Yet in a recent one, which tracked patients over six years, intermittent fasting wasn’t linked with lasting weight loss.

Whether other benefits can be attributed to fasting remains unclear. Assertions that it might improve insulin sensitivity, obesity, cardiovascular health, Alzheimer’s, and Parkinson’s disease are largely based on preclinical and animal studies, according to a 2019 review in The New England Journal of Medicine. Although its authors argue that fasting is broadly favorable, they concluded that whether people who fast over a span of years can ever accrue the health benefits seen in animals remains to be determined.

Incomplete evidence is typical for dieting fads, which tend to come and go pretty quickly in a way that intermittent fasting hasn’t. (Does anyone remember the Special K and Zone diets? Exactly.) What really sets the practice apart is how hard it is. Skipping meals can send a person into a tailspin; willfully avoiding food for hours or even days on end can feel like torture. The gnawing hunger, crankiness, and reduced concentration associated with fasting usually takes at least a month to dissipate.

That may be why intermittent fasting is hard for America to shake. When it comes to diets, “the more extreme they are, the more they are perceived to be extremely efficacious,” Chrzan said. The self-discipline required to persevere through a fast is commonly glorified. Abstaining from food for 36 hours, a particularly intense form of the diet, is known as the “monk fast”; one intermittent-fasting app is called Hero.

That pushing your body to the limit has benefits isn’t a new idea. In 1900, the American physician Edward Hooker Dewey published The No Breakfast Plan and the Fasting-Cure, which promoted fasting as a virtuous act that could remedy physical and mental ailments. The extreme self-sacrifice and resulting moral fortitude that fasting wrought, Dewey believed, would turn people into “better, stronger men and women.” Even before that, there was the era of “heroic medicine,” which held that a treatment had to match the severity of the illness. That’s how you get bloodletting, purging, and leeching. Such a harsh approach to health is still a part of “how we think about medicine,” Chrzan said. “It has to be hard, because perfecting the self is a worthy goal.”

To that end, more research into intermittent fasting may not matter much for its popularity. In fact, some adherents of the diet don’t seem to care about the mixed evidence, Kima Cargill, a clinical-psychology professor at the University of Washington at Tacoma and the other co-author of Anxious Eaters, told me. Consciously or not, maybe the point of intermittent fasting isn’t health, but something else entirely. Maybe dieters see results, or maybe they don’t. In either case, surviving a period of fasting is a test of fortitude, proof that the mind can overcome the body. “It’s not all about deprivation,” Forman said. “Part of it is about this chase for optimization”—a kind of bodily transcendence. According to Cargill, fasting gives people a way “to feel structured and contained.” If these are the unspoken reasons people practice intermittent fasting, it’s no wonder that the diet has proved to be so much more than a fad.

When you consider the food environment that Americans have to contend with, the appeal of a program as drastic as intermittent fasting makes sense. Amid a glut of ultra-processed options, constant invitations to snack, and general bewilderment about what anyone is actually supposed to eat, “people feel so out of control about their diet that extreme ideas just have a lot more traction,” Chrzan said. Intermittent fasting offers a simple rule to eat by—and a means to rise above the chaos.