In 2013, a British journalist and a doctor introduced an obscure dietary protocol to the wider culture. The idea was simple: Two days a week, eat almost nothing - fewer than 600 calories. The rest of the time, eat normally. The writers, Dr. Michael Mosley and Mimi Spencer, claimed "The Fast Diet" could help you shed fat, reverse Type 2 diabetes and stave off age-related diseases of mind and body. Early studies had shown it had outsize benefits in lab mice, and scientists were enthusiastic about its prospects for humans.

This simple intermittent fast (today referred as a 5:2 diet) was not the first of its kind to go mainstream, but it became an international sensation. Today there are dozens more variations, with books, apps, wearables and supplements to support them. And they're very popular: The International Food Information Council, a nonprofit tied to the food industry, surveyed 3,000 American adults and found that 13 percent have experimented with intermittent fasting in 2024.

All intermittent fasts alternate periods when you're allowed to eat with periods when you should eat nothing or very little. In perhaps the most popular variant, the 16:8 diet, you fast most of the day and only eat during an eight-hour window. Proponents of the diets praise them for being easier to follow than regular diets and offering greater weight-loss and metabolic benefits. But researchers have been growing disillusioned with them. As more studies have piled up, the gaps between the claims made for intermittent fasting and the evidence behind them has grown.

"It is oversold, honestly," said Krista Varady, a professor of nutrition at the University of Illinois at Chicago. So what can intermittent fasting actually offer? Are any of the variations any better than other diets? Where did intermittent fasting come from? Since the turn of this century, studies have shown that short periods of radical calorie restriction has outsized benefits for animals.

"If you only let a mouse eat in a four or six hour window, it will live longer, it won't get cancer, it won't get dementia, it won't get diabetes, you name it," said Michelle Harvie, a nutrition researcher at the University of Manchester, who created the 5:2 diet that Dr. Mosley described in his book. The same was true if they were only allowed to eat every other day. In theory - and in mice - intermittent fasting works because going without food means blood sugar drops. In response, cells start feeding on fat stores and switch into a kind of low-power mode. Now, instead of dividing, cells focus on self repair, gobbling up aged and nonfunctioning components and recycling them in a process called autophagy. This cleanup leads to better working cells and an array of health benefits.

But mice have a different and far higher metabolism than people do, Dr. Harvie said. The metabolic mismatch means the animal evidence "doesn't necessarily translate to human beings." Does it trigger weight loss? The most common claim about intermittent fasting is that it's a better way to lose weight than other diets. Early mouse and rat experiments suggested that something interesting was going on beyond simple calorie restriction. The animals lost weight and stayed healthier than mice that ate normally, no matter how many calories they binged between fasts.

But in humans, the idea that intermittent fasts offer special weight loss benefits "really hasn't been borne out by the data," said James Betts, a professor of metabolic physiology at the University of Bath in the United Kingdom. At first glance, it would appear that the research is mixed - some studies find that intermittent fasting is no more effective for weight loss than other diets, while others report a small additional benefit. But many of the latter are based on small, low quality studies, Dr. Betts said. He and his colleagues have reviewed many studies published in the past few years and found that much of the pro-fasting research is corrupted by studies that make massive errors, like counting study subjects twice or misinterpreting their own data.

"There are only probably about 20 or 30 studies out there that are good," Dr. Varady said. "It's not bad, but it's like all diets," she added. "It can't produce more than 5 percent weight loss." The diet's major selling point is that it's easier to stick to than simple calorie restriction, but even this may be overstated, according to two recent papers. In other words, Dr. Harvie said, people seem to get bored with intermittent fasting just as they get bored with other diets.

How does it affect metabolic health? Proponents of intermittent fasting say that it improves your metabolic health in addition to shedding pounds. Research showed that diabetic people who tried it were able to take less of their medicine because of improved insulin sensitivity and that people with fatty liver disease reversed the fat accumulation. A study at Heidelberg University Hospital, in Germany, found that people with diabetes-related kidney problems saw improvements after a five-day fast. But these metabolic benefits are about the same as what you would expect from any calorie restriction diet that leads to weight loss, said Stephan Herzig, the researcher who ran the Heidelberg study.

He started his research hoping to find some kind of special chemical or cellular changes that result from intermittent fasting, which might become the basis for new drugs - but he didn't find any. He concluded that the metabolic benefits of intermittent fasting likely ended the moment you start eating again. Several studies even suggest some forms of fasting may be bad for your metabolism. In a 2021 paper comparing alternate day fasting to normal calorie restriction, people in both groups lost equivalent amounts of weight, but those who fasted lost far more muscle, which negatively affects metabolism.

Can it have cognitive benefits? Some claim that the benefits of fasting extend beyond the body to the mind. The health influencer Andrew Huberman has said that intermittent fasting can be "great for focus and concentration." In lab studies, mice that have been starved are faster to find food and less prone to learning and memory deficits associated with Alzheimer's disease than wellfed counterparts. Some of these claims may be true for long intermittent fasts. Only a few such experiments have been done, but they suggest that some cognitive enhancement is reported after about four to five days without food, as cells start metabolizing fats, said Mirjam Bloemendaal, a neuroscientist at Goethe University in Frankfurt.

But shorter, less intense fasting regimens seem to have no effect on cognition and are unlikely to provide a boost to those with cognitive decline, Dr. Bloemendaal said. A brutal four-day fast is completely different from the popular varieties, Dr. Varady added. What about cancer? There is some intriguing preliminary research suggesting that fasting might improve cancer prognoses, experts said, though not for the reason once theorized. Around 2008, the gerontologist Valter Longo at the University of Southern California proposed that intense four- or five-day fasts could delay tumor growth.

The theory, suggested by mouse trials, was fasting would make healthy cells more resilient while pushing cancerous cells into a weakened state, making them more vulnerable to treatments. In 2020, a major study failed to find this effect but supported another idea: Intermittent fasting reduced the side effects of treatment, like nausea, bone marrow loss and nerve damage. Such effects make some patients reluctant to choose chemotherapy and stay with it, said Tanya Dorff, a medical oncologist at City of Hope in Duarte, California, who has worked with Dr. Longo. Fewer symptoms, she said, might simply "allow them to stay on their chemo at full dose and on schedule."

A small study further suggested that a version of the 5:2 fast might let women with metastatic breast cancer live longer. For the women on the diet, it took on average 42 weeks for their cancer to progress, Dr. Harvie said, compared to 28 weeks for the women who were not on the diet, though this still needs to be confirmed by larger studies. For those with advanced breast cancer, "it won't do you any harm," Dr. Harvie said. "And maybe it will keep your tumor smaller for longer." As for all the other still-contested claims around intermittent fasting - longevity, improved heart health and lower risk of Parkinson's disease - a clear trend is developing, said Dr. Varady: The better the study, the less it finds.

"I just don't think we ever should have made these promises to begin with based on mouse trials," she said.