Ancient Romans found the idea of breakfast repellent. They were “obsessed with digestion,” according to the historian Caroline Yeldham, and believed eating more than one meal a day was unhealthy and gluttonous.
If that’s the case, the likes of Cicero and Marcus Aurelius were early adherents of “intermittent fasting,” which is a catchall term for a handful of related diets that either restrict food intake to certain hours of the day or limit intake several days each week. The nomenclature can get confusing, but the most popular and evidence-backed of these fasting plans are known as time-restricted feeding, alternate-day fasting, and the 5:2 diet.
The first — time-restricted feeding — involves compressing the day’s snacks and meals into a narrow window of time, usually six or eight hours. The operating theory here — one that, to an extent, nearly all nutrition experts support — is that the human body wasn’t designed to consume and digest food all day, every day.
“Most people are putting something caloric in their mouths essentially every minute they’re up, and we know that from an evolutionary perspective, this is not how humans or animals are geared to eat,” said Mark Mattson, a fasting researcher with the National Institutes of Health and an adjunct professor of neuroscience at the Johns Hopkins University School of Medicine.
In both human beings and mice, studies have found that constraining food intake to an eight-hour window promotes weight loss, regardless of diet quality. “If you restrict the time window of eating, you can put animals on a McDonald’s diet and they don’t get fat,” Mattson said.
No one is suggesting a McDonald’s-only plan. “I think quality of diet is important in the long term for reduction of heart disease and diabetes risk,” said Krista Varady, an associate professor of nutrition at the University of Illinois at Chicago. “But in the short term, even if people don’t eat healthier, they still lose weight.”
Varady coauthored a 2018 study that found obese men who ate only between 10 a.m. and 6 p.m. lost an average of 3 percent of their body weight after three months, and also improved their blood-pressure scores.
Apart from that 2018 study, much of Varady’s research has focused on alternate-day fasting. These diets involve eating freely one day and restricting food intake to 500 calories the next. “Alternate-day fasting produces faster weight loss, but it’s harder to follow,” Varady said. In three months, she said someone on a time-restricted diet can expect to lose five to 10 pounds, while someone on an alternate-day fasting regimen would likely lose 10 to 15 pounds.
In three months, she said someone on a time-restricted diet can expect to lose five to 10 pounds, while someone on an alternate-day fasting regimen would likely lose 10 to 15 pounds.
Weight loss aside, the metabolic and disease-lowering benefits appear to be similar when comparing time-restricted feeding and alternate-day fasting diets, Varady said. And the same goes for 5:2 plans, which involve eating normally five days a week but mixing in two non-consecutive days of caloric restriction — usually defined as 500 to 600 calories or fewer. A 2018 study in the American Journal of Clinical Nutrition found people lost an average of 7 percent of their body weight after 12 weeks on the 5:2 diet, and a 2018 study in JAMA found that following a 5:2 plan for one year improved blood-sugar scores among people with Type 2 diabetes. Both of these studies found that the 5:2 plan either matched or bested traditional diet strategies that involved cutting calories on a daily basis.
All this evidence has nutrition experts buzzing. Traditionally, popular weight-loss plans have concerned themselves with the types and amounts of foods a person eats. Most have involved either cutting carbs or cutting fat. But time and again, research has shown that these approaches fail in the long run. While most diets work in the short term, inevitably, the weight comes back. The appeal of intermittent fasting is that it concerns the timing of meals, not the content. “It’s not good to constantly bombard our bodies with nutrients,” Varady said. “Fasting gives the body a break from having to deal with food coming in all the time.”
At this point, Varady said, the research doesn’t reveal which of the popular fasting plans is optimal for health or weight loss. There aren’t good head-to-head studies comparing these diets. But time-restricted feeding — because it does not involve severe restriction or counting calories — seems to be nosing ahead of its competitors. “People like it because all they have to do is watch the clock and pick their window,” Varady said. (A common practice is to skip breakfast and morning snacks, and then eat freely from noon to 8 p.m.) Experts usually cite poor adherence as the prime reason that diets fail in the long-term. If people find it easier to stick with time-restricted feeding compared to other fasting diets, as studies to date suggest, that’s a big selling point.
Another unsettled question: Are fasting diets beneficial for healthy people who aren’t trying to lose weight? Here, the data are murkier.
“We only have good evidence that intermittent fasting is a good option for overweight and obese people,” said Michelle Harvie, a research dietitian at Manchester University in the U.K. who studies the effects of intermittent fasting on health and disease. “We don’t know of [intermittent fasting’s] benefits in normal-weight people as it has not been studied.”
Varady shared this view. “There’s not that much evidence out there at this point on healthy adults,” she said. Some studies found that certain groups who practice intermittent fasting for religious reasons — such as Seventh-day Adventists and Orthodox Christians — enjoy health benefits. But Varady said these groups tend to lead healthy lifestyles, at least compared with the average American, and so it’s tough to tell whether to credit fasting. “I’m hopeful that intermittent fasting will have more general health benefits, but we need more long-term studies,” she said.
But while most of the work on intermittent fasting involved sick or obese adults, there is some evidence that periodic, long-duration fasts may also benefit healthy folks.
“The longer you fast, the more you basically kill cells,” said Valter Longo, a professor of biological sciences and gerontology at the University of Southern California. “That sounds like a bad thing, but the cells that die are unhealthy ones.”
According to Longo, dysfunctional cells and disused cell components steadily accumulate in the body as a person ages, and these ailing cells contribute to the aging process and age-related diseases like cancer. But when the body gets an extended break from food and digestion — something on the order of five days — it has to break down its own tissue for sustenance. And in so doing, it ends up clearing away unhealthy cells and making room for new ones to flourish. “So fasting kills cells, but with refeeding [following the fast], the cells not only come back but are healthier,” he said.
Some of Longo’s work — most of it on mice — found that an extended fast can trigger a number of beneficial biochemical changes, including the regeneration of healthy cells and a retardation in the growth or development of cancer cells and tumors. More of his work has found that fasting reduces inflammation and oxidative damage, and also “reprograms” an individual’s metabolism in ways that may combat Type 2 diabetes.
It normally takes several days of water-only fasting for the body to initiate these processes, and this kind of fasting can be dangerous without close medical supervision. But some of Longo’s research on humans found that a specially designed, temporary fasting diet — known as the fasting-mimicking diet, or FMD — can provide people with sustenance without interrupting these cell-regenerating operations. “It’s not pure fasting — it’s not water-only,” he said. “People can eat nuts and non-starchy vegetables, raw or cooked, dressed with a tablespoon of olive or canola oil and lemon, vinegar, and salt.” But the FMD is very low in calories — as low as 300 per day, depending on a person’s health status — and no proteins or carbohydrates from grains are permitted, he said.
A 2017 study of Longo’s found that people who stuck with FMD for five consecutive days a month for three months were slimmer, had lower blood pressure, and improved cholesterol scores. They also had lower circulating levels of hormones associated with inflammation and disease risk. Here again, FMD benefited adults at risk for disease more than healthy ones. But Longo said most Americans fall into the “at-risk” group, and he believes FMD can help prevent or lower a person’s risk for a number of age-related diseases.
“If you’re very healthy, I would say do the fasting-mimicking diet two to three times a year,” he said. “If you’re unhealthy, once a month, but only with a doctor’s recommendation.” (Based on his research, Longo helped formulate diet products that are sold commercially under the name ProLon. These provide people with the nutrients they need to safely complete a five-day fast without undue risk. He donates his share of profits to charity and does not receive consulting fees from the company.)
In the not-too-distant future, the evidence backing fasting diets may be so solid that doctors recommend these plans to sick and well patients alike. The science isn’t there yet. But for overweight or obese Americans looking for new, research-backed diet strategies, few are as promising as those that incorporate elements of fasting.
Markham Heid, a reporter and writer, is a longtime contributor at TIME, Men’s Health, Food & Wine, and other outlets. He has received reporting awards from the Society of Professional Journalists and the Maryland, Delaware, and D.C. Press Association.