Why Fasting Works

Ancient Romans were onto something

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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.