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As anyone seeking to lose weight knows, diets come in and out of fashion. The Sexy Pineapple diet, launched by a Danish psychologist in 1970, never really took off. Kellogg’s no longer promotes the Special K diet, which swaps out two meals a day for a bowl of the breakfast cereal of that name. These days, you don’t hear much about eating according to blood type, cutting out acidic foods, or following the potato diet.

Intermittent fasting has, however, had unusual staying power for more than a decade—and has grown even more popular in the past few years. One survey found that almost one in eight adults in the United States had tried it in 2023.

The enduring popularity of intermittent fasting has been fed by celebrity endorsements, news coverage, and a growing number of books, including several written by researchers in the field. More than 100 clinical trials in the past decade suggest that it is an effective strategy for weight loss. And weight loss generally comes with related health improvements, including a reduced risk of heart disease and diabetes. What is less clear is whether there are distinct benefits that come from limiting food intake to particular windows of time. Does it protect against neurodegenerative diseases such as Alzheimer’s disease, enhance cognitive function, suppress tumours, and even extend lifespan? Or are there no benefits apart from those related to cutting back on calories? And what are the potential risks?

Neuroscientist Mark Mattson at the Johns Hopkins School of Medicine in Baltimore, Maryland, and author of the 2022 book The Intermittent Fasting Revolution, has been studying fasting for 30 years. He argues that, because ancient humans went for long periods without food as hunter-gatherers, we have evolved to benefit from taking breaks from eating. “We’re adapted to function very well, perhaps optimally, in a fasted state,” he says.

Fasting’s deep roots

Fasting is far from new. Periodic abstentions from food have long been practised in many religions. In the fifth century bc, the Greek physician and philosopher Hippocrates prescribed it for a range of medical conditions.

Recent scientific interest in fasting has its roots in questions raised by research on calorie restriction. Since the 1930s, studies have shown that putting rodents on low-calorie diets can increase their lifespans. Hypotheses proposed to explain this effect include that calorie restriction slows growth, lowers fat intake or reduces cellular damage caused by unstable free radicals.

But an observation made in 1990 by researcher Ronald Hart, who was then studying ageing, nutrition and health at the US National Center for Toxicological Research in Jefferson, Arkansas, highlighted another intriguing possibility. Calorie-restricted rodents fed once daily consumed all their food in a few hours. Perhaps the calorie-restricted rodents lived longer because they repeatedly went for 20 or so hours without eating.

In the immediate aftermath of a meal, cells use glucose from carbohydrates in food as fuel, either straight away or following storage in the liver and muscles as glycogen. Once these sources are depleted—in humans, typically around 12 hours after the last meal—the body enters a fasted state during which fat stored in adipose tissue is converted to ketone bodies for use as an alternative energy source.

‘Intermittent fasting’ generally refers to various diets that include repeated periods of zero- or very low-calorie intake that are long enough to stimulate the production of ketone bodies. The most common are time-restricted eating (TRE), which involves consuming all food in a 4- to 12-hour window, usually without calorie counting; alternate-day fasting (ADF), whereby people either abstain from food every other day or eat no more than around 500 calories on that day; and the 5:2 diet, which stipulates a 500-calorie limit on 2 days per week (see ‘Three forms of fasting’).

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