Does intermittent fasting work?
Summary
- It does for weight loss. Its other supposed benefits are debatable
Diets come and diets go. One of the most popular today is “intermittent fasting" in which, as the name suggests, the idea is to limit one’s food intake to certain time windows. One popular variant, the “5-2 diet", requires people to eat either very small amounts, or nothing at all, on two days a week, but imposes no restrictions on the other five.
Intermittent fasting is popular. And as a weight-loss strategy, it has several things going for it. One is that it is uncomplicated. There is no need to weigh the ingredients of every meal, as some diets demand, nor to change what you eat drastically. Limiting the restrictions to a couple of days a week, or several hours a day (most of which are spent asleep) also requires less willpower, which might make it easier to stick with.
Working out whether that actually translates into greater weight loss than other diets is difficult. Most studies find limited data and mixed results. The general consensus, says Nichola Ludlam-Raine, a dietitian and spokeswoman for the British Dietetic Association, is that intermittent fasting seems to work roughly as well for weight loss as traditional calorie-counting does.
Other health benefits might also beckon. Forcing lab animals to fast (albeit not intermittently) can increase their lifespans by up to 40%. It also appears to mitigate the physical decline that comes with old age, boost various markers of metabolic health and even reduce susceptibility to cancer.
Exactly how it does all that is not entirely clear. One important factor seems to be autophagy, the process by which cells break down and recycle parts of themselves. Cells become much keener on autophagy when nutrients are scarce. At the same time, autophagy seems to have a preference for attacking damaged and degraded parts of cells—and the accumulation of such cellular detritus is one of several mechanisms thought to underlie the decrepitude that comes with ageing.
The hope is that intermittent fasting might provoke a similar response in humans. There are theoretical reasons to think it might: the cellular mechanisms triggered by food shortages seem to have been conserved by evolution in all sorts of different animals. But running definitive human trials of the sort done on lab animals is impossible. “When we say ‘calorie restriction’ we mean nearly starving [the animals]," says Adam Collins, a nutrition researcher at the University of Surrey.
That leaves scientists reliant, for now, on smallish, short-lived studies that use less drastic diets and which rely on proxy measures of health such as insulin response or cholesterol levels. Their results are mixed. Dr Collins’s team, for instance, has published a randomised-control trial (the most rigorous sort) suggesting that intermittent fasting improves the metabolism of fats more than ordinary dieting does. A review paper published in April 2024 looked at 23 other studies and concluded that intermittent fasting was slightly better than ordinary dieting for overweight people when it came to improving levels of cholesterol and insulin. A similar article, published in January, found no meaningful difference for either weight loss or cardiovascular health.
There are also risks. A study in mice published in Nature in October 2024 found that severe fasting (where calories were cut by 40%) had downsides, including muscle mass loss and, possibly, weakened immune systems. Moderation, too, should be taken in moderation.
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