Fasting diets not a ‘magical’ solution for weight loss, study finds

Researchers conclude that the amount of calories matters more than when you eat them.

The study groups lost similar amounts of weight, while fasting made no significant difference to blood pressure. Illustration: The Sunday Times

Trendy intermittent fasting diets offer no “magical” weight loss solution and it is more important to stay focused on cutting calories, scientists have found.

The practice of intermittent fasting – eating only during a set time window each day such as noon to 8pm – has been credited by celebrities as the secret to staying slim.

But a trial in obese patients found that restricting eating to certain hours offered no additional weight-loss benefit compared with eating normally throughout the day, if people consumed the same calories overall.

Scientists said it showed “there is nothing magical about time-restricted eating” and there was “not a shred of evidence” that it ramped up metabolism or boosted fat burning. While fasting may be effective for weight loss, this was simply because it helped people to eat less, the study concluded.

Many popular diets, including the 5:2 and 16:8 plans, are based on the principle of restricting food to specified periods. Previous research has suggested that fasting helps people to lose weight, as well as providing other health benefits such as cutting inflammation.

Among the disciples of intermittent fasting is Rishi Sunak, who fasts for 36 hours at the start of each week. Other high-profile fans include George Osborne, Jennifer Aniston and Hugh Jackman.

Rishi Sunak starts each week by fasting for 36 hours. Picture: WPA Pool/Getty Images

Jennifer Aniston is another disciple of intermittent fasting. Picture: Getty Images

The trial, led by Johns Hopkins University in Baltimore, involved 41 obese patients in America, who were given exactly the same daily meals over a three-month period. Half of the participants were allowed to eat only within a 10-hour window, whereas the others spread their meals over the course of the day, between 8am and midnight.

After 12 weeks, both groups had lost similar amounts of weight, with the fasting group losing 2.3kg on average, and the normal-eating-habits group losing slightly more at 2.6kg.

The scientists also did tests to see if fasting helped with other measures of health, but found no significant difference between the two groups in blood pressure, blood sugar levels or waist circumference.

Researchers from the University of Chicago said that the paper, published in Annals of Internal Medicine, had “important clinical implications”. They said that time-restricted eating “is no more effective than other diet interventions for weight reduction” but that it could offer an easy way to help people eat less.

Commenting on the findings, Keith Frayn, emeritus professor of human metabolism at the University of Oxford, said: “The finding that time-restricted eating results in no more weight loss than expected for the reduction in calories taken in should surprise nobody. If time-restricted eating were to have an effect on body weight beyond calorie restriction, that would imply that it ‘ramps up metabolism’, for which there is not a shred of evidence in humans.”

Frayn cautioned that the study was relatively small, and therefore could not determine whether or not fasting helped with blood sugar control and heart health.

Professor Naveed Sattar, professor of cardiometabolic medicine at the University of Glasgow, said: “This relatively small trial seems well done and tells us what we expected – that there is nothing magical about time-restricted eating on weight change other than effects to reduce caloric intake. In the end, we cannot reinvent physics – less calories in, however achieved, means lower weight over time. If time-restricted eating helps some people eat less calories than they would otherwise, great.”

Fasting has become fashionable over the past decade, leading to growing numbers of people in Britain skipping breakfast and eating within a set “window”. Proponents of the diet, including Professor Tim Spector and Dr Michael Mosley, argue that it helps with weight loss, as well as boosting gut health and immunity.

Some studies have shown that some types of fasting could reduce belly fat and can cut inflammation, but the mechanism by which this works is unclear. Several scientists suspect that long periods of fasting may give our bodies time to perform the repair and maintenance tasks that keep cells healthy. This can reduce inflammation, which is linked to chronic diseases including heart disease and dementia.

The Times