In a blow to believers in time-restricted eating – a form of intermittent fasting whereby people eat only during a restricted window of time each day – researchers in China have found the method has no significant benefits compared with simple calorie restriction in battling obesity.
“Our data suggest that caloric intake restriction explained most of the beneficial effects of a time-restricted eating regimen,” the researchers at Southern Medical University in Guangzhou, China, said.
“Bottom line, the determinant of weight loss, as well as reductions in body fat, visceral fat, blood pressure, and glucose and lipid levels, is dependent on reducing calorie intake, regardless of the distribution of food and beverages consumed throughout the day,” Tufts University Cardiovascular Nutrition Laboratory director and senior scientist Alice Lichtenstein said. She was not involved in the study.
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The study, published Thursday in the New England Journal of Medicine, split 139 overweight to significantly obese adults in Guangzhou into two study groups that were followed for a year.
One group was told to limit their daily food intake to 1500-1800 calories a day for men, and 1200-1500 calories a day for women. Men and women in the other group were told to eat the same amount of calories, but only between 8am and 4pm.
To track compliance, study participants were asked to keep food diaries and photograph everything they ate.
At the end of the year, both groups lost weight – about 6.4 to 8.2kg – but it was found that eating on a time-restricted schedule did not produce any significant difference in weight loss between the two groups.
Nor was there any real difference in other markers of weight loss, such as BMI (body mass index, a popular way of measuring weight), waist circumference, body fat or metabolic risk factors such as insulin resistance and blood pressure.
Prior research showed benefits
Prior research had shown advantages to time restriction.
A 2020 review of human and animal studies had found benefits to restricting calories to a shortened period of the day, including a reduction in blood pressure, weight loss and improved longevity.
However, a number of those studies were in mice while those in humans were of much shorter duration – mere months.
The results of the year-long clinical trial are not surprising, according to Dr David Katz, a specialist in preventive and lifestyle medicine and nutrition, who was not involved in the study.
“Restricting time instead of calories does not contribute metabolic magic; rather, it is a form of discipline that helps to control calories,” Katz said.
“My favourite approach is to focus on the quality of foods and diet, and then let quantity control take care of itself. A diet made up mostly of whole, wholesome plant foods is intrinsically satiating at low calorie levels,” he said.
“Decisions on the approach used to restrict calorie intake should be determined by what works best for an individual within the context of their lifestyle,” Lichtenstein, who is also a Gershoff Professor of Nutrition Science and Policy, said.
“The most important factor is identifying an approach that is sustainable in the long term.”
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