There is no additional weight-loss benefit from restricting eating times among adults who are already following a calorie-restricted diet, according to a new study published in the New England Journal of Medicine.
Time-restricted eating — also sometimes known as intermittent fasting, which is a less specific term — has gained awareness in recent years as an eating strategy that proponents say can have several health benefits — including improved memory, better cardiovascular health, and less body fat. While there is no set definition for time-restricted eating, many people practice it by limiting their food intake to an eight-hour or 12-hour period each day. This means, of course, that they’re not eating for a period lasting 12 hours or 16 hours each day. Some research suggests that when people don’t eat for such a long period of time, the body switches its main energy source from glucose to stored body fat, which may potentially improve blood glucose levels as well as aid in weight loss. One recent study showed that participants with obesity who restricted their eating to an eight-hour period each day, but made no other changes, lost significantly more weight than participants who didn’t rigorously practice this time restriction.
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But studies like the one just mentioned leave an important question unanswered — do time-restricted diets lead to weight loss because of the time restriction, or simply because people tend to eat less when they have less time to eat? To help answer this question, researchers at Southern Medical University in Guangzhou, China, designed a study in which two groups of participants with obesity followed a diet with the same calorie restrictions — both groups were limited to 1,500 to 1,800 calories per day for men and 1,200 to 1,500 calories per day for women. But some of the 139 participants were randomly assigned to also restrict their daily eating to an eight-hour period — 8:00 a.m. to 4:00 p.m. — for 12 months. In addition to overall weight loss, researchers were interested in any differences between he two groups when it came to waist circumference, body-mass index (BMI, a measure of body weight that takes height into account), body fat percentage, and metabolic risk factors like blood pressure, blood lipids (cholesterol and triglycerides), blood glucose, and insulin sensitivity, as noted in an article on the study at The New York Times.
No clear weight-loss benefit found to intermittent fasting
Among the 118 participants who completed a follow-up visit after 12 months, there was an average weight loss of 8.0 kilograms (17.6 pounds) in the time-restricted eating group and 6.3 kilograms (13.9 pounds) in the group that only followed calorie restrictions. But this difference wasn’t large enough to reach statistical significance — meaning that it could have been due to chance. The researchers also found no significant differences between the two groups in the secondary outcomes — waist circumference, BMI, body fat, or metabolic risk factors. There were also no substantial differences between the two groups in adverse events like a heart attacks or stroke.
These results are consistent with those of past studies on time-restricted diets, in the sense that it showed a potential benefit — but a very small one, if it was in fact an actual benefit. The much larger weight-loss benefit in this study came from restricting the total number of calories that participants consumed. It’s possible, though, that time-restricted eating could help some people follow a calorie-restricted diet because of the added structure it offers, leaving fewer opportunities for snacking and ensuring that most calories come from planned meals. And if you’re not rigorously limiting the number of calories you consume, restricting your eating period might help you reduce your caloric intake — as long as you don’t “make up” for the limited eating period by eating more food than you normally would during meals or snacks.