In a new study published Wednesday in the New England Journal of Medicine, researchers from Southern Medical University in China found that restricting calorie consumption to an 8-hour window did not result in significantly higher weight loss.
Study details and key findings
For the study, researchers conducted a randomized trial with 139 participants with obesity over a 12-month period. Participants were randomly assigned either to a group that followed time-restricted eating—only eating between 8 a.m. and 4 p.m.—with a daily calorie restriction or a group that only followed a daily calorie restriction.
During the study period, participants followed a calorie-restricted diet of 1,200 to 1,500 calories per day for women and 1,500 to 1,800 calories per day for men. To ensure each participants' compliance, individuals in both groups were required to keep food diaries and photograph everything they ate.
Overall, 97.1% of participants adhered to their assigned diets for six months, and 84.9% completed the 12-month follow-up visit.
Over the course of the 12-month study, both groups lost a significant amount of weight. On average, participants who followed the time/calorie-restricted diet lost eight kilograms, and participants who only followed a calorie-restricted diet lost around six kilograms.
At six and 12 months, there were no significant differences between the two groups for other metabolic outcomes, including BMI, waist circumference, and lean body mass. However, participants from the time-restricted eating group did show slightly higher improvements at 12 months.
The bottom line, according to Ethan Weiss, a diet researcher at the University of California, San Francisco, "[t]here is no benefit to eating in a narrow window."
"These results indicate that caloric intake restriction explained most of the beneficial effects seen with the time-restricted eating regimen," Weiss added.
In an accompanying editorial, Blandine Laferrère, of the Columbia University Irving Medical Center, and Satchidananda Panda, of the Salk Institute for Biological Studies called the baseline window of 10 hours and 23 minutes "relatively short." According to the editorial, the fact that the time-restricted diet only reduced the average window of eating by about two hours could be the reason there was not a significant difference between the two diets.
"Persons whose habitual time period for eating is long are likely to benefit the most from time-restricted eating," Laferrère and Panda noted, while adding that "the reduced caloric intake may have blunted any effect of time-restricted eating independent of calorie restriction."
In addition, the authors of the editorial said that the study's participants were generally healthy at baseline, which could have "left little room to test whether time-restricted eating plus calorie restriction has a significantly greater effect on cardiometabolic risks than calorie restriction alone, as has been observed in studies that involved patients with metabolic syndrome."
For future research, Laferrère and Panda suggested further investigating the specifics of time-restricted eating. In particular, identifying the best window of time for time-restricted eating—including whether there is a difference between eating early or late in the day—and determining who could benefit most from this strategy.
"From a public health point of view, time-restricted eating may turn out to be an approach to accomplish calorie restriction and improve metabolic health without the resource-intensive approach of intentional calorie restriction," the authors concluded. (Kolata, New York Times, 4/20; Monaco, MedPage Today, 4/20; Liu et al., New England Journal of Medicine, 4/21)