The popular new weight-loss approach of eating within a restricted window of time during the day, allowing for an extended period of fasting – also known as intermittent fasting – does not result in greater weight loss, compared with nonrestricted meal timing, results from a randomized clinical trial show.
“I was very surprised by all of [the results],” senior author Ethan J. Weiss, MD, said in an interview.
“Part of the reason we did the study was because I had been doing time-restricted eating myself for years and even recommending it to friends and patients as an effective weight-loss tool,” said Dr. Weiss, of the Cardiovascular Research Institute, University of California, San Francisco.
“But no matter how you slice it, prescription of time-restricted eating – at least this version –is not a very effective weight-loss strategy,” Dr. Weiss said.
The study, published online in JAMA Internal Medicine by Dylan A. Lowe, PhD, also of the University of California, San Francisco, involved 116 participants who were randomized to a 12-week regimen of either three structured meals per day or time-restricted eating, with instructions to eat only between 12:00 p.m. and 8:00 p.m. and to completely abstain from eating at other times.
The participants were not given any specific instructions regarding caloric or macronutrient intake “so as to offer a simple, real-world recommendation to free-living individuals,” the authors wrote.
Although some prior research has shown improvements in measures such as glucose tolerance with time-restricted eating, studies showing weight loss with the approach, including one recently reported by Medscape Medical News, have been small and lacked control groups.
“To my knowledge this is the first randomized, controlled trial and definitely the biggest,” Dr. Weiss. “I think it is the most comprehensive dataset available in people, at least for this intervention.”
Participants used app to log details
At baseline, participants had a mean weight of 99.2 kg (approximately 219 lb). Their mean age was 46.5 years and 60.3% were men. They were drawn from anywhere in the United States and received study surveys through a custom mobile study application on the Eureka Research Platform. They were given a Bluetooth weight scale to use daily, which was connected with the app, and randomized to one of the two interventions. A subset of 50 participants living near San Francisco underwent in-person testing.
At the end of the 12 weeks, those in the time-restricted eating group (n = 59) did have a significant decrease in weight, compared with baseline (−0.94 kg; P = .01), while weight loss in the consistent-meal group (n = 57) was not significant (−0.68 kg; P = .07).
But importantly, the difference in weight loss between the groups was not significant (−0.26 kg; P = .63).
There were no significant differences in secondary outcomes of fasting insulin, glucose, hemoglobin A1c, or blood lipids within or between the time-restricted eating and consistent-meal group either. Nor were there any significant differences in resting metabolic rate.
Although participants did not self-report their caloric intake, the authors estimated that the differences were not significant using mathematical modeling developed at the National Institutes of Health.
Rates of adherence to the diets were 92.1% in the consistent-meal group versus 83.5% in the time-restricted group.