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, told Medscape Medical News.
“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 Weiss, of the Cardiovascular Research Institute, University of California, San Francisco (UCSF).
“But no matter how you slice it, prescription of time-restricted eating — at least this version — is not a very effective weight loss strategy,” Weiss said.
The study, published online in JAMA Internal Medicine by Dylan A. Lowe, PhD, also of UCSF, 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 pm and 8:00 pm 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 write.
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,” Weiss told Medscape Medical News. “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 (app) 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, A1c, or blood lipids within or between the time-restricted eating and consistent meal timing 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-timing group versus 83.5% in the time-restricted group.
Not All Diets Are Equal: Time-Restricted Eating Group Lost More Lean Mass
In a subset analysis, loss of lean mass was significantly greater in the time-restricted eating group, compared with the consistent meals group, in terms of both appendicular lean mass (P = .009) and the appendicular lean mass index (P = .005).
In fact, as much as 65% of the weight lost (1.10 kg of the average 1.70 kg) in the time-restricted eating group consisted of lean mass, while much less was fat mass (0.51 kg).
“The proportion of lean mass loss in this study (approximately 65%) far exceeds the normal range of 20% to 30%,” the authors say. “In addition, there was a highly significant between-group difference in appendicular lean mass.”
Appendicular lean mass correlates with nutritional and physical status, and its reduction can lead to weakness, disability, and impaired quality of life.
“This serves as a caution for patient populations at risk for sarcopenia because time-restricted eating could exacerbate muscle loss,” the authors assert.
Furthermore, previous studies suggest that the loss of lean mass in such studies is positively linked with weight regain.
While a limitation of the work is that self-reported measures of energy or macronutrient or protein intake were not obtained, the authors speculate that the role of protein intake could be linked to the greater loss of lean mass.
“Given the loss of appendicular lean mass in participants in the time-restricted eating arm and previous reports of decreased protein consumption from time-restricted eating, it is possible that protein intake was altered by time-restricted eating in this cohort, and this clearly warrants future study,” they say.
Weiss said the findings underscore that not all weight loss in dieting is beneficial.
“Losing 1 kg of lean mass (is not equal) to a kg of fat,” he said. “Indeed, if one loses 0.65 kg of lean mass and only 0.35 kg of fat mass, that is an intervention I’d probably pass on.”
Time-Restricted Eating Is Popular, Perhaps Because It‘s Easy?
Time-restricted eating has gained popularity in recent years.
The approach “is attractive as a weight-loss option in that it does not require tedious and time-consuming methods such as calorie-counting or adherence to complicated diets,” the authors note.
“Indeed, we found that self-reported adherence to the time-restricted eating schedule was high; however, in contrast to our hypothesis, there was no greater weight loss with time-restricted eating compared with the consistent meal timing.”
They explain that the 12 pm to 8 pm window for eating was chosen because they thought people might find it easier culturally to skip breakfast than dinner, the more social meal.
However, an 8 pm cut-off is somewhat late given there is some suggestion that fasting several hours before bedtime is most beneficial, Weiss noted. So it may be worth examining different time windows.
“I am very intrigued about looking at early time-restricted eating – 6 am to 2 pm,” for example, he said. “It is on our list.”
Meanwhile, the study results support previous research showing no effect on weight outcomes in relation to skipping breakfast.
The study received funding from the UCSF Cardiology Division’s Cardiology Innovations Award Program and the National Institute of Diabetes and Digestive and Kidney Diseases, with additional support from the James Peter Read Foundation. Weiss has reported nonfinancial support from Mocacare and nonfinancial support from iHealth Labs during the conduct of the study. He also is a cofounder and equity stakeholder of Keyto, and owns stock and was formerly on the board of Virta. Disclosures for the other authors are listed in the article.
JAMA Intern Med. Published September 28, 2020. Abstract