Time-restricted eating — that is, reducing the number of hours a person is allowed to eat during the day — may produce a modest 1% to 4% weight loss, even without cutting calories, early studies in humans suggest. But more research is needed to provide definitive evidence.
This type of intermittent fasting also appears to improve blood glucose, blood pressure, and oxidative stress, said Courtney M. Peterson, PhD, a researcher at the University of Alabama at Birmingham, summarizing what is known about the potential weight-loss strategy at the virtual American Diabetes Association (ADA) 81st Scientific Sessions.
The best results were seen with early time-restricted eating (that is, ending the nighttime fasting early in the day) and allowing a person to eat 8 to 10 hours each day (eg, 8 AM to 4 PM or 8 AM to 6 PM), with fasting and only water allowed the remaining hours, she reported.
However, the three dozen or so studies in humans to date are mainly small, pilot, or single-arm studies lasting up to 3 months, and there are only three main randomized controlled trials with 25 or more participants in each group.
Large trials with around 260 participants are needed, Peterson said, “before drawing definitive conclusions” about the weight-loss and cardiometabolic benefits of time-restricted eating.
Invited to comment, session chair Lisa S. Chow, MD, an associate professor of medicine in the endocrine and diabetes division at the University of Minnesota in Minneapolis, similarly said: “I think time-restricted eating is promising because of its simple message and noted weight loss benefit, yet more data are needed.”
“Many uncertainties remain,” she added, “including the potential concern that time-restricted eating may be associated with lean [muscle] mass loss (Obesity. 2020;28:860-869; JAMA Intern Med. 2020;180:1491-1499) and identifying the populations most likely to benefit from time-restricted eating,” she said in an email.
36 Small Studies, a Review, a Meta-Analysis, Three RCTs
There have been about three dozen small studies of time-restricted eating in humans, which examined 4- to 11-hour eating windows, Peterson explained.
A systematic review of 23 trials of time-restricted eating reported that, on average, participants lost 3% of their initial weight (Nutrients. 2020;12:3770). And a meta-analysis of 19 trials in 475 participants found a –0.9 kg mean difference effect for weight loss (Nutrients. 2020;12:1267).
However, those two analyses did not compare time-restricted eating with a control treatment, she stressed.
The largest randomized controlled trial is a 12-week study in 271 adults with nonalcoholic fatty liver disease in China (BMC Gastroenterol. 2019;19:219), Peterson said.
The researchers compared three groups:
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Alternate-day modified fasting: healthy meal provided.
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Time-restricted eating: 8-hour window, healthy meal provided.
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Control: 20% calorie reduction, no meal provided.
At 4 and 12 weeks, adults in the two treatment groups lost more weight than those in the control group, but “this was not a fair comparison” due to the lack of a provided meal in the control group, Peterson pointed out.
The next largest randomized controlled study is the 12-week TREAT trial, published online in JAMA Internal Medicine in October 2020, as reported by Medscape Medical News.
The researchers, from the University of California, San Francisco, randomized 116 adults into two groups:
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8-hour time-restricted eating from noon to 8 PM.
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Control: 3 meals/day.
Time-restricted eating did not lead to greater weight loss compared with three structured meals a day, which was not surprising, Chow said, as “participants just reported whether they were engaged in time-restricted eating in a yes/no answer.”
Moreover, “there was no objective measure of their eating window. From our study, we showed that the extent of eating window restriction matters, not just time-restricted eating participation (Obesity. 2020;28:860-869).”
Also, in TREAT, the eating window was noon to 8 PM (considered late for time-restricted eating), and the trial also allowed noncaloric beverages outside the window, whereas most studies only allow water and medications.
Lastly, TREAT showed that time-restricted eating reduced weight compared with baseline, but the weight loss was not significant compared with the control group, and there was a wide spread of effects (ie, some lost a lot of weight, others didn’t lose much weight).
“That being said, the JAMA Internal Medicine paper is the largest paper to date of time-restricted eating randomized versus control, so its findings need to be acknowledged and recognized,” Chow said.
Peterson reported that her group recently completed a 14-week intervention in 90 adults with obesity divided into two groups:
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Control: Continuous energy restriction, self-selected ≥ 12-hour window.
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Early time-restricted eating: 8-hour window from 7 AM to 3 PM.
The findings will provide further insight into the benefits of time-restricted eating.
How Might Time-Restricted Eating Lead to Weight Loss?
Peterson concluded by presenting data suggesting how time-restricted eating may induce weight loss.
In a 4-day crossover study in 11 overweight adults, time-restricted eating did not affect energy expenditure, but it lessened swings in subjective hunger, improved appetite hormones including ghrelin, and increased fat oxidation (Obesity. 2019;27:1244-1254).
Most trials have reported that time-restricted eating improves one or more cardiometabolic endpoints (J Transl Med. 2016;14:290), she noted.
Early time-restricted eating was associated with improved insulin sensitivity and secretion, blood pressure, and oxidative stress, but not better lipid levels (Metabolism. 2007;56:1729-1734).
In contrast, compared with eating 3 meals/day (control), late time-restricted eating (eating 1 meal/day from 5 PM to 9 PM) was associated with worsened cardiometabolic health (glucose, insulin, blood pressure, and lipid levels) in an 8-week crossover study in 15 participants (Metabolism. 2007;56:1729-1734).
Peterson and Chow have reported no relevant financial relationships.
ADA 2021 Scientific Sessions. Presented on June 25, 2021.
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