Intermittent Fasting: Metabolic Hack or Mind Trick for Weight Loss?

EBM Focus - Volume 17, Issue 22

Reference: N Engl J Med. 2022 Apr 21;386(16):1495-1504

Intermittent fasting is a weight loss method which involves taking regular breaks from eating (typically 12 hours or longer) that has gained attention for not only weight loss but its potential to reduce oxidative stress and improve metabolic markers and gut health. Despite intermittent fasting’s ancient origins across numerous cultures and growing popularity, evidence for its effect on weight loss and other benefits remains uncertain. A trial recently published in NEJM studied the effects of time-restricted eating (a type of intermittent fasting) on weight loss and metabolic factors in adults with overweight and obesity.

Investigators in Guangzhou, China randomized 139 adults (mean age 32 years old, mean weight 88 kg) to calorie restriction plus time-restricted eating (with an 8-hour feeding window between‌ 8‌ am and 4‌ pm) or calorie restriction alone for 12 months. In both groups, daily calorie limits were 1500-1800 kcal for men and 1200-1500 kcal for women, with 40-55% carbohydrates, 15-20% protein, and 20-30% fat. Participants were asked to regularly photograph and log food intake and the time at which they ate on a mobile app. They also received dietary counseling via phone calls, messages through the app, and both individualized and group coaching sessions. Both groups were provided with a daily protein shake for the first 6 months of the trial and were instructed to maintain normal physical activity throughout. The primary outcome was change from baseline body weight at 12 months. Secondary outcomes included changes in waist circumference, body composition, and metabolic risk factors including serum glucose, insulin sensitivity, serum lipids, and blood pressure.

Of the 139 participants randomized, 118 (84.9%) completed the study. Adherence was > 80% in both groups and the average caloric deficit and macronutrient intake were similar. At baseline, the enrolled participants ate during a 10.5 hour window during the day. At one year, the mean weight change from baseline was -8.0 kg (17.6 lbs) in the time-restricted group and -6.3 kg (13.9 lbs) in the daily calorie restriction group, but the difference between the two groups was not significant (-1.8 kg; 95%‌ CI, -4.0‌ to 0.4; P = 0.11). Both groups had similar improvement in BMI, waist circumference, body composition, metabolism and blood pressure. There were no serious adverse events in either group.

This trial was designed to assess whether adding time-restriction to calorie reduction improves weight loss. While no extra benefit of time-restricted eating was demonstrated in this trial, participants in each group lost about 15 lbs and continued the diet for a year, which is nothing to scoff at. Certain aspects of the study design may pose some limitations, however. It’s possible that reducing eating time from 10.5 to 8 hours wasn’t a large enough change to make a difference. We also don’t know how cultural-specific eating patterns may have influenced these results. The amount of effort participants did recording their food intake may also limit real-world applicability. Attention control or time-restriction-without-calorie-restriction arms would have been informative. Participants in the study were also relatively healthy which may have blunted any additional effects of time-restriction on metabolic risk factors. Finally, it’s also possible that the study was underpowered, as greater weight loss (and most secondary outcomes) favored the time-restricted group. Although the present results suggest that the addition of time-restriction to calorie limits may not be all that helpful, we can at least say that this approach seems to be a safe and sustainable way to lose about 15 lbs in a year. We could use a little more time, however, to parse out the specific effects of this particular weight loss method on its own.

For more information, see the topic Diets for Weight Loss in DynaMed.

DynaMed EBM Focus Editorial Team

This EBM Focus was written by Nicole Jensen, MD, Family Physician at WholeHealth Medical. Edited by Alan Ehrlich, MD, Executive Editor at DynaMed and Associate Professor in Family Medicine at the University of Massachusetts Medical School; Katharine DeGeorge, MD, MS, Deputy Editor at DynaMed and Associate Professor of Family Medicine at the University of Virginia; Dan Randall, MD, Deputy Editor at DynaMed; Carina Brown, MD, Assistant Professor at Cone Health Family Medicine Residency; Vincent Lemaitre, PhD, Senior Medical Writer at DynaMed; and Sarah Hill, MSc, Associate Editor at DynaMed.