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A three-month study challenges the weight-loss claims of the popular intermittent-fasting (IF) protocol — and suggests that pounds shed was mostly lean muscle mass.

Practitioners of IF, also known as time-restricted eating, eat only during a shortened window — such as noon to 8 p.m. — resulting in a fast that often occurs overnight.

The randomized clinical study by University of California, San Francisco researchers, pub­lished in JAMA Internal Medicine, included 116 adults considered overweight or obese by BMI standards. Participants followed the 16:8 protocol (fasting hours to eating hours) and were allowed to eat and snack as they wished, except during the nighttime fast. The study did not control for other factors, such as smoking, drinking, exercise, stress, or sleep — all of which play roles in insulin sensitivity and weight.

Scientists found that during the three-month study, IF resulted in a modest average loss of just 2 to 4 pounds — only slightly more than a control group. “Time-restricted eating, in the absence of other interventions, is not more effective in weight loss than eating throughout the day,” they concluded.

More troubling was that most of the weight shed was not body fat but lean mass, which includes muscle. The results echo a similar four-month randomized controlled trial of 309 participants in 2014, published in the American Journal of Clinical Nutrition.

Other studies, however, have found IF to be a more promising weight-loss strategy.

Proponents maintain that no eating protocol works for everyone. They also argue that weight loss is not IF’s only benefit, pointing to potential pluses for digestive health, hormone regulation, mitochondrial health, and cancer prevention. (For more on the protocol, see “Everything You Need to Know About Intermittent Fasting“.)

This article originally appeared as “A Closer Look at Intermittent Fasting and Weight Loss” in the April 2021 issue of Experience Life.

Michael Dregni

Michael Dregni is an Experience Life deputy editor.

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