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October 9, 2020

Does intermittent fasting work? Here's what a new study reveals.

Daily Briefing
    Editor's note: This popular story from the Daily Briefing's archives was republished on Sep. 14, 2021.

    Intermittent fasting has emerged as one of the most-hyped diet trends in recent years but, to date, there have been almost no human studies evaluating the practice. Now, a study in JAMA Internal Medicine provides some insight into whether intermittent fasting helps people lose weight.

    Cheat sheets: Evidence-based medicine 101

    Study details

    For the study, 116 participants who had a body mass index (BMI) that categorized them as overweight or obese were randomly assigned to one of two groups. In one group, researchers instructed participants to follow a time-restricted eating (TRE) plan, under which they were told that they could eat whatever they wanted between 12:00 p.m. and 8:00 p.m., but they couldn't take in any calories between 8:00 p.m. and 12:00 p.m. the following day.

    For the other group, which the researchers called the "consistent meal timing (CMT) group," researchers instructed participants "to eat three structured meals per day," according to the study. The researchers also allowed the CMT group to snack between meals if they desired.

    The researchers did not provide either group with recommendations for caloric or nutritional intake or physical activity. Each participant received a Bluetooth-connected scale, which the researchers used to track the participants' weight and calculate participants' BMIs.


    After 12 weeks, the researchers found that participants in the TRE group lost an average of about two pounds, while those in the control group lost an average of about 1.5 pounds—a difference that the researchers said was not "statistically significant."

    The researchers also found no significant difference between the two groups when it came to changes in whole body fat mass, lean mass, or cardiovascular health markers, including systolic and diastolic blood pressure.

    However, the researchers found that participants in the TRE group appeared to have lost more muscle mass than those in the CMT group, though Ethan Weiss, a cardiologist at the University of California-San Francisco and lead author on the study, said that finding wasn't definitive and requires more research.

    What do the findings mean?

    The researchers wrote that the results of their study are "consistent with a prior study demonstrating that a recommendation to skip breakfast does not affect weight outcomes in patients trying to lose weight." Further, they noted that their findings "contradict previous reports describing the beneficial effects of TRE on weight loss and other metabolic risk markers."

    Weiss said he believes the so-called "placebo effect" may have led to both groups losing weight, as people often will pay closer attention to their diets when they're enrolled in a nutrition study. He added that, overall, people should be skeptical of the findings generated by any nutritional study on weight loss that does not include a control group (as does this latest study).

    And Weiss added that he's not yet ready to give up on intermittent fasting, which he previously practiced. Weiss said his study had participants skip meals in the morning, but it did not study the effects of skipping meals at night, and he theorized that there may be benefits to practicing intermittent fasting at different times of the day.

    But Weiss said he won't be recommending intermittent fasting to his patients for the time being. "Just losing weight alone doesn't mean good things are happening for your health," he said (Farr, CNBC, 9/28; Monaco, MedPage Today, 9/28).

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