Weight Loss

Intermittent Fasting Update

Slim down. Cure your cancer. Prevent heart attacks. Live longer. Improve joint health. Glucose control. Reports that intermittent fasting could help with the biggest health concerns facing people have inspired millions to take it up. Initially the data seemed to agree- it works. I know many patients and friends who advocate the benefits and tout the results. But a new randomized clinical study just published in JAMA 9/28/20 says otherwise.

Intermittent fasting does not result in greater weight loss compared to non restricted meal timing. The participants were told to eat only between 12:00 pm and 8:00pm and abstain from eating at all other times. They were not given any specific instructions regarding caloric or macronutrient intake “so as to offer a simple, real-world recommendation to free-living individuals,” according to the authors.

Although some prior research has shown improvements in measures such as glucose tolerance with time-restricted eating, studies showing weight loss with this approach, including one recently reported by Medscape Medical News, have been small and lacked control groups.

In the study published in JAMA, participants had a mean weight of 99.2 kg (approximately 219 lbs). 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 (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 lost on average 0 .94 kg  while weight loss in the consistent meal group averaged 0.68 kg. But more importantly, the difference in weight loss between the groups, .26 kg was not clinically significant. And there were no significant differences in secondary outcomes of fasting insulin, glucose, HgA1c (measures diabetic control), or blood Lipids within or between the time-restricted eating and consistent meal timing group either. There were also no 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.

The 12 pm to 8 pm window for eating was chosen because the authors thought people might find it easier culturally to skip breakfast than dinner, the more social meal. In retrospect the authors speculated that an 8 pm cut-off is somewhat late given there is some suggestion that fasting several hours before bedtime is more beneficial. So it may be worth examining different time windows. The study results did support previous research showing no effect on weight outcomes in relation to skipping breakfast. 

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 fact, as much as 65% of the weight lost in the time-restricted eating group consisted of lean mass, while only a third was fat mass.

According to the authors, “The proportion of lean mass loss in this study (approximately 65%) far exceeds the normal range of 20% to 30%. In addition, there was a highly significant between-group difference in the type of lean mass lost. This group lost more 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 patients because time-restricted eating could exacerbate muscle loss,” the authors assert. Which is even more concerning since previous studies have suggested that the loss of lean mass is positively linked with weight regain.

Not all weigh loss is beneficial

Losing 1 kg of lean mass is not equal to a kg of fat. In fact, if one loses 0.65 kg of lean mass and only 0.35 kg of fat mass, that is probably an intervention they should pass on.

Although the prescribed (12-8 p.m.) eating window is likely more attractive and more amenable to long-term adherence, it might have impacted the results. Researchers recommended future studies should be aimed at understanding the effects of early vs late time-restricted eating and protein intake or timing as a means to offset the loss in appendicular lean mass.


Time-restricted eating 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. In fact the JAMA study showed self-reported adherence to the time-restricted eating schedule was high; it just didn’t result in greater weight loss when compared with the consistent meal timing and the weight loss was not all fat. Just losing weight alone doesn’t mean good things are happening for your health. 


-jamanetwork.com/journals/jamainternalmedicine/article-abstract/2771095

-medscape.com/viewarticle/902221?src=ppc_google_rlsa-traf_mscp_news-perspectives-hdle-cohort_md_us#vp_2

-advisory.com/daily-briefing/2020/10/09/intermittent-fasting

-health.harvard.edu/blog/intermittent-fasting-surprising-update-2018062914156

-nejm.org/doi/full/10.1056/NEJMra1905136

-medpagetoday.com/primarycare/dietnutrition/88850

-pubmed.ncbi.nlm.nih.gov/29754952/

-insider.com/doctor-no-longer-supports-fasting-diet-study-found-few-benefits-2020-10

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