Intermittent fasting study find dieters shed muscle, not fat

October 2020

Intermittent fasting, also known as time restricted eating, is the “in” diet right now, but a new study revealed surprising results. Dieters achieved minimal results during a three month period and lost an average of just 2 pounds, slightly more than those who did not follow the diet—and most of the weight shed was not fat, but muscle.

University of Hawaiʻi Cancer Center researcher John Shepherd worked in collaboration with primary investigator Ethan Weiss, a cardiologist, and graduate student Dylan Lowe, both from the University of California San Francisco, and other colleagues on the Time-Restricted Eating (TREAT) study published in JAMA Internal Medicine . The article was rated one of the most popular among readers after its release on September 28.

During the study, researchers observed 116 overweight and obese participants and split them in two groups. Time restricted eaters (TRE) were instructed to eat their meals during an 8-hour window between 12 p.m. and 8 p.m. and abstain from caloric intake from 8 p.m. to 12 p.m. the following day, giving them a 16-hour fast. Those in the consistent meal timing (CMT) group ate three structured meals during the day.

The study only included requirements to the timing of food intake and did not provide requirements for calorie and macronutrient intake or physical activity. Participants were given a digital scale and asked to track their weight for 12 weeks. Fifty of the participants in the TRE group underwent in-person testing to measure fat and muscle mass, blood sugar levels and energy expenditure.

“Both groups lost weight but there wasn’t a significant difference in weight loss between the TRE and the CMT groups, and that was surprising,” said Shepherd. “The weight loss on average was very modest, about 2 pounds in 12 weeks for the TRE group compared to no statistically significant loss for the CMT group.”

According to Shepherd and the researchers, even more significant was reduction in lean mass in the TRE group. For those tracked in-person, 65% of the weight they lost was due to lean mass loss, more than double of what is considered normal. Loss of lean mass during weight loss is typically 20% to 30% of total weight loss.

Shepherd provided expertise on body composition on this study. By collaborating with Shepherd’s Shape Up Study , the TREAT study was able to track changes in muscle, visceral fat and other regional body composition measures in 50 local participants. All other participants were scattered throughout the nation and only monitored weight change.

“There have been encouraging studies previously, but this is one of the few randomized control studies,” said Shepherd. “Intermittent fasting needs to be thoroughly studied given the surprising loss in muscle in this particular study.”

Shepherd and Weiss are working together on additional studies to further assess body composition during dietary interventions that may reduce cardiovascular and cancer risk.