Lean mass questions move to the center of GLP-1 practice

Doctors presenting at the 2026 American Diabetes Association Scientific Sessions say a growing body of research shows that a meaningful share of the weight lost on GLP-1 medicines comes from lean body mass rather than fat. Ian Neeland, MD, a cardiovascular prevention specialist at University Hospitals Harrington Heart & Vascular Institute, told the ADA symposium that lean mass typically makes up about 25% of total weight loss with any weight-loss method, including diet, surgery, or medication. With newer, higher-dose GLP-1 compounds, he said, that share can climb to 40% or more ^[2].

The concern is not just cosmetic. Neeland explained that each kilogram of muscle lost lowers resting energy expenditure by about 13 kilocalories a day, compared with only 4 kilocalories for each kilogram of fat lost ^[2]. That means losing muscle can make it harder for the body to burn calories at rest, which matters for people trying to keep weight off over time.

Researchers are testing ways to offset this. The S-LITE trial found that combining liraglutide with supervised resistance and aerobic exercise let patients lose more weight overall while actually gaining lean mass, something drug treatment alone did not achieve ^[2]. Neeland pointed to protein intake of 1.2 to 1.6 grams per kilogram of body weight per day as the range shown to help preserve muscle, with little added benefit above that level ^[2].

A newer approach targets the biology directly. In the phase 2 BELIEVE trial, adding the myostatin inhibitor bimagrumab to semaglutide cut the lean-mass share of total weight loss from about 21% down to roughly 7% ^[2]. Still, Neeland cautioned that large studies measuring actual muscle function, not just muscle mass, are lacking, and that clinics do not yet have a clear, standard way to measure muscle function in patients ^[2].

Why it matters for patients

For people taking Ozempic, Wegovy, Mounjaro, Zepbound, or other GLP-1 medicines, this research means that a real portion of the pounds lost is muscle, not just fat. Because these drugs typically produce more total weight loss than diet alone, the amount of muscle lost in absolute terms can be larger even if the percentage is similar to what happens with lifestyle-only weight loss ^[2]. Since muscle affects strength, daily function, and how many calories the body burns at rest, this is a factor patients and their care teams may want to track over the course of treatment, alongside the number on the scale.

It's also worth noting what is not yet settled. The sources reviewed here do not include a study that directly compares lean-mass loss between different GLP-1 drugs head-to-head, so it is not yet known whether one medicine preserves more muscle than another under similar conditions. Findings on tirzepatide's effects on muscle fat, cited by Neeland, come from separate imaging studies rather than a direct lean-mass comparison across drug classes ^[2].

What happens next

Research on preserving muscle during GLP-1 treatment is still developing. The phase 2 BELIEVE trial pairing semaglutide with the myostatin inhibitor bimagrumab represents an early step toward drug combinations aimed specifically at protecting lean mass, though functional outcomes from that approach are not yet well studied ^[2]. Neeland and other presenters called for future trials to include clearer measures of muscle strength and function, not just body composition scans, so that patients and clinicians can better judge the practical impact of muscle loss during treatment ^[2].