What the Studies Actually Show

The most useful studies do not show that GLP-1 medications “eat muscle.” They show that substantial weight loss is made up mostly of fat mass, with a smaller average reduction in measured lean mass. They also show why one percentage should never be pulled out of context.

In an exploratory DXA analysis from the STEP 1 semaglutide trial, 140 participants had body-composition measurements. After 68 weeks, average body weight in the semaglutide group was 15.0% lower, total fat mass was 19.3% lower, visceral fat was 27.4% lower, and total lean body mass was 9.7% lower. Because fat mass fell more, lean mass made up a larger proportion of the body at the end of the study.

That result does not mean each participant lost 9.7% of skeletal muscle. It describes the average change in DXA-measured lean body mass in a small exploratory subset. It also cannot tell us how much of the change was water, organ tissue, connective tissue, or muscle.

In the SURMOUNT-1 tirzepatide DXA substudy, 160 participants had evaluable scans. At 72 weeks, average body weight was 21.3% lower, fat mass was 33.9% lower, and lean mass was 10.9% lower. The researchers estimated that roughly 75% of the weight lost was fat mass and roughly 25% was lean mass.

STEP 1 and SURMOUNT-1 should be shown in separate panels. They enrolled different participants, used different drugs and designs, and were not a head-to-head test of muscle preservation. Their common lesson is more important than a drug comparison: fat loss predominated, while absolute lean mass also declined.

Two DXA substudies, read separately

STEP 1 — semaglutide

Exploratory subset: n=140 at 68 weeks. Average body weight −15.0%, total fat mass −19.3%, and lean body mass −9.7%.

SURMOUNT-1 — tirzepatide

Evaluable subset: n=160 at 72 weeks. Average body weight −21.3%, fat mass −33.9%, and lean mass −10.9%. About 75% of weight lost was fat mass and 25% lean mass.

Interpretation

Fat loss predominated and absolute DXA-measured lean mass also declined. These were not head-to-head studies, and lean mass is not the same as skeletal muscle.

There is also an evidence gap. We have strong trials showing that semaglutide and tirzepatide reduce weight and fat mass. We have smaller body-composition substudies. We do not yet have a large completed trial proving one exact protein-and-resistance program prevents a specified amount of skeletal-muscle loss with these medications.

That uncertainty is not a reason to do nothing. It is a reason to use the best-supported general weight-loss principles, label the extrapolation honestly, and monitor outcomes that matter to the person.