Muscle Loss on a GLP-1: What the Data Shows

Last updated October 20, 2026

Short answer: Some of the weight lost on a GLP-1 is lean mass, but that is true of weight loss in general. In the SURMOUNT-1 body-composition substudy, about 74% of the weight lost on tirzepatide was fat and 26% was lean mass, nearly the same split as the placebo group (75% and 25%). Lean mass also includes water and organs, so the portion that is actual muscle is smaller. A 2025 joint advisory puts it at roughly 20% of the weight lost in the semaglutide trial.

What the body-composition studies measured

Both major trials ran a smaller substudy in which some participants had DXA scans, which estimate fat mass and lean mass, at the start and the end. They are different studies with different participants, so the two columns below should not be read as a head-to-head comparison.

In the tirzepatide substudy, 74% of the weight lost was fat and 26% was lean mass. The placebo group, which lost much less weight, split 75% and 25%. The joint advisory reports that in STEP 1 the average loss was 13.6 kg, of which 5.3 kg (38%) was lean body mass.

Lean mass is not the same as muscle

DXA counts everything that is not fat or bone as lean mass, including water and organ tissue. The joint advisory says the STEP 1 figure corresponds to about 20% of total weight reduction being muscle loss. It also cites modeling that estimates muscle loss of 10% to 15% of the weight lost in females and 20% to 25% in males when people lose weight without structured strength training. Those are estimates, not measurements from a trial of GLP-1 users.

Who may be more prone to it

The advisory flags older adults, people in perimenopause or menopause, people with lower testosterone, and people who are sedentary or do no resistance training as more prone to muscle loss during weight reduction.

What the advisory recommends

• Regular strength training at least three times a week, plus at least 150 minutes of moderate-intensity aerobic activity weekly.
• Adequate protein, because increased protein alone is likely inadequate without resistance training. See our protein guide for the ranges and a calculator.
• Clinician guidance for anyone at higher risk, such as older adults or people with kidney disease.

Our protein on a GLP-1 guide covers the numbers. For related visible changes, see Ozempic face and hair loss.

Limits of this data

• The substudies were small. The tirzepatide one was designed around fat mass, not lean mass, and its subgroup results were post hoc.
• DXA is less accurate than the most precise body composition methods, and visceral fat was estimated from one region of the scan.
• Physical activity and food intake were not measured in the tirzepatide substudy.
• The tirzepatide substudy was funded by Eli Lilly and the STEP 1 trial by Novo Nordisk.
• These are two separate studies, so they cannot show which medication preserves more lean mass.