Tracking Muscle Loss on GLP-1 Medications

GLP-1 medications — semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound) — are effective at reducing body weight. But the scale only tells you how much weight you’ve lost, not what kind. Some of the loss is fat. Some is lean mass, which includes muscle. Knowing the split matters, because preserving muscle is part of staying strong and metabolically healthy as you lose weight.

What the trials found. In a body-composition substudy of the STEP-1 trial of semaglutide, participants lost about 15% of their body weight over 68 weeks, and total lean body mass decreased by roughly 9.7% from baseline — though because fat fell faster, lean mass as a proportion of body weight actually rose. In a DEXA substudy of the SURMOUNT-1 trial of tirzepatide, participants lost about 21% of their weight over 72 weeks, and of the total weight lost, roughly 75% was fat mass and 25% was lean mass. Across the research, the share of weight lost as lean mass has ranged from roughly a quarter to nearly half, depending on the medication, the study, and the person.

Why this matters. Losing some lean mass during weight loss is normal and expected — it happens with almost any method, not just GLP-1s. What you don’t want is to lose more muscle than necessary. Muscle supports strength, mobility, and metabolic health, and it’s harder to rebuild than to keep. Seeing whether your loss is mostly fat — or tilting too far toward lean mass — gives you and your prescriber something concrete to work with.

What DEXA measures. A DEXA scan separates your weight into fat, lean mass, and bone, and reports lean mass region by region. Scanned before you start and again a few months in, it shows how much of your change is fat versus muscle — objective data instead of guessing from the scale.

What we do and don’t do. Wayne DEXA measures body composition. We don’t prescribe, adjust, or manage GLP-1 therapy — but if you’re looking for a prescriber, we work alongside vetted practices that take a holistic, whole-body approach to medicine, including Forward Family Medicine, with whom we share our Wayne location. The measurement itself is yours to review with whichever clinician manages your care, and nothing here is medical advice or treatment guidance.