Some weight lost on semaglutide is lean mass, including muscle, but fat loss dominates.
In the STEP 1 body composition substudy (140 participants, DEXA scans, 68 weeks), semaglutide 2.4 mg produced a 15.0% weight loss versus 3.6% on placebo. Total fat mass dropped 19.3% and regional visceral fat dropped 27.4%. Total lean body mass also dropped, by 9.7% from baseline, which is expected whenever weight falls quickly. The reassuring detail: because fat fell faster, the proportion of lean mass relative to total body weight rose by 3.0 percentage points, and more weight loss meant a better lean-to-fat ratio. This is not a reason to avoid the medication. It is a reason to protect muscle while you are on it. The two levers that matter most are protein intake and resistance training. Older adults and anyone with low muscle mass at the start should treat this as a real planning conversation, not an afterthought.
What to do
Common questions
Does Ozempic cause muscle loss?
Yes, some of the weight you lose on Ozempic or Wegovy (semaglutide) comes from lean mass, which includes muscle, not only fat. That is true of any weight loss, but the numbers are worth knowing. In a STEP 1 trial substudy that measured body composition with DEXA scans over 68 weeks, people on semaglutide 2.4 mg lost 15.0% of their body weight. Total fat mass fell 19.3% and visceral (belly) fat fell 27.4%, while total lean body mass fell 9.7% from baseline. Because fat fell faster than lean mass, lean tissue actually made up a larger share of the remaining body, up 3 percentage points. So the drug is not melting muscle, but rapid weight loss always costs some lean tissue. You can blunt it: eat enough protein at every meal, do resistance training two to three times a week, and avoid crash-level calorie deficits. If you are older or already low on muscle, bring this up with your prescriber before you start.