GLP-1 weight-loss drugs mainly burn fat, not muscle — and women lose more than men

Researchers pooled body-composition data from randomised trials of GLP-1 receptor agonists — the drug class that includes semaglutide (Ozempic, Wegovy) and tirzepatide. Crucially, they looked at the fat-versus-lean-mass breakdown, not just the number on the scale, which is what actually tells you whether weight loss is coming from fat or from muscle.

Across the pooled trials, the large majority of the weight lost came from fat mass, with lean (muscle) tissue making up a smaller share than the alarming early headlines had implied. Some lean mass is still lost — that happens with almost any weight loss, including plain dieting — but the fat-to-muscle ratio was more favourable than the "these drugs waste your muscle" narrative suggested.

One finding stands out for women: on the same drugs, women lost proportionally more weight than men. That fits what's known about sex differences in body composition and how these medicines act, and it's a useful reminder that trial averages — often weighted toward male physiology — don't always map onto a woman's experience.

None of this means muscle looks after itself. Losing weight quickly at any age thins muscle and bone unless you actively defend them — which is exactly where enough protein and resistance training earn their place, and it matters even more through perimenopause and beyond, when muscle and bone are already under pressure. The drugs tilt the fat-to-muscle ratio in your favour; they don't remove the need to train and eat well while the weight comes off.

What the research says