Weight-loss jabs shed muscle too: semaglutide users lost 5.44 kg of lean mass

GLP-1 receptor agonists produce substantial weight loss at the doses used for obesity management, but total weight is a blunt measure. It bundles together fat and lean tissue, and what happens to muscle during drug-driven weight loss has not been fully understood. Muscle matters for strength, metabolic health and long-term function, so a treatment that quietly stripped it away would carry a hidden cost. This review set out to establish what the randomised trial evidence actually shows about lean mass in people taking these drugs for obesity.

The researchers systematically searched PubMed, Embase and LILACS in March 2025 for randomised controlled trials in people with obesity treated with GLP-1 receptor agonists at obesity doses, compared with placebo. Seven trials covering 821 patients met the criteria. Using a random-effects model, they pooled the change in lean mass as a proportion of total weight, the absolute and relative changes in lean mass, and adverse effects. Results for individual drugs, including semaglutide, were also examined separately.

Lean mass as a proportion of total body weight improved by 1.81% with GLP-1 treatment, meaning muscle made up a larger share of a lighter body. At the same time, absolute lean mass fell by an average of 1.74 kg, and the relative change in lean mass was a drop of 3.06%. Semaglutide showed the largest absolute loss at 5.44 kg. The trials agreed closely on the proportional improvement but disagreed substantially on the size of the absolute losses, which softens confidence in the exact figures.

The result cuts both ways, which is why coverage of these drugs and muscle can sound contradictory: relative body composition improves, yet some genuine lean tissue is lost, and both statements are true at once. The authors conclude that lean mass loss should not be treated as a reason to avoid these drugs in obesity. At the same time they stress that treatment should be accompanied by nutrition and exercise interventions to preserve or improve muscle. Because the comparator in every trial was placebo, whether resistance training or higher protein intake actually prevents the loss remains the key open question.

If you or someone you know is using a GLP-1 drug for weight loss, this is a reason to look past the number on the scale. Part of the weight that comes off is lean tissue, on average 1.74 kg across these trials, even though the overall balance of muscle to total weight improves. It is also a reason to take the authors' conclusion seriously: they see nutrition and exercise support as essential companions to the drug rather than optional extras. None of this suggests the drugs fail at their job; it clarifies what the lost weight is actually made of.