Will You Lose Muscle on a GLP-1? The 10 Papers (2026)

The reference point for this whole question: 160 of SURMOUNT-1's 2,539 participants scanned by DEXA at baseline and week 72. Weight fell 21.3%, fat mass 33.9% and lean mass 10.9% on tirzepatide, against 5.3%, 8.2% and 2.6% on placebo. ★ The finding that matters most is the one usually left out of the coverage: the split ran roughly three parts fat to one part lean in both groups, and it held across sex, age and how much weight came off. The drug did not shift the composition of weight loss — it produced more of it.

PMID 39996356 ↗NCT04184622 ↗DOI 10.1111/dom.16275 ↗

A scan substudy nested inside retatrutide's phase 2 diabetes program, reading out at 36 weeks. Fat mass fell dose-dependently and far more than lean mass, with the lean share of weight lost landing in the same range tirzepatide and semaglutide produce, and visceral and liver fat dropping sharply. It was the result that cleared retatrutide to proceed without a body-composition question hanging over it. ⚠ Grip strength and physical function were not endpoints, so the functional question is untouched here.

PMID 40609566 ↗DOI 10.1016/S2213-8587(25)00092-0 ↗

The only synthesis built specifically around semaglutide and lean mass: six studies, 1,541 adults with overweight or obesity, summarized narratively rather than pooled statistically. Weight came off predominantly as fat, and lean mass as a share of total body weight went up. But the lean fraction of the weight lost ranged from close to zero in some trials to roughly 40% in others, with the largest reductions appearing in the largest trials. ⚠ Six heterogeneous studies and no meta-analysis is a weak design. Read it as a map of how widely the estimates scatter, which is the honest answer to how much muscle any individual should expect to lose.

PMID 38629387 ↗DOI 10.1080/14656566.2024.2343092 ↗

The trial that actually tests the thing everyone recommends. 195 adults dieted down over eight weeks first, then spent a year in one of four arms: liraglutide 3.0 mg, a supervised training program, the two together, or neither. The combination lost the most weight and held onto the most lean mass — roughly double the fat loss of either alone, with the lean-mass penalty blunted relative to the drug by itself. ★ It is the strongest randomized evidence that training alongside these drugs changes what kind of tissue comes off, not just how much.

PMID 33951361 ↗NCT04122716 ↗DOI 10.1056/NEJMoa2028198 ↗

The follow-up a year after everything stopped, and the part people rarely hear. The exercise group held the most of its fat-mass reduction and lean-mass preservation; the liraglutide-alone group regained the most; the combination sat in between. ⛔ Read together with the withdrawal trials elsewhere on this site, it says something uncomfortable: the favorable body composition is a property of continuing, not an achievement banked. Habits built during treatment are what survive it.

PMID 38544798 ↗NCT04122716 ↗DOI 10.1016/j.eclinm.2024.102475 ↗

The review that put a number on the mitigation argument. Across the GLP-1 and dual-agonist trials, it estimates 25 to 40% of weight lost is lean mass — proportionally similar to dieting, but larger in absolute terms simply because these drugs remove more weight. Its case is that resistance training is the best-supported countermeasure available, and its complaint is that nobody has run a trial powered on muscle function rather than scan-measured mass.

PMID 38687506 ↗DOI 10.2337/dci23-0100 ↗

The network meta-analysis pooling body composition across liraglutide, semaglutide, tirzepatide and retatrutide trials. Fat-mass reductions were large, lean-mass reductions smaller in absolute terms, and the ratio comparable to diet-induced loss, with tirzepatide and retatrutide looking most favorable. ⚠ The authors flag the limitation that undercuts every entry on this page: DEXA fat-free mass is not skeletal muscle, and not one included trial made muscle function a primary endpoint.

PMID 39719170 ↗DOI 10.1016/j.metabol.2024.156113 ↗

The paper that supplies the vocabulary the rest of the field now uses: adaptive lean-mass loss, proportional to fat loss and physiologically expected, against maladaptive loss that would cost someone function. Its reading of the trial data is that what has been observed so far is the adaptive kind. It also assembles the practical list — protein at 1.2 to 1.6 g/kg a day, resistance training, slower titration in older patients, and the muscle-sparing agents in development.

PMID 38937282 ↗DOI 10.1111/dom.15728 ↗

A direct argument that the alarm has outrun the evidence, drawing on MRI and DEXA across the STEP, SURMOUNT and SURPASS substudies. Fat infiltration within thigh muscle improves, visceral fat falls disproportionately, and the lean-mass decline tracks fat loss rather than running ahead of it. ★ Its practical demand is the one worth remembering: obesity trials should be reporting grip strength, gait speed and lean mass indexed to height, not a single fat-free-mass number that cannot distinguish muscle from water.

PMID 39401279 ↗DOI 10.1161/CIRCULATIONAHA.124.067676 ↗

The preclinical work behind the strategy everyone is watching. In obese mice, semaglutide combined with an antibody blocking the activin type II receptor stripped more fat than semaglutide alone while leaving skeletal muscle fully intact. That is the mechanism underlying bimagrumab and the other muscle-sparing combinations now in human obesity trials. ⛔ Mice, not people. It explains why the approach is being tested; it is not evidence that it works in humans.

PMID 38218536 ↗DOI 10.1016/j.molmet.2024.101880 ↗