# GLP-1 muscle mass: what the trials measured. By [Max HP](https://maxhp.me/about) · Updated 3 November 2026 GLP-1 trials measure body weight, and some also measure what that weight was made of. The results are more specific than the headlines: fat mass falls more than lean mass, and the proportions vary. What the numbers mean for one person is a different question. ## What the trials measured Two body-composition substudies are worth knowing because they used DEXA, a scan that estimates fat mass, lean mass and bone. The STEP 1 substudy looked at semaglutide 2.4 mg over 68 weeks. The SURMOUNT-1 substudy looked at tirzepatide over 72 weeks. Both compared the medication with placebo and reported the change in total fat mass and total lean body mass. In the STEP 1 analysis, the average weight change was large: “Percentage change in body weight from baseline to week 68 was -15.0% with semaglutide vs -3.6% with placebo.” Fat mass fell more than lean mass, but lean mass did fall: “Total lean body mass decreased from baseline (-9.7%)”. The paper also reported that the proportion of lean mass relative to total body mass increased, because fat mass fell by a larger percentage. In SURMOUNT-1, the average changes were larger: “The change in body weight, fat mass and lean mass from baseline to Week 72 was -21.3%, -33.9% and -10.9% with tirzepatide and -5.3%, -8.2% and -2.6% with placebo, respectively.” The authors summarized the composition this way: “Of the body weight lost, approximately 75% was fat mass and 25% was lean mass for both tirzepatide and placebo.” ## What the proportions mean The 75/25 figure is the clearest single number in the discussion, and it is also the easiest to misread. It describes the composition of the weight lost, not the composition of the body. A person who loses 10 kg with 75% fat and 25% lean has lost 7.5 kg of fat and 2.5 kg of lean mass. The lean share is not nothing, and it is not all muscle. Lean mass includes water, organ tissue, bone and glycogen as well as skeletal muscle. A DEXA scan estimates the whole compartment; it does not label each kilogram. That is why a change in lean mass is not the same as a change in muscle strength or function, and why the trials’ numbers cannot tell you what happened in one person’s legs or arms. The proportions also varied by subgroup and by the amount of weight lost. The SURMOUNT-1 authors reported that the proportions remained consistent across most subgroup analyses, but the STEP 1 paper noted that the lean-to-fat ratio improved more in people with greater weight loss. A single average hides that variation. DEXA itself has error. The scan estimates a compartment from a model, and the estimate can shift with hydration, positioning and the machine. A change of a kilogram of lean mass in one person is not necessarily a kilogram of muscle. That is why the trial reports group averages with confidence intervals, not individual certainties. The trials also did not measure protein intake, resistance training or physical function as part of the body-composition substudy. Those factors can influence lean mass, but the study was not built to test them. A reader who wants to know whether a particular program preserves muscle will not find the answer in these numbers; that is a different study, and possibly a different question. The trial numbers describe what happened on average, not what any one person can expect from a plan, and they do not replace a conversation about your own history, goals, function and preferences. ## What the labels say The prescribing information states the direction without the DEXA detail. The Wegovy® label says: “Semaglutide lowers body weight with greater fat mass loss than lean mass loss.” The Zepbound® label says: “Tirzepatide lowers body weight with greater fat mass loss than lean mass loss.” Those sentences are consistent with the substudies: fat mass falls more, in absolute and proportional terms, while lean mass also falls. What the labels do not say is that the lean loss is harmless, or that it is avoidable, or that a particular exercise or protein intake changes it. The trials were not designed to test those questions. The labels report what was measured, and the rest is a conversation for a prescriber, a dietitian or an exercise professional who knows your history. That conversation can include resistance training, protein intake and the difference between weight and function. It cannot include a guarantee. Body composition changes with age, activity, illness and genetics, and a GLP-1 is one input among several. ## What the numbers cannot answer They cannot tell you whether your own lean mass changed, because you did not have a DEXA scan before and after under trial conditions. They cannot tell you whether a change in the scale is fat, water or lean tissue. They cannot tell you what your strength, balance or energy will be at the end of a step. They also cannot tell you to stop or continue a medication. That is a prescriber’s decision, made with the whole history. The useful thing a log can do is record the facts the prescriber needs: the dose steps, the weight trend, the side effects and any change in strength or function you notice. The labels also list signs that need urgent care, and those apply whatever the body-composition question is. The Wegovy® Medication Guide says: “Stop using WEGOVY and get medical help right away, if you have any symptoms of a serious allergic reaction including: swelling of your face, lips, tongue or throat; severe rash or itching; very rapid heartbeat; problems breathing or swallowing; fainting or feeling dizzy.” If any of those appear, contact your prescriber, or get emergency help. ## Questions **Does a GLP-1 cause muscle loss?** The trials show that lean mass falls along with fat mass. In SURMOUNT-1, about 25% of the weight lost was lean mass. Lean mass is not the same as muscle, and the trials did not measure strength or function in the DEXA substudy. **Is 25% lean mass loss a lot?** It is the average proportion in that trial, not a threshold for an individual. The proportion varied by subgroup and by the amount of weight lost. A prescriber can put it beside your own history and any changes you notice in strength. **Can I prevent lean mass loss?** The GLP-1 trials were not designed to test exercise or protein interventions, so they cannot answer that question. Resistance training and adequate protein are reasonable topics for a conversation with your prescriber or a dietitian, not a promise from a trial. **Do I need a DEXA scan?** That is a clinical decision. A DEXA scan gives an estimate of fat and lean mass, with error and without a muscle-specific label. Ask your prescriber whether it would change anything in your care. ## Sources 1. [Wilding JPH, et al. “Impact of Semaglutide on Body Composition in Adults With Overweight or Obesity: Exploratory Analysis of the STEP 1 Study.”](https://pmc.ncbi.nlm.nih.gov/articles/PMC8089287/) _Journal of the Endocrine Society_ 5, Supplement 1 (2021): A16–A17. Read 3 November 2026. 2. [Look M, et al. “Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight.”](https://pubmed.ncbi.nlm.nih.gov/39996356/) _Diabetes, Obesity and Metabolism_ 27, no. 5 (2025): 2720–2729. Read 4 November 2026. 3. [Wegovy® (semaglutide) prescribing information and Medication Guide](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=ee06186f-2aa3-4990-a760-757579d8f77b)⁠, DailyMed. Revised June 2026. Read 3 November 2026. 4. [Zepbound® (tirzepatide) prescribing information](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=487cd7e7-434c-4925-99fa-aa80b1cc776b)⁠, DailyMed. Revised August 2026. Read 3 November 2026. ## Read more - [Why your weight jumps](https://maxhp.me/blog/weight-trend-7-day-average) - [What to track on a GLP-1](https://maxhp.me/blog/what-to-track-on-glp-1) - [GLP-1 dose steps, explained](https://maxhp.me/blog/glp-1-dose-steps) - [The Clinic Report](https://maxhp.me/clinic-report) Coming soon to the App Store --- Canonical: https://maxhp.me/blog/glp-1-muscle-mass · Updated 2026-11-03