October 9, 2026

How Much Muscle Do GLP-1 Drugs Take? A Close Look at the Body Composition Trials

By Dr. Zakira Naureen, Associate Professor, University of Nizwa
Metabolic Health Body Composition Young Adults

In Simple Terms

  • If I lose a lot of weight on a GLP-1 drug, how much of it will actually be muscle?
  • Is losing muscle on these drugs really a problem, or does the scan just make it look worse than it is?
  • Does losing muscle quickly in my twenties or thirties set me up for sarcopenic obesity later?
  • What does the research say I can do to hold on to muscle while the fat comes off?

Headlines about weight loss drugs tend to swing between two extremes: miracle cure or muscle wasting. The trial data sit somewhere in between, and the details matter. Our earlier short post on GLP-1 drugs and muscle introduced the issue. This article goes deeper into what the body composition studies actually measured, how large the effect is, and where the evidence is weaker than the headlines suggest. The question is especially relevant to sarcopenic obesity, which is defined by the combination of low muscle and excess fat, because any treatment that removes fat while also removing muscle can leave the underlying ratio unchanged or worse.

What these drugs do and why lean mass falls

GLP-1 receptor agonists such as semaglutide, and the related dual agonist tirzepatide, reduce appetite and food intake. The result is a sustained energy deficit, and any large energy deficit causes the body to lose both fat and fat free tissue. This is not unique to medication. Diets, bariatric surgery, and illness all do the same thing, as we describe in our article on unsupervised weight loss and sarcopenic obesity. What is new is the speed and scale. Trials of the newer drugs routinely produce weight losses of 15 to 20 percent of body weight, so even a modest lean share translates into a large absolute loss of tissue.

One point of terminology is important. In most of these studies, the measurement tool is dual energy X-ray absorptiometry (DXA), which divides the body into fat mass, bone mineral, and lean mass. DXA lean mass is not the same as muscle. It also includes water, organs, skin, and connective tissue. That limitation shapes how every number below should be read.

What the trials show

The largest pooled analysis to date is a focused meta-analysis by Kristen Beavers and colleagues, published in the journal Obesity. It pooled nine placebo controlled randomized trials with 659 participants (419 on GLP-1 based therapy and 240 on placebo), of which seven reported lean mass. Compared with placebo, GLP-1 therapy produced about 6.9 kg more weight loss and about 1.9 kg more lean mass loss, which the authors summarize as roughly 30 percent of the weight lost coming from lean tissue. The analysis did not pool fat mass separately. Individual studies varied widely, and the authors describe the analysis as focused and exploratory.

The tirzepatide data point the same way. A DXA substudy of the SURMOUNT-1 trial, led by Michelle Look and published in Diabetes, Obesity and Metabolism in 2025, measured body composition in 160 of the trial’s 2,539 participants at baseline and at week 72. Those on tirzepatide lost 21.3 percent of body weight, including 33.9 percent of their fat mass and 10.9 percent of their lean mass, compared with 5.3, 8.2, and 2.6 percent in the placebo group. About three quarters of the weight lost was fat and about one quarter was lean mass, and that ratio was similar in the placebo group and across most subgroups the authors examined.

For semaglutide, a DXA subset of the STEP 1 trial (140 of 1,961 participants) was presented by John Wilding at the 2021 ENDO meeting. Summaries of that presentation report that total and abdominal body fat fell and that the proportion of lean mass in the body rose, but the report we reviewed gave no figures for the lean mass change itself, and it noted the work was industry sponsored.

Put together, a reasonable summary is that somewhere between a quarter and a third of weight lost on these drugs is lean mass as measured by DXA. That is broadly in line with what is seen with calorie restriction alone, which is one reason some researchers argue the drugs are not uniquely harmful to muscle. It is also why the headline figure of “one quarter lean mass” is not reassuring in itself, since a person who loses 20 kilograms may lose five kilograms or more of lean tissue.

Is lean mass loss necessarily muscle loss, and does it matter?

This is where the evidence becomes genuinely uncertain. Because DXA lean mass includes water and organs, and because people with obesity carry extra lean tissue to support a larger body, some of the lean mass lost during weight loss may be a normal, proportionate adjustment rather than a loss of functional muscle. The Beavers analysis states plainly that its trials did not measure muscle quality or function, so the clinical significance of the lean mass reduction remains unclear. A commentary on this issue by Carla Prado, Stuart Phillips, and colleagues in The Lancet Diabetes & Endocrinology makes the broader point that muscle should be treated as an outcome to monitor during medically induced weight loss, not an afterthought.

The concern for sarcopenic obesity is about function and reserve, not only the scan. Sarcopenic obesity is diagnosed on low muscle mass together with low strength or physical performance, as we explain in our piece on how the definitions disagree. A person starting weight loss with already limited muscle, or one who gains little strength back, has less margin than a person with a large reserve. For adults in their twenties and thirties, the worry is a long term one: entering midlife with a lower muscle reserve than they would otherwise have had, at the point when age related decline begins.

What the evidence says about protecting muscle

Evidence on how to protect muscle during drug assisted weight loss is still thin, and most of it comes from exercise research more broadly. The most useful randomized evidence comes from the trial by Lundgren and colleagues in the New England Journal of Medicine, which compared control, exercise, liraglutide, and combined treatment for maintaining weight loss. As summarized in a British Journal of Sports Medicine commentary by Alysha D’Souza, Kyle Lau, and Stuart Phillips, all active treatments reduced fat mass and waist circumference, with about twice the reduction in the exercise plus liraglutide group, but the commentary reports that the exercise only group was the only one to preserve lean mass relative to placebo. The combination produced the largest fat loss, but this summary does not show it protecting lean mass any better than exercise alone. We cover training specifics in our article on resistance training volume, and the practical principle is simple: regular strength training is the best supported lever for keeping lean mass during weight loss.

Protein intake is the other commonly recommended lever, discussed in our piece on the protein paradox. It is reasonable to ensure adequate protein, which can be difficult when a drug suppresses appetite, but we are not aware of large trials in people taking GLP-1 drugs that isolate the protein effect on muscle outcomes. That is a gap, and it is why advice here should be framed as sensible practice rather than proven prescription.

Practical implications for young adults

Most of the trials above enrolled adults with established obesity, often in their forties or fifties, and young adults are under represented. So we should be careful not to assume the same percentages apply to a 24 year old. What we can say is that rapid loss without resistance training is the scenario in which the risk is highest, and that anyone considering these medications should plan muscle protection from the start, not after the fact. In practical terms that means discussing strength training and protein targets with a clinician, tracking strength (for example, whether your lifts or grip strength are holding up) and not only the scale, and aiming for gradual rather than extreme loss where the clinical situation allows. Our complete guide to sarcopenic obesity in young adults covers the broader prevention picture.

Limitations and open questions

Several caveats apply. First, the Beavers meta-analysis included only nine trials of mixed populations (including people with polycystic ovary syndrome, type 1 diabetes, and schizophrenia), several of which offered diet or exercise counseling to both groups, and its authors describe the analysis as exploratory. Second, the SURMOUNT-1 body composition study covered only a small subset of participants, pooled tirzepatide doses, and relied on post hoc subgroup analyses. Third, DXA cannot separate muscle from other lean tissue, and none of these studies tell us whether the lost lean mass reduced strength or function in daily life. Fourth, nearly all data cover roughly one to a year and a half of treatment, so we do not know what happens to muscle after drug discontinuation, when weight is often regained, or whether regained weight is more fat than muscle. These are exactly the questions that long term, function based trials would need to answer, and they matter for any serious assessment of sarcopenic obesity risk.

This post is for general education and is not medical advice. Anyone considering or currently using a GLP-1 medication should discuss muscle preservation strategies and monitoring with their healthcare provider.

References

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