Muscle loss on GLP-1s

How much of the weight lost is lean mass?

The best-measured example is tirzepatide High. In a SURMOUNT-1 substudy, 160 people had body-composition (DXA) scans at the start and at 72 weeks. On tirzepatide, weight fell 21.3%, fat mass 33.9% and lean mass 10.9%; on placebo the figures were 5.3%, 8.2% and 2.6% ¹. The share of weight lost as fat or lean mass stayed much the same in most subgroups, across sexes, age groups and amounts of weight lost ¹. For semaglutide High, a DXA substudy within the STEP programme found fat mass fell more than on placebo ².

The clearest comparison comes from a 2026 meta-analysis of 20 randomized trials with 15,782 participants ³. Lean mass made up 35.2% of the weight lost on semaglutide, 25.4% on tirzepatide and 26.8% on liraglutide. Lifestyle interventions without drugs lost a similar share, 26.2%, and the difference was not significant ³. An earlier meta-analysis likewise found GLP-1-based drugs cut both fat (2.25 kg more than controls) and lean mass (1.02 kg more), while the lean percentage of the body stayed comparable ⁴. The pooled numbers for each drug come from different trials, so they are not a head-to-head ranking.

Put simply, these drugs cause lean-mass loss in proportion to the weight lost, not out of proportion to it. That is still a real loss, and it is the reason the question matters.

Lean mass is not muscle

Most trials report lean mass, and lean mass is not a measure of muscle. A 2024 review of lean-mass changes on GLP-1 therapies points out that it includes organs, bone, fluids and the water held in fat tissue as well as muscle, so a fall in lean mass does not always mean the same fall in muscle ⁵. Some of what a scan records as lost lean mass is the water and supporting tissue that shrinks along with fat.

Some of it is muscle, though, and muscle matters: skeletal muscle is essential for cardiometabolic health and for preventing frailty ⁶. That is why the concern is sharpest for people who start with little muscle to spare.

Strength, not just scans

Body composition is only half the story; what people care about is whether they stay strong. A REDEFINE-1 analysis of CagriSema High presented at ECO 2026 paired DXA scans in 252 participants with sit-to-stand strength testing in 1,038 over 68 weeks, to see whether changes in body composition track changes in physical function ⁷; our Wire report covers it.

What protects muscle: training and protein

Exercise is the best-supported answer. A 2026 meta-analysis of 34 randomized trials (1,455 participants) found that adding exercise to calorie restriction kept 0.87 kg more fat-free mass than calorie restriction alone, preventing nearly half (45.7%) of the loss; mixed strength and endurance training did best, with 1.20 kg ⁶. In the incretin meta-analysis, lifestyle programmes that included resistance training lost only 17.5% of their weight as lean mass, the most favourable result of any approach ³.

Protein helps too, more modestly. Across 18 studies, eating more than the recommended minimum of 0.8 g per kg of body weight a day added an average of 0.32 kg of lean mass compared with eating at that minimum ⁸, and in older adults losing weight, higher-protein diets kept more lean mass and lost more fat ⁹.

Muscle-sparing drugs: bimagrumab

Drug makers are testing ways to keep muscle while losing fat. The most studied is bimagrumab Medium, an investigational antibody that blocks activin type II receptors ¹⁰, a signal that holds back muscle growth ¹¹. In a 48-week trial, 75 adults with type 2 diabetes and obesity received monthly infusions or placebo ¹¹. A 2026 meta-analysis of four trials (268 participants) found bimagrumab cut weight by 4.85 kg and fat mass by 4.72 kg more than placebo while adding 1.66 kg of lean mass, but it also raised LDL cholesterol by 0.47 mmol/L ¹². A 2026 randomized phase 2 trial paired it with semaglutide ¹³; our entry covers the results. As of October 2, 2026, it is still in clinical trials, not an approved treatment.

Frequently asked questions

Do GLP-1 drugs cause muscle loss?

They cause lean-mass loss along with fat loss. A 2026 meta-analysis of 20 trials found lean mass was 25% to 35% of the weight lost, about the same share as with diet and lifestyle alone.

Does tirzepatide cause less muscle loss than semaglutide?

In pooled trials, lean mass was 25.4% of weight lost on tirzepatide and 35.2% on semaglutide, but those figures come from different trials, not a head-to-head comparison.

Is lean mass the same as muscle?

No. Lean mass also includes organs, bone, fluids and the water in fat tissue, so a lean-mass loss on a scan overstates the muscle lost.

How can I protect muscle on a GLP-1?

Exercise has the best evidence: adding it to calorie restriction prevents nearly half the loss of fat-free mass, and resistance training gave the most favourable lean-mass result in the incretin meta-analysis. Protein above 0.8 g per kg a day helps modestly.

Is there a drug that prevents muscle loss?

Not an approved one. Bimagrumab added 1.66 kg of lean mass in a 2026 meta-analysis of four trials but raised LDL cholesterol; as of October 2, 2026 it is still investigational.

Related on Grey Peptides

• Coming off a GLP-1: the transition guide
• Eloralintide vs retatrutide
• What peptide therapy really costs in 2026

Sources

1. Look, M., Dunn, J. P., Kushner, R. F., et al. (2025). Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes Obes Metab, 27(5), 2720-2729. PMID: 39996356
2. O'Neil, P. M., Rubino, D. M. (2022). Exploring the wider benefits of semaglutide treatment in obesity: insight from the STEP program. Postgrad Med, 134(sup1), 28-36. PMID: 36691307
3. Eisa, N., Barood, O. (2026). Lean Mass Changes With Incretin Therapy Versus Lifestyle Intervention: A Systematic Review and Meta-Analysis of Randomised Controlled Trials. Diabetes Obes Metab, 28(6), 4818-4827. PMID: 41877354
4. Jiao, R., Lin, C., Cai, X., et al. (2025). Characterizing body composition modifying effects of a glucagon-like peptide 1 receptor-based agonist: A meta-analysis. Diabetes Obes Metab, 27(1), 259-267. PMID: 39431379
5. Neeland, I. J., Linge, J., Birkenfeld, A. L. (2024). Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes Obes Metab, 26 Suppl 4, 16-27. PMID: 38937282
6. Deller, M., Weiershaus, J., Held, S., et al. (2026). Effects of Calorie Restriction With and Without Strength, Endurance or Mixed Training on Fat-Free and Skeletal Muscle Mass in Overweight or Obese Individuals-A Systematic Review With Pairwise Meta-Analysis and Network Meta-Analysis of Randomized Controlled Studies. Diabetes Obes Metab, 28(8), 6810-6823. PMID: 42144246
7. Grey Peptides Wire. CagriSema REDEFINE-1 body-composition analysis at ECO 2026 (/news/cagrisema-redefine-1-body-composition-eco2026/), with its sources; read October 2, 2026.
8. Hudson, J. L., Wang, Y., Bergia, I., et al. (2020). Protein Intake Greater than the RDA Differentially Influences Whole-Body Lean Mass Responses to Purposeful Catabolic and Anabolic Stressors: A Systematic Review and Meta-analysis. Adv Nutr, 11(3), 548-558. PMID: 31794597
9. Kim, J. E., O'Connor, L. E., Sands, L. P., et al. (2016). Effects of dietary protein intake on body composition changes after weight loss in older adults: a systematic review and meta-analysis. Nutr Rev, 74(3), 210-24. PMID: 26883880
10. Kaur, M., Misra, S. (2024). Bimagrumab: an investigational human monoclonal antibody against activin type II receptors for treating obesity. J Basic Clin Physiol Pharmacol, 35(6), 325-334. PMID: 39385353
11. Heymsfield, S. B., Coleman, L. A., Miller, R., et al. (2021). Effect of Bimagrumab vs Placebo on Body Fat Mass Among Adults With Type 2 Diabetes and Obesity: A Phase 2 Randomized Clinical Trial. JAMA Netw Open, 4(1), e2033457. PMID: 33439265
12. Shao, C., Lin, H., Yu, J., et al. (2026). Efficacy and Safety of Bimagrumab in Adults With Obesity and Metabolic Dysfunction: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Diabetes Obes Metab, 28(10), 9433-9442. PMID: 42530342
13. Heymsfield, S. B., Aronne, L. J., Montgomery, P., et al. (2026). Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial. Nat Med, 32(3), 869-882. PMID: 41772149

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