Food & protein
Keeping muscle while you lose weight
By Joshua Nichvolodoff · Fact-checked against sources November 2, 2026 · Next review due November 2, 2027
Clinical review: not yet reviewed by a clinician. How review works · Editorial policy
When you lose weight, some of what you lose is lean tissue, not just fat. Here is what the GLP-1 trials measured, why "lean mass" isn't the same as muscle, and what the 2025 joint nutrition advisory recommends to protect it.
What the trials measured
| Item | Weight | Fat mass | Lean mass |
|---|---|---|---|
| STEP 1, semaglutide (68 weeks) | −15.0% | −19.3% | −9.7% |
| STEP 1, placebo | −3.6% | — | — |
| SURMOUNT-1, tirzepatide (72 weeks) | −21.3% | −33.9% | −10.9% |
| SURMOUNT-1, placebo | −5.3% | −8.2% | −2.6% |
In STEP 1, lean mass fell in absolute terms, but its share of body weight rose by 3.0 percentage points, because fat fell faster[4]. In SURMOUNT-1, about 74% of the weight lost on tirzepatide was fat and 26% lean mass[1].
Lean mass isn't only muscle
DXA scans count organs, bone, fluids and the water in fat tissue as "lean." Across GLP-1 trials, lean mass has ranged from about 15% or less to 40–60% of the weight lost[2]. A larger lean loss on a scan doesn't necessarily mean the same loss of muscle.
What helps keep muscle
Resistance exercise is an effective way to limit, or even prevent, muscle loss during weight loss. High protein intake helps preserve lean mass but does not, on its own, improve strength[5]. A 2024 review names resistance exercise, alongside adequate protein, as the main strategy[2].
The 2025 joint advisory recommends strength training at least 3 times a week plus at least 150 minutes a week of moderate aerobic activity, together with adequate protein[3]. See protein on a GLP-1 for targets.
General information, not medical or exercise advice. If you have a heart condition, joint problems or another reason to be careful with exercise, ask your clinician before starting strength training.
Sources
- Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes, Obesity and Metabolism, February 25, 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC11965027/ · DOI 10.1111/dom.16275 · PMID 39996356. Accessed November 2, 2026. Back to the text
- Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes, Obesity and Metabolism (repository copy, Helmholtz Munich), 2024. https://push-zb.helmholtz-munich.de/deliver.php?id=36021 · DOI 10.1111/dom.15728. Accessed November 2, 2026. Back to the text
- Nutritional priorities to support GLP-1 therapy for obesity: a joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society. American Journal of Lifestyle Medicine, May 30, 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC12125019/ · DOI 10.1177/15598276251344827 · PMID 40452753. Accessed November 2, 2026. Back to the text
- Impact of semaglutide on body composition in adults with overweight or obesity: exploratory analysis of the STEP 1 study (ENDO 2021 abstract). Endocrine Society (ENDO 2021), abstract PDF via Newswise, March 2021. https://www.newswise.com/pdf_docs/16159288255831_Wilding%20Abstract.pdf. Accessed November 2, 2026. Back to the text
- Preserving healthy muscle during weight loss. Advances in Nutrition, May 5, 2017. https://pmc.ncbi.nlm.nih.gov/articles/PMC5421125/ · DOI 10.3945/an.116.014506 · PMID 28507015. Accessed November 2, 2026. Back to the text
Figures marked “as reported” are historical: they show what the source said on that date, not today's price or rule.
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