⚕️ Medical Disclaimer
⚕️ Medical Disclaimer: This is general education, not medical advice. Written by the Kymata Health editorial team; medical review pending. Talk with your prescriber before changing your diet, exercise or medicines. Evidence grades: A = several randomized trials or a meta-analysis; B = at least one randomized trial; C = small or pilot human data, or mechanism only; D = animal or cell data, or anecdote.
How Much of the Weight Lost Is Lean Mass?
Weight loss by any method removes some lean mass along with fat. "Lean mass" includes not only muscle but also organs, bone, body fluids and the water inside fat tissue. So a drop in lean mass is not the same as an equal drop in muscle (Neeland et al., Diabetes Obes Metab 2024; PMID: 38937282).
SURMOUNT-1 body-composition substudy (Look et al., Diabetes Obes Metab 2025; PMID: 39996356): in 160 adults scanned at baseline and week 72, about 75% of the weight lost on tirzepatide was fat and about 25% was lean mass. The split was about the same in the placebo group, which lost much less weight. Evidence: B (randomized trial substudy).
STEP 1 DXA substudy (Wilding et al., N Engl J Med 2021; PMID: 33567185): people on semaglutide 2.4 mg lost about 10.4 kg of fat and 6.9 kg of lean mass, roughly 60% fat and 40% lean, as summarized in the SURMOUNT-1 substudy paper above. Evidence: B (randomized trial substudy).
Across GLP-1 studies, reported lean-mass shares vary widely with the population and the measuring method (PMID: 38937282). Losing some lean mass is expected. The aim is to keep as much muscle and strength as you can while you lose fat.
Preservation Strategies
Protein: with a smaller appetite it is easy to under-eat protein. Include a protein food at every meal, starting with breakfast. Ask your clinician or a registered dietitian what daily target fits you, especially if you have kidney disease.
Resistance training 2-3 times a week, gradually making it harder over time, gives your muscles a reason to stay while you lose fat. Bands, machines or bodyweight exercises all count.
Creatine monohydrate is sometimes used alongside resistance training. It has not been studied specifically in people taking GLP-1 medicines, so check with your clinician before adding it. Evidence for these strategies in GLP-1 users specifically: C.