“Will I lose muscle?” is one of the most common questions people ask about GLP-1 medications, and the honest answer has been “some, probably, but it depends.” A 12-month French study called SEMALEAN put actual measurements behind that answer. It is worth reading closely, partly for what it found and partly for what a study of this design cannot find.
What the study did
Researchers at Rouen University Hospital followed 106 adults who completed a year of semaglutide 2.4 mg weekly. Participants were about 69% female, averaged 52 years old, and had a mean BMI of 46.3, so this was a group with severe obesity. Dosing was stepped up over the first 16 weeks.
The measurement method matters. Body composition was assessed with dual-energy X-ray absorptiometry (DXA), which separates fat mass from fat-free mass and estimates appendicular skeletal muscle, the muscle in the arms and legs. Scans happened at baseline, 7 months, and 12 months. Handgrip strength was also measured, which is a useful addition: how much lean tissue you have and how well it works are not the same question.
What it found
- Fat mass fell 14.3% by month 7 and 18.9% by month 12.
- Lean mass dropped about 3.0 kg in the first seven months, then stabilized through month 12 rather than continuing to fall.
- Handgrip strength was up 4.1 kg at 12 months compared with baseline.
- The share of participants meeting the study’s definition of sarcopenic obesity went from 49% to 33%.
The pattern, a lean mass dip early on followed by a plateau, is the interesting part. It suggests that the loss was front-loaded rather than a steady drain, and that function did not decline alongside it.
What it does not establish
There was no control group. Everyone in SEMALEAN took semaglutide. Without a comparison arm, the study cannot say how much lean mass the same people would have lost from weight loss by other means, or whether the grip strength gain came from the medication, from the clinical program around it, or from changes in how active people became.
People who stopped were not counted. Participants who discontinued early were excluded. A study that only analyzes completers tends to look better than the full group of people who started, because people who feel worse or see less benefit are more likely to leave.
Twelve months is one year. It says nothing about what happens to muscle and strength in year two, or after stopping.
A starting BMI of 46 is not a typical starting point. Someone with severe obesity carries more lean mass at baseline, and a loss of 3 kg means something different for them than for a smaller person. The results do not transfer neatly to people at other body sizes.
A DXA “lean mass” number is not pure muscle. It includes water, organs, and connective tissue, so a drop can partly reflect fluid shifts. That is one reason researchers pair it with a functional measure like grip strength.
The wider picture
SEMALEAN fits into a growing set of body composition studies, including a 2026 systematic review and meta-analysis in the International Journal of Obesity pooling GLP-1 trials. The overall theme across this literature is that most of the weight lost is fat, that some lean mass is lost too, and that how much varies by person, by medication, and by how lean mass is measured. Pooled averages hide a lot of individual spread.
A separate finding from ENDO 2026 adds a caution: in a study of people taking GLP-1 medications, physical activity tended to decrease. That matters because movement and adequate nutrition are the levers most associated with holding on to muscle during weight loss, and an observational study of this kind cannot tell us whether the reduced activity was caused by the medication or by something else. Either way, this is a conversation to have with a licensed clinician about your own situation, including protein intake and strength-oriented activity, not something a single study settles.
The part that applies to compounded medications
SEMALEAN studied branded, manufactured semaglutide 2.4 mg dosed under a specific program. That evidence covers that product and does not transfer to a compounded preparation.
Compounded semaglutide and tirzepatide are made by state-licensed 503A pharmacies for individual patients. They are not FDA-approved, and no study like this one has been run on them. Their strength and composition can differ from the branded product. If a company tells you studies like SEMALEAN prove what a compounded product will do to your body composition, they are stretching the data past what it measured.
What to take from it
The reasonable reading is modest: in one real-world cohort of people who stayed on semaglutide for a year, lean mass fell early and then leveled off, and grip strength did not get worse. The reasonable limits are equally clear: no control group, completers only, one starting population, and a measurement that cannot see exactly what you care about.
At NoBsRx, how the program works puts a licensed provider in charge of your plan, and questions like muscle, nutrition, and activity are exactly what that review is for. The FAQ covers what to expect from provider review.
This post is general information, not medical advice. It does not describe a benefit of any NoBsRx program and it is not a claim that any medication will preserve muscle or produce any result. Compounded medications are not FDA-approved. Treatment is never guaranteed. Eligibility and every treatment decision are made by independent licensed providers based on an individual clinical assessment, and availability varies by state. Do not start, stop, or change any treatment based on an article. If you have questions about muscle loss or body composition, talk to a licensed clinician.