Ozempic, Wegovy, Mounjaro, Zepbound: do you lose fat or muscle?
Every GLP-1 forum asks it. Here’s what the trial body scans found, drug by drug, and how to check your own.
Short answer
Mostly fat, plus some lean mass. In trial body scans (DXA), about 40% of the weight lost on semaglutide (Ozempic, Wegovy) and about 25% on tirzepatide (Mounjaro, Zepbound) was lean mass, which includes water and organs, not only muscle. Strength training two to three times a week and enough protein reduce the muscle share.
Not medical advice. GLP-1 medications are prescribed and supervised by a clinician. This page explains published trial results and how to track your own body; talk to your prescriber about your dose, protein and training.
Do you lose fat or muscle on a GLP-1?
Mostly fat, and some lean mass. In the DXA substudies of the big trials, about three-fifths to three-quarters of the weight people lost was fat; the rest was lean mass. “Lean mass” isn’t all muscle: it includes water, glycogen and organ tissue, so actual muscle loss is smaller than the lean-mass figure. The share is in the same range as weight loss from diet alone; what’s different on a GLP-1 is that the total loss is bigger and faster, so the kilos of lean mass can add up.
| Brand | Drug | Trial with body-composition data | Share of weight lost that was lean mass (DXA) |
|---|---|---|---|
| Ozempic, Wegovy (injection), Rybelsus, Wegovy pill | Semaglutide | STEP 1 (2.4 mg, 68 weeks) | About 40% |
| Mounjaro, Zepbound | Tirzepatide | SURMOUNT-1 (up to 15 mg, 72 weeks) | About 25% (roughly 3 parts fat to 1 part lean) |
| Diet alone (for comparison) | — | Many studies | Commonly about 20–30%, more when lean or losing fast |
Ozempic and Wegovy (semaglutide)
Ozempic, Wegovy and the oral forms are all semaglutide. In STEP 1, people on 2.4 mg lost about 15% of their bodyweight over 68 weeks, and in the DXA substudy about 40% of the weight lost was lean mass. Lower doses such as Ozempic’s haven’t been studied as carefully for body composition, but there’s no reason to expect a different pattern.
Mounjaro and Zepbound (tirzepatide)
Mounjaro and Zepbound are both tirzepatide. In SURMOUNT-1, the highest dose averaged about 21% weight loss over 72 weeks; in the DXA substudy fat mass fell about 34% and lean mass about 11%, so roughly a quarter of the weight lost was lean. See semaglutide vs tirzepatide muscle loss for why these two numbers aren’t a fair head-to-head.
Signs you’re losing muscle, not just fat
- Your arms and shoulders shrink about as fast as your waist. Fat loss usually takes more off the waist.
- Strength drops for several weeks: fewer reps at the same weight, harder stairs.
- You’re losing more than about 1% of bodyweight a week for months, eating little protein, and not lifting.
- Photos look smaller but softer rather than leaner.
Measure waist, arms and shoulders weekly and take matched photos every two weeks; the comparison over a month tells you more than any single number. Our free fat vs muscle calculator gives a quick estimate from two body fat readings.
How to keep more muscle on a GLP-1
- Lift two to three times a week, the whole body, with progressive weights.
- Eat enough protein; many clinicians suggest about 1.2–1.6 g per kg of bodyweight a day, more if you train hard. Ask yours.
- Don’t chase the fastest loss. Talk to your prescriber if you’re losing very fast or barely eating.
- Track arms and shoulders, not just the scale. Read how to preserve muscle on a GLP-1.
FAQ
Frequently asked questions
Do you lose fat or muscle on Ozempic?
Mostly fat. In the semaglutide STEP 1 trial, about 60% of weight lost was fat and about 40% lean mass (which includes water and organs, not only muscle).
Do you lose fat or muscle on Mounjaro?
Mostly fat. In SURMOUNT-1, roughly three-quarters of the weight lost on tirzepatide was fat and a quarter lean mass.
Do you lose fat or muscle on Zepbound?
Zepbound is tirzepatide, the same drug as Mounjaro, so the SURMOUNT-1 result applies: about three parts fat to one part lean mass.
Do you lose fat or muscle on Wegovy?
Wegovy is semaglutide 2.4 mg, the dose tested in STEP 1, where about 40% of the weight lost was lean mass.
Does semaglutide cause muscle loss?
Some lean mass loss happens with any large weight loss, including on semaglutide. Strength training and enough protein reduce it.
How do I know if I’m losing muscle on a GLP-1?
If your arms and shoulders shrink as fast as your waist and your strength drops, you’re likely losing muscle. Track both weekly; ProgressLens flags this pattern in its Progress Report.
Sources
- Wilding JPH et al. (2021). Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med 384:989–1002, with DXA substudy.
- Jastreboff AM et al. (2022). Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med 387:205–216, with DXA substudy.
- Aronne LJ et al. (2025). Tirzepatide as compared with semaglutide for the treatment of obesity (SURMOUNT-5). N Engl J Med.
- Forbes GB. Body fat content influences the body composition response to nutrition and exercise. Ann N Y Acad Sci (2000).