If you're in your 40s, 50s or 60s, muscle isn't vanity. It's what keeps you strong, steady on your feet and independent later on. So it's a fair worry that a drug which takes off 15% to 20% of your weight might take muscle with it. The most detailed data so far comes from body scans in one large tirzepatide trial, and it's more reassuring than the headlines, with real limits [1].
What the body scans measured
SURMOUNT-1, the main weight trial behind Zepbound, ran a substudy using DXA, an X-ray body scan that splits the body into fat, lean tissue and bone. Of the people scanned at the start and at week 72, 160 had both scans: 124 on tirzepatide and 36 on placebo. Most were women (73%), with an average starting weight of about 226 lb (102.5 kg) [1].
The study was funded by Eli Lilly, which makes Zepbound and Mounjaro, and several authors were Lilly employees. A later correction fixed only a chart label in one figure, from percent to kilograms; the headline numbers didn't change [2].
Three-quarters of the loss was fat, the same as on placebo
By week 72, people on tirzepatide had lost 21.3% of their body weight, 33.9% of their fat mass and 10.9% of their lean mass. The placebo group lost 5.3%, 8.2% and 2.6% [1]. Put as shares of the weight that came off, about 75% was fat and 25% was lean, "for both tirzepatide and placebo."
| Measure | Tirzepatide | Placebo |
|---|---|---|
| Body weight | −21.3% | −5.3% |
| Fat mass | −33.9% | −8.2% |
| Lean mass | −10.9% | −2.6% |
| Share of weight lost that was fat | About 75% | About 75% |
Look 2025, pooled tirzepatide doses (n=124) vs placebo (n=36) [1].
That's the key finding. Tirzepatide took off far more weight than placebo, so it took off more lean mass in absolute terms. But the mix was the same: it didn't take a bigger share from lean tissue than ordinary weight loss did. The authors note that diet-only studies commonly report the same rough 75/25 split.
The split held for people 65 and older
The researchers checked the split by age group. On tirzepatide, fat made up 74% of the weight lost in people under 50, 75% in people 50 to 64, and 76% in people 65 and older [1]. On placebo it was 77%, 75% and 63%. These were post hoc checks in small groups, so treat them as a pattern rather than proof, but they don't suggest older adults lost proportionally more lean tissue on the drug.
Men, women, dose and how much you lose barely moved the split
The same pattern showed up in the other cuts the researchers made. On tirzepatide, fat made up 75% of the weight lost in women and 73% in men [1]. By dose, it was 75% on 5 mg, 72% on 10 mg and 75% on 15 mg. By how much weight people lost, the fat share was 70% in the group that lost the least (about 34 lb, 15.3 kg, or less), 73% in the middle group and 76% in the group that lost the most.
| Subgroup | Fat share of weight lost |
|---|---|
| Under 50 / 50 to 64 / 65 and older | 74% / 75% / 76% |
| Women / men | 75% / 73% |
| 5 mg / 10 mg / 15 mg | 75% / 72% / 75% |
| Smallest / middle / largest weight loss | 70% / 73% / 76% |
Look 2025, post hoc subgroup analyses [1].
In pounds, the split still adds up to something. A 226-lb person who loses the study's average 21.3% drops about 48 lb; a quarter of that is roughly 12 lb of lean tissue, by our arithmetic. That's why the conversation about keeping strength is worth having early, not after the weight is off.
What the labels say: more fat lost than lean
Both weight-loss labels carry the same line in their pharmacology sections. Zepbound's: "Tirzepatide lowers body weight with greater fat mass loss than lean mass loss" [3]. Wegovy's: "Semaglutide lowers body weight with greater fat mass loss than lean mass loss" [4]. Neither label lists muscle loss as a side effect.
For semaglutide, STEP 1 also ran a body-scan substudy, but its numbers sit in the full paper rather than the summary, and this page sticks to figures it could check. The label line above is the semaglutide statement on the record.
"Lean mass" isn't the same thing as muscle
A DXA scan can't see muscle on its own. The SURMOUNT-1 paper defines lean mass as "total body fat-free mass minus total bone mineral content, also known as lean soft tissue mass" [1]. That's everything that isn't fat or bone, so it includes muscle along with organs, skin and body water. A 10.9% drop in lean mass doesn't mean a 10.9% drop in muscle; the scan can't say how much of it was muscle.
Why it still matters after 40
Even at the normal 75/25 split, a large weight loss means a real amount of lean tissue. The paper's authors note that losing too much lean mass, especially muscle, can reduce strength and physical function, and that it may be undesirable for older adults at risk of sarcopenic obesity, low muscle combined with excess fat [1]. They suggest nutrition and physical-activity plans tailored to the person.
Those are questions for your prescriber, not a protocol for this page. Worth asking: how much protein makes sense for you, whether strength training fits your health, and whether a body-composition scan is worth tracking. For changes in the face and skin with fast weight loss, see Ozempic face. For a broader long-term view, see metformin and longevity.
Bone: Wegovy's label reports more fractures in some groups
Bone is the other half of the worry. Zepbound's label doesn't report bone or fracture outcomes [3]. Wegovy's does. In its heart-outcomes trial, hip and pelvis fractures were more common on Wegovy than placebo in women (1% vs 0.2%) and in people 75 and older (2.4% vs 0.6%). In a trial in adults with fatty liver disease (MASH), fractures happened in 4.4% on Wegovy against 3.3% on placebo [4].
| Group | Wegovy | Placebo |
|---|---|---|
| Women, heart-outcomes trial (hip and pelvis) | 1% (24 of 2,448) | 0.2% (5 of 2,424) |
| Age 75 and older, heart-outcomes trial (hip and pelvis) | 2.4% (17 of 703) | 0.6% (4 of 663) |
| Adults with MASH (all fractures) | 4.4% | 3.3% |
Wegovy prescribing information, section 6.1, "Fractures" [4].
The label doesn't say why; falls, bone density and other causes weren't separated. If you already have thin bones, a fall history or are 75 or older, raise it with your prescriber before starting. The full Wegovy safety picture is in Wegovy side effects.
Compounded versions and other drugs
No body-scan study has looked at compounded semaglutide or tirzepatide. Compounded drugs aren't FDA-approved, the FDA doesn't review them for safety or effectiveness, and it doesn't check their quality before they ship. The tirzepatide numbers above belong to the brand trial. Metformin, often discussed alongside GLP-1s for longevity, has no reported effect on muscle mass on its label; muscle pain appears there only as a possible sign of lactic acidosis [5]. For what else is on the tirzepatide label, see tirzepatide side effects, and for the molecules, the tirzepatide guide and semaglutide guide.
Results build over months, so a muscle plan matters most if you'll be on treatment for a while; see how long Zepbound takes to work and what happens when you stop. Programs that also cover longer-term health are scored on longevity prescription programs.