When semaglutide or tirzepatide takes off 15% or 20% of a person's weight, not all of that weight is fat, and the worry is that the drugs strip muscle along with it. The answer depends on what is measured, against what comparison, and whether lean tissue is the same thing as muscle.
For the size of the losses involved, see how much weight you can lose on tirzepatide and on semaglutide.
What the labels say
Both the Wegovy and Zepbound labels use the same sentence in their pharmacology sections: the drug “lowers body weight with greater fat mass loss than lean mass loss” [1] [2].
That confirms lean mass is lost too, but it does not say how much, or whether the lean tissue lost is muscle, which is why the scan studies below matter.
How much of the weight lost is lean mass?
The clearest single dataset is the SURMOUNT-1 DXA substudy, in which 160 participants had whole-body scans at the start and at week 72 [3]. On pooled tirzepatide doses, body weight fell 21.3%, fat mass 33.9% and lean mass 10.9%.
On placebo the figures were 5.3%, 8.2% and 2.6%. In both groups, roughly 75% of the weight lost was fat and 25% was lean mass, and that split held across most subgroups of sex, age and amount lost.
| Source | Scope | Lean share of weight lost |
|---|---|---|
| SURMOUNT-1 DXA substudy | Tirzepatide, 160 people, 72 weeks | About 25% |
| Karakasis 2025 | 22 trials, 2,258 people, all GLP-1 drugs | About 25% |
| Eisa 2026 | Semaglutide trials | 35.2% |
| Eisa 2026 | Tirzepatide trials | 25.4% |
| Eisa 2026 | Lifestyle intervention alone | 26.2% |
| Eisa 2026 | Lifestyle plus resistance training | 17.5% |
A 2025 network meta-analysis of 22 randomized trials found lean mass made up about 25% of weight lost on GLP-1 drugs [4]. Relative lean mass, lean tissue as a share of body weight, did not fall.
It ranked semaglutide 2.4 mg and tirzepatide 15 mg as the most effective for weight and fat loss but among the least effective at preserving lean mass, while liraglutide lowered weight without a significant lean loss.
A 2026 meta-analysis of 20 trials with 15,782 people set drug trials beside lifestyle trials [5]. Lean mass was 25% to 39% of the weight lost on incretin drugs, and the lifestyle share was comparable (p = 0.42).
Lean mass is not the same as muscle
A 2024 review points out that lean mass includes not only muscle but also organs, fluids and the water held in fat tissue itself [6]. So a fall in lean mass does not always mean an equal fall in muscle.
Drawing on MRI studies, the same review argues that muscle changes on these drugs look adaptive: the volume lost is roughly what age, disease and the amount of weight lost would predict, and fat infiltration inside muscle improves.
The same review flags older age and more severe disease as reasons for caution, because of the risk of sarcopenia, the age-related loss of muscle mass and strength.
A 2026 systematic review in the Annals of Internal Medicine is less reassuring [7]. Across 35 randomized trials, the median share of weight lost from muscle-based measures was 28.3%, and about two thirds of drug studies exceeded the benchmarks the authors set in advance.
Its sharpest finding is a gap: no study it included reported an objective measure of physical function. So whether this lean loss translates into weaker or less mobile people remains unmeasured.
What limits it
The strongest signal in the 2026 meta-analysis came from resistance training: lifestyle programs that added it lost 17.5% of weight as lean mass, the lowest share of any group [5].
The 2025 joint nutrition advisory lists resistance training as part of care for people on GLP-1 drugs, to help preserve muscle and bone [8]. On protein, it notes the general adult allowance of 0.8 g per kg per day and says targets of 1.2 to 1.6 g/kg have been proposed during active weight loss.
Because body weight overstates needs in obesity, it also offers an absolute target of 80 to 120 g a day. It suggests eating protein-rich foods first in a meal, since appetite is reduced.
Food choices on these drugs are covered in what to eat on semaglutide. The evidence on peptides marketed for muscle is in peptides for muscle growth.
Related reading
Visible changes that follow fast weight loss are covered in Ozempic face. Whether NAD+ has any role in weight is in NAD for weight loss, and the timeline of loss is in how long it takes for tirzepatide to work.
To compare prescribers, see the weight-loss prescriber rankings.