GLP-1 muscle loss is real but often overstated: lean mass makes up about 25-35% of the weight people lose on these drugs, roughly the same share as with diet-only weight loss. A 2026 meta-analysis of 20 randomized trials (15,782 people) found lean mass accounted for 35.2% of weight lost on semaglutide, 25.4% on tirzepatide and 26.2% with lifestyle programs.[1] Adding resistance training cut that share to 17.5%.
For the basics of these drugs, see the GLP-1 basics hub.
Bottom line
| Approach | Share of weight lost that was lean mass |
|---|---|
| Semaglutide | 35.2%[1] |
| Liraglutide | 26.8%[1] |
| Tirzepatide | 25.4%[1] |
| Lifestyle (diet and activity) | 26.2%[1] |
| Lifestyle plus resistance training | 17.5%[1] |
Because people lose much more total weight on GLP-1 drugs, they also lose more lean mass in kilograms, even when the proportion is similar.
Why lean mass falls
When you lose weight, your body carries less load, so it needs less muscle and supporting tissue. Eating far fewer calories, and often less protein, adds to the loss. GLP-1 drugs reduce appetite strongly, so protein intake can drop without people noticing. Our GLP-1 diet guide covers ways to keep protein up when appetite is low.
An important caveat: lean mass is not the same as muscle. DXA scans, the usual tool in trials, count everything that isn’t fat or bone as lean mass, including water, organs and connective tissue. Some of the “lean” loss is from these, so DXA likely overstates true muscle loss to some degree.
The key studies
SURMOUNT-1 body composition substudy (2025). In 160 adults with obesity who had DXA scans, tirzepatide reduced body weight by 21.3%, fat mass by 33.9% and lean mass by 10.9% over 72 weeks. Placebo reduced them by 5.3%, 8.2% and 2.6%. About 75% of weight lost was fat and 25% lean, in both groups, and this held across age and sex subgroups.[2]
STEP 1 (2021). This trial of 1,961 adults found 14.9% average weight loss with semaglutide 2.4 mg over 68 weeks, versus 2.4% on placebo. A subgroup had DXA scans, and the trial reported a larger fall in fat mass than lean mass.[3]
Lean mass meta-analysis (2026). Pooling 20 trials of semaglutide, tirzepatide, liraglutide and lifestyle programs, the share of weight lost as lean mass was similar for drugs and diet alone. Programs that included resistance training preserved the most.[1]
Muscle health meta-analysis (2026). Across 7 trials (821 people), GLP-1 drugs at obesity doses lowered absolute lean mass by an average of 1.74 kg but raised lean mass as a percentage of body weight by 1.81 points, because fat fell faster. Semaglutide showed the largest absolute lean loss, about 5.4 kg.[4]
Exercise plus liraglutide (2024). In a Danish trial, people who first lost weight on a diet were assigned to supervised exercise, liraglutide, both, or placebo. One year after all treatment stopped, those who had done exercise plus liraglutide kept off more weight and had a lower body-fat percentage than those on liraglutide alone. People who stopped liraglutide alone regained 6 kg more than those who stopped exercise alone.[5]
Established vs preliminary
Established:
- Lean mass falls during GLP-1 weight loss, at about a quarter to a third of total weight lost.[1][2]
- Fat falls more than lean, so body composition usually improves overall.[4]
Preliminary:
- Whether muscle quality (fat inside muscle, strength per kilogram) improves or worsens.
- Long-term effects on strength, falls and fractures, especially after 65.
- Whether muscle-sparing drugs add meaningful benefit.
Protecting muscle: what’s recommended
A 2025 joint advisory from four US societies (lifestyle medicine, nutrition, obesity medicine and The Obesity Society) recommends assessing muscle strength and body composition before starting a GLP-1 drug. During treatment, it calls for resistance training and an appropriate diet to preserve muscle and bone mass, plus support from a registered dietitian where possible.[6]
In practice, that usually means:
- strength training two or more times a week, built up gradually
- protein at most meals, especially when appetite is low
- watching for weakness or extreme tiredness; our GLP-1 fatigue guide covers common causes
Muscle-sparing drugs in development
Bimagrumab is an investigational antibody that blocks activin type II receptors, signals that limit muscle growth. In the 507-person BELIEVE phase 2 trial, people on bimagrumab plus semaglutide 2.4 mg lost 17.8 kg at 48 weeks, versus 14.2 kg on semaglutide alone. Muscle spasms, diarrhea and acne were common with bimagrumab.[7] A 2026 meta-analysis of 4 bimagrumab trials found it increased lean mass by about 1.7 kg versus placebo, but also raised LDL cholesterol and dropout rates.[8] An older experimental drug that targeted the same pathway is profiled on our ACE-031 page.
Limitations
- DXA can’t separate muscle from water and organs.
- Most trials lasted 1-1.5 years and measured body composition in small subgroups.[2]
- Few studies tracked strength, walking speed or falls.
- Lean mass regained after stopping may not match what was lost; see our guide to stopping a GLP-1.
What’s next
- LEAN-PREP will randomize 232 adults on semaglutide or tirzepatide to resistance exercise, protein supplements, both, or neither, with MRI-measured thigh muscle size as the main outcome.[9]
- Bimagrumab plus tirzepatide: a 252-person Lilly phase 2 trial is active but no longer recruiting.[10]
Questions to discuss with a clinician
- Should I have a body composition scan or strength test before starting?
- How much protein should I aim for, given my kidneys and other conditions?
- What kind of strength training is safe for me?
- If I’m over 65, should my dose or weight-loss goal be adjusted?
