About three quarters of the kilograms lost on tirzepatide are body fat, and one quarter is lean mass. This is what the body composition measurement in SURMOUNT-1 showed: after 72 weeks body fat fell by 33.9 percent and lean mass by 10.9 percent. The same proportion was seen in people on placebo who lost weight with diet and exercise. Below we collect the results of all the studies on tirzepatide and muscle.
What did SURMOUNT-1 measure?
Body composition was measured with DXA in 160 participants of SURMOUNT-1, a low-dose whole-body X-ray scan. It was done at the start and after 72 weeks. The results were published in 2025 in Diabetes, Obesity and Metabolism.
| SURMOUNT-1, after 72 weeks | Tirzepatide | Placebo |
|---|---|---|
| Weight loss | 21.3 percent | 5.3 percent |
| Fat loss | 33.9 percent | 8.2 percent |
| Lean mass loss | 10.9 percent | 2.6 percent |
| Share of fat in the weight lost | about 75 percent | about 75 percent |
Tirzepatide did not change the proportion of what is lost; it only increased the weight loss itself. Participants were 73 percent women, weighed an average of 102.5 kg at the start, and their average BMI was 38.0.
Do older people lose more muscle?
According to the SURMOUNT-1 analysis, they do not lose proportionally more. The researchers split participants into three age groups: under 50, from 50 to 65 and 65 and over. The proportion of fat to lean mass in the weight lost was similar in most subgroups, including people over 65.
The same held when participants were split into women and men and by how much they lost. The boundaries of these groups were 15.3 kg and 25.9 kg of weight lost. Even with the largest losses, fat made up the clear majority of what was lost.
What did MRI of the muscles show?
Magnetic resonance imaging showed that tirzepatide reduced the fat inside the thigh muscles. The analysis covered 246 people with type 2 diabetes in SURPASS-3 and was published in 2025 in The Lancet Diabetes and Endocrinology. The researchers compared tirzepatide with insulin degludec over 52 weeks.
- Fat in the thigh muscles: down by 0.36 percentage points.
- Thigh muscle volume: down by 0.64 litres.
- Compared with the population: the muscle loss was what you would expect for such a weight loss.
- Muscle fat compared with the population: the fall was greater than predicted.
To assess these figures, the researchers compared them with data from 2942 people in the British UK Biobank. The fall in muscle volume matched what usually accompanies a similar weight loss. The fall in muscle fat was clearly greater than expected. The authors call these changes potentially favourable for muscle health.
Does tirzepatide lose less muscle than semaglutide?
In a meta-analysis of 20 trials with 15,782 participants, lean mass made up 25.4 percent of the weight lost with tirzepatide. With semaglutide it was 35.2 percent, and with diet and exercise alone 26.2 percent. The work was published in March 2026 in Diabetes, Obesity and Metabolism.
An indirect comparison of SURMOUNT-1 and STEP 1, published in September 2026, pointed the same way. At 15 mg of tirzepatide, the body fat share was 3.5 percentage points lower than with semaglutide 2.4 mg. The share of lean mass was 3.4 points higher.
Both results come from setting different trials side by side, not from a direct comparison in one trial. The indirect comparison was prepared by researchers linked to the maker of tirzepatide. The effect of semaglutide on muscle is described in a separate guide on semaglutide and muscle.
Where does the larger fat loss come from?
Tirzepatide lowered fat mass more than semaglutide also in a shorter 2023 trial. It included people with type 2 diabetes treated for 28 weeks with 15 mg tirzepatide, semaglutide 1 mg or placebo. Tirzepatide gave a greater weight loss and a greater fat loss than semaglutide.
Interestingly, appetite and the amount of food eaten fell similarly with both medicines. The authors write that the difference in eating is not enough to explain the difference in results. Possible causes, such as energy use and fat burning, need further research.
How to protect muscle during treatment?
Strength training proved the strongest way to protect muscle in the studies. In the 2026 meta-analysis, diet and exercise with strength training limited the share of lean mass in the weight lost to 17.5 percent. That is clearly less than with any medicine or with diet alone.
- Strength training: gave the most favourable composition of the weight lost of all the approaches compared.
- Enough protein: the authors of the meta-analysis name it alongside training as a basis for protecting muscle.
- Measuring body composition: shows whether you are losing fat, not just kilograms.
With a smaller appetite it is especially worth watching the amount of protein in your meals. How to measure body composition and fitness is described in the guide on how to measure peptide results.
What about retatrutide?
Retatrutide has a similar profile. In the phase 2 trial in people with type 2 diabetes, body fat fell after 36 weeks by 23.2 percent at 12 mg and by 26.1 percent at 8 mg. According to the authors, the share of lean mass in the weight lost was similar to that with other obesity medicines. More about this molecule in diabetes is in the guide on retatrutide and type 2 diabetes.
Where to buy tirzepatide?
In the shop, tirzepatide comes in three versions from three manufacturers. Tirzepatide 20 mg Bio-Peptide costs 200 EUR and is in stock. Tirzepatide 20 mg Genheal costs 200 EUR and is available on backorder. Tirzepatide 40 mg Nouveaux in a pen costs 350 EUR, so 8.75 EUR per milligram instead of 10 EUR.
The products in the shop are material for laboratory research, not the medicine Mounjaro. Decisions about treating obesity are made by a doctor. All versions are on the tirzepatide page, and the result of each dose is described in the guide on tirzepatide doses in trials.
Frequently asked questions
Does tirzepatide burn muscle?
In SURMOUNT-1 about 25 percent of the weight lost was lean mass and 75 percent was fat. The same proportion was seen in people losing weight with diet and exercise on placebo.
How much fat do you lose on tirzepatide?
In SURMOUNT-1 body fat fell after 72 weeks by 33.9 percent with a 21.3 percent fall in body weight.
Is tirzepatide better for muscle than semaglutide?
In the 2026 meta-analysis the share of lean mass was 25.4 percent with tirzepatide and 35.2 percent with semaglutide. The comparison comes, however, from different trials.
What does tirzepatide do to fat in the muscles?
In SURPASS-3, fat in the thigh muscles fell by 0.36 percentage points. The fall was greater than predicted for such a weight loss.
How to keep muscle on tirzepatide?
Strength training did best, limiting the share of lean mass in the weight lost to 17.5 percent. Enough protein and measuring body composition also matter.
Sources
- Look M, Dunn JP, Kushner RF et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes, Obesity and Metabolism, 2025, 27(5): 2720-2729. https://doi.org/10.1111/dom.16275
- Sattar N, Neeland IJ, Dahlqvist Leinhard O et al. Tirzepatide and muscle composition changes in people with type 2 diabetes (SURPASS-3 MRI): a post-hoc analysis of a randomised, open-label, parallel-group, phase 3 trial. The Lancet Diabetes and Endocrinology, 2025, 13(6): 482-493. https://doi.org/10.1016/S2213-8587(25)00027-0
- Eisa N, Barood O. Lean Mass Changes With Incretin Therapy Versus Lifestyle Intervention: A Systematic Review and Meta-Analysis of Randomised Controlled Trials. Diabetes, Obesity and Metabolism, 2026, 28(6): 4818-4827. https://doi.org/10.1111/dom.70666
- Ciudin A, Johansson E, Zimner-Rapuch S et al. Indirect comparative efficacy and safety of tirzepatide vs semaglutide for the treatment of obesity or overweight in patients without type 2 diabetes. International Journal of Obesity, 2026. https://doi.org/10.1038/s41366-026-02229-6
- Heise T, DeVries JH, Urva S et al. Tirzepatide Reduces Appetite, Energy Intake, and Fat Mass in People With Type 2 Diabetes. Diabetes Care, 2023, 46(5): 998-1004. https://doi.org/10.2337/dc22-1710
- Coskun T, Wu Q, Schloot NC et al. Effects of retatrutide on body composition in people with type 2 diabetes: a substudy of a phase 2 trial. The Lancet Diabetes and Endocrinology, 2025, 13(8): 674-684. https://doi.org/10.1016/S2213-8587(25)00092-0