What the trial body scans showed
In SURMOUNT-1, 160 people had DXA body scans: 124 on tirzepatide and 36 on placebo. Over 72 weeks, the tirzepatide group lost 33.9% of their fat mass and 10.9% of their lean mass, on average (SURMOUNT-1 DXA substudy, 2025).
| Group | Weight | Fat mass | Lean mass | Share of weight lost that was lean |
|---|---|---|---|---|
| Tirzepatide (all doses) | −21.3% | −33.9% (−15.9 kg) | −10.9% (−5.6 kg) | 26% |
| Placebo | −5.3% | −8.2% (−3.6 kg) | −2.6% (−1.2 kg) | 25% |
From the SURMOUNT-1 DXA substudy (SURMOUNT-1 DXA substudy, 2025). Averages in the trial, not a prediction.
The placebo group, who had diet and activity counselling, lost a similar share as lean mass (SURMOUNT-1 DXA substudy, 2025). A widely cited rule of thumb for diet-based weight loss is that about a quarter of the weight lost is lean (fat-free) mass, though the review that describes it also critiques it (Heymsfield, Obes Rev, 2014).
Semaglutide's numbers point the same way. In the STEP 1 body-scan group, people on semaglutide lost an average of 9.3 kg of fat and 5.8 kg of lean mass (STEP 1 results, ClinicalTrials.gov, 2021).
Lean mass is not the same as muscle
A DXA scan sorts the body into fat, bone and everything else. "Lean mass" is that everything else, so it includes muscle but also organs and water. The trials didn't measure muscle on its own.
What may help
A 2025 joint advisory from four US obesity and nutrition societies recommends strength (resistance) training alongside GLP-1 treatment, and says 1.2 to 1.6 g of protein per kg of body weight a day has been proposed during weight loss (Joint nutrition advisory, Obesity, 2025). Guy's and St Thomas' NHS Foundation Trust suggests strength exercise twice a week for people on weight-loss medicines (Guy's and St Thomas' NHS, 2025).
If you have a health condition, or you're unsure what is safe for you, ask your doctor, dietitian or a physiotherapist first. More on protein on a GLP-1.
Protect what you can.
- Strength train on a regular schedule
- Eat protein at each meal
- Track strength sessions, not just weight
- Watch your waist and photos
- Ask about a body-composition scan if it matters for your care
Where these facts come from.
- Look M et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes Obes Metab 2025;27:2720-2729.
- Heymsfield SB et al. Weight loss composition is one-fourth fat-free mass: a critical review and critique of this widely cited rule. Obes Rev 2014;15:310-321.
- STEP 1 (NCT03548935) results, including the DXA body-composition substudy. ClinicalTrials.gov.
- Mozaffarian D et al. Nutritional priorities to support GLP-1 therapy for obesity: a joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association and The Obesity Society. Obesity 2025;33:1475-1503.
- Dietary advice for people taking weight-loss medicines: food and nutrition. Guy's and St Thomas' NHS Foundation Trust. Reviewed Aug 2025.
Checked 25 Oct 2026. Trial figures are average results in the trial, not predictions. Labels change; your prescriber and the leaflet in your pack have the current version.
Pip Body Scan is a personal progress record. It is not a medical device and does not give medical advice, diagnose conditions or recommend doses. Speak to your prescriber or another qualified healthcare professional about symptoms, side effects or changes to your treatment.