Quick answer: the only published retatrutide muscle loss data come from a phase 2 body scan study in 189 adults with type 2 diabetes. Total fat mass fell by up to 26.1% on retatrutide, and the authors reported that the share of weight lost as lean mass was similar to other obesity treatments.[1] No phase 3 body composition results have been published yet, so how much muscle people lose on the highest doses over 80 weeks is still unknown.
Retatrutide is an investigational drug from Eli Lilly and is not FDA-approved. For its background, see the retatrutide profile.
Why muscle loss is a question for retatrutide
When people lose a large amount of weight, by any method, some of what they lose is lean mass, not just fat. Lean mass includes muscle, organs, water and other tissue that is neither fat nor bone. Losing too much muscle can affect strength, balance and metabolism, especially in older adults.
Retatrutide raises the question more sharply than older drugs because it produces very large weight loss. In the phase 3 TRIUMPH-1 trial, people on 12 mg lost 25.0% of their body weight over 80 weeks on average.[5] Our guide to GLP-1 muscle loss explains how this compares across the drug class.
The body composition study: what it measured
The study was a substudy of the 36-week phase 2 trial in adults with type 2 diabetes.[1][2]
- Who: 189 adults aged 18 to 75 with type 2 diabetes and a BMI of 25 to 50, at 42 US centers.[1]
- What they took: weekly retatrutide (0.5, 4, 8 or 12 mg), dulaglutide 1.5 mg, or placebo.[1]
- How: dual-energy X-ray absorptiometry (DXA), a low-dose scan that splits the body into fat, lean and bone mass.[1]
- Main measure: percent change in total fat mass at week 36.[1]
Only 103 people completed treatment and had both a starting and a week-36 scan, so the numbers per dose group are small.[1]
Results: fat mass by dose
| Treatment | Total fat mass change at 36 weeks |
|---|---|
| Retatrutide 0.5 mg | −4.9% |
| Retatrutide 4 mg | −15.2% |
| Retatrutide 8 mg | −26.1% |
| Retatrutide 12 mg | −23.2% |
| Dulaglutide 1.5 mg | −2.6% |
| Placebo | −4.5% |
Source: Coskun et al., Lancet Diabetes & Endocrinology 2025.[1]
Compared with placebo, the 4 mg, 8 mg and 12 mg doses reduced fat mass significantly; the 0.5 mg dose did not.[1] Retatrutide also reduced fat mass more than dulaglutide.[1]
What happened to lean mass
The authors concluded that the share of weight lost as lean mass was in line with other obesity treatments.[1] In plain terms:
- People on retatrutide did lose lean mass, because they lost a lot of weight overall.
- The lean share of that weight loss was not larger than seen with other treatments.
- The study did not show that retatrutide protects muscle better than other drugs.
The published summary does not give lean mass in kilograms by dose, and we could not access the full paper to confirm those figures. You may see specific numbers quoted online; treat any that are not traceable to the paper with caution.
Side effects in the substudy were similar between groups, mostly gastrointestinal, and no deaths were reported.[1]
Where the fat came from
A different phase 2 substudy, in adults with obesity and fatty liver disease, used MRI to measure fat in specific places:[3]
- Visceral fat (deep belly fat around the organs) fell by up to 48.3% at 48 weeks, while it rose 2.5% on placebo.
- Belly fat under the skin fell by up to 43.5%.
- Liver fat fell by up to 86.0% on 12 mg.
Visceral fat is linked to heart and metabolic risk, so losing it is a meaningful part of the overall result. Lilly also reported that waist size fell by 24.1 cm (9.5 inches) on 12 mg in TRIUMPH-1, versus 3.6 cm on placebo, among people who stayed on treatment.[6] See retatrutide fatty liver for the liver results.
What we don’t know yet
- Phase 3 data: TRIUMPH-3, in adults with severe obesity and heart disease, includes a DXA substudy of about 100 participants to measure changes in fat and lean mass.[4] Results had not been published as of October 2026.
- People without diabetes: the published DXA study enrolled only people with type 2 diabetes.[1]
- Strength and function: DXA measures tissue, not how strong someone is. The published retatrutide results we reviewed do not report grip strength or physical performance tests.
- Older adults: the effect on muscle in people over 75 has not been studied, since they were excluded from the DXA study.[1]
Muscle gain claims
Some people searching for retatrutide ask about muscle gain. No retatrutide trial has measured or reported muscle gain.
A 2026 JAMA Network Open study analyzed posts from people describing their own use of unapproved retatrutide online. Among 7,823 who described at least one effect, 4.1% mentioned muscle hypertrophy (muscle growth), while 2.0% mentioned muscle atrophy (wasting) and 1.8% muscle weakness.[7]
FDA says products sold as “research” retatrutide are of unknown quality and that retatrutide cannot be used in compounding.[8]
Protecting muscle while losing weight
The trials gave retatrutide alongside advice on healthy eating and physical activity.[4] General approaches clinicians often discuss with anyone losing a large amount of weight include:
- regular resistance (strength) training
- enough protein spread through the day
- not losing weight faster than your care team advises
- checking strength or body composition over time if you are older or frail
None of these has been tested specifically with retatrutide. Ask a clinician or dietitian to set targets that fit your health.
Questions to ask a clinician
- How much lean mass might I lose with the weight loss I am aiming for?
- Should I have a body composition scan before and during treatment?
- What protein and exercise targets suit my age and health?
For more on the drug’s effects, see retatrutide weight loss results, retatrutide side effects and how retatrutide works.
