Does Retatrutide cause muscle loss?

A common concern with any weight-loss drug is whether it burns fat or burns muscle. For retatrutide, the triple GLP-1/GIP/glucagon receptor agonist developed by Eli Lilly, the short answer is: the body composition data published so far shows fat mass declining substantially more than lean mass, with the proportion of lean mass lost looking similar to other obesity treatments. That is reassuring, but the dataset is still limited to Phase 2 trials, and the full picture will only emerge when the larger Phase 3 TRIUMPH results are published.

What the Phase 2 body composition substudy found

The most direct evidence comes from Coskun and colleagues (Lancet Diabetes and Endocrinology, 2025), who ran a body composition substudy using dual-energy X-ray absorptiometry (DXA) within the Phase 2 retatrutide type-2 diabetes trial. DXA scans were taken at baseline and at week 36. Total fat mass reductions were substantial and dose-dependent: 15.2% on the 4 mg dose, 26.1% on the 8 mg dose, and 23.2% on the 12 mg dose, compared with 2.6% on dulaglutide 1.5 mg and 4.5% on placebo^[1]. The key conclusion on muscle: the authors stated that "the proportion of lean mass loss to weight loss was similar to other obesity treatments" and that the findings "could provide reassurance that a greater proportion of lean mass is not lost with retatrutide."

The Phase 2 obesity trial: weight loss without body composition breakout

The headline Phase 2 obesity trial, Jastreboff et al. (NEJM, 2023), enrolled 338 adults with obesity or overweight and reported 24.2% mean body-weight reduction at 48 weeks on the 12 mg dose, 22.8% at 8 mg, and 17.1% at 4 mg, against 2.1% on placebo^[2]. The published abstract does not include a dedicated DXA or body composition analysis for the obesity population. The Coskun 2025 substudy covers the diabetes trial specifically. A body composition analysis of the obesity-trial population has not yet been published as of the date of this article, which is a gap in the evidence.

How this fits the broader GLP-1 class picture

Retatrutide belongs to the incretin drug class, and the body composition pattern observed in its Phase 2 data is consistent with what systematic reviews have found across semaglutide, liraglutide, and tirzepatide. A 2026 systematic review and meta-analysis by Sawicka-Gutaj and colleagues, pooling data from GLP-1 receptor agonist trials, concluded that "fat mass decline predominated, whereas reductions in lean body mass were modest," characterising the overall finding as selective fat mass reduction with relative preservation of lean tissue^[3].

That relative preservation does not mean zero lean mass loss. Any significant weight reduction, whether from medication, surgery, or dietary restriction, involves some loss of lean body mass. The question is proportion: does GLP-1 class treatment remove more muscle than the same magnitude of weight loss would through other means? Current evidence does not support that conclusion, but the datasets remain smaller and shorter than the obesity surgery literature.

Why the glucagon receptor component matters here

Retatrutide adds the glucagon receptor to the GLP-1 and GIP arms that tirzepatide already covers. Glucagon agonism raises energy expenditure, which is one rationale for the larger weight-loss numbers. There is a theoretical question of whether glucagon-driven energy expenditure increases the catabolism of lean tissue. Based on the available Phase 2 data, that concern has not materialised into a measurably worse lean mass outcome compared with dual or single agonists, but the Phase 2 dataset (roughly 600 participants across both trials, observed for 36 to 48 weeks) is not large enough to rule out a small difference. Phase 3 results from the TRIUMPH programme will provide the more definitive answer^[4].

Resistance exercise and protein intake

The literature on GLP-1 class drugs consistently raises the same practical question: can resistance training and adequate dietary protein offset the lean mass loss that occurs alongside fat loss? The direct trial evidence for this in retatrutide users is non-existent, because resistance exercise protocols were not a primary variable in the Phase 2 trials. The broader weight-loss literature supports resistance training for lean mass preservation during caloric restriction, and that rationale extends plausibly to pharmacological weight loss, but the data are in semaglutide and tirzepatide populations, not in retatrutide specifically.

For context on what the class evidence looks like, see the semaglutide body composition article. The tirzepatide clinical profile also covers lean mass data from the SURMOUNT programme, which is the closest approved comparator to retatrutide.

What we do not yet know

The body composition question for retatrutide has several open edges. The DXA substudy by Coskun et al. covered the diabetes population at 36 weeks: we do not have equivalent published data for the obesity population, for longer observation windows, or for the highest Phase 3 doses. We do not know whether the glucagon arm produces any signal at scale that differs from the class baseline. And we do not know what happens to lean mass over multi-year continuous treatment, which is the clinically relevant timeline for a drug being positioned as long-term obesity management.

The bottom line

The Phase 2 DXA data from Coskun et al. 2025 shows retatrutide driving large fat mass reductions with lean mass loss in proportion to what other obesity treatments produce. That is the best available answer to the muscle-loss question as of July 2026. The Phase 3 TRIUMPH programme, which enrolled thousands of participants with longer follow-up, will provide the data needed to confirm or qualify that finding. Until those results are published and peer-reviewed, the body composition picture for retatrutide remains based on a relatively small Phase 2 dataset.

Medical disclaimer: this article is for educational and journalistic purposes only and does not constitute medical advice. Retatrutide is an investigational compound not approved by the FDA, EMA, MHRA, or any regulatory agency we track. Always consult a qualified healthcare professional before considering any peptide or investigational compound. PeptideMethods.com does not sell, distribute, or facilitate the sale of any peptide product.