Does Retatrutide Cause Muscle Loss?

The Phase 2 Body Composition Sub-Study

Study design

• Sub-study within the Phase 2 T2D trial (NCT04867785)
• Body composition measured by DXA scans (dual-energy X-ray absorptiometry) — the clinical gold standard for distinguishing fat mass from lean mass
• 36 weeks of treatment
• Primary endpoint: percent change in total fat mass at 36 weeks vs. placebo and dulaglutide (an existing GLP-1 drug)
• Comparators: placebo and dulaglutide 1.5mg

Who was actually scanned

56% of sub-study participants were female; 85% were White, 13% Black, and 3% Asian.

Why this data matters

This is the first time retatrutide's effect on body composition has been measured with DXA, providing objective data on how much of the weight lost is fat versus lean tissue. Prior to this, all retatrutide weight loss data reported only total body weight — we could not determine the composition of that weight loss.

What the sub-study reported

The authors concluded that fat loss "outpaced lean mass loss across all effective dose groups" and that lean mass loss "did not exceed expected proportions" for weight-loss therapies.

The authors' published interpretation: the findings "could provide reassurance that a greater proportion of lean mass is not lost with retatrutide despite the overall increased weight loss."

Caveats: only 103 of the 189 enrolled participants completed both DXA scans, and the sub-study was not powered as a primary endpoint analysis. The attenuated fat loss at 12 mg (-23.2%) versus 8 mg (-26.1%) likely reflects sample attrition at the highest dose rather than a true plateau, and these were participants with type 2 diabetes — the obesity-only Phase 3 population may show different ratios.

The Muscle Loss Question in Context

Why muscle loss matters during weight loss

When the body is in caloric deficit — whether from a drug that reduces appetite or from eating less — it draws energy from both fat stores and lean tissue. The consequences of excessive lean mass loss include:

• Reduced resting metabolic rate — muscle is metabolically active tissue, so losing it lowers the number of calories your body burns at rest
• Impaired physical function — particularly concerning in older adults, where sarcopenia (age-related muscle loss) is already a risk
• Potential for weight regain — a lower metabolic rate after weight loss can create conditions favorable for regaining weight if the drug is stopped
• Bone density reduction — significant weight loss can also reduce bone mineral density, increasing fracture risk

What we know from other GLP-1 drugs

For context, the "ideal" ratio is debated, but most clinicians consider it acceptable if lean mass loss accounts for less than ~25% of total weight lost. Semaglutide's STEP 1 DXA data showing ~39% lean mass loss generated concern and fueled the "Ozempic body" narrative in consumer media.

Why Retatrutide Might Differ

There are theoretical reasons to believe retatrutide's triple-agonist mechanism could produce a more favorable body composition outcome than GLP-1-only drugs:

The glucagon hypothesis

Glucagon receptor activation — unique to retatrutide among advanced clinical candidates — has several effects that could preferentially target fat over muscle:

• Promotes lipolysis — the breakdown of stored fat for energy, potentially directing the body to draw more energy from fat stores rather than muscle
• Increases thermogenesis — energy expenditure through heat production, primarily in brown and beige adipose tissue, not muscle tissue
• Increases hepatic fatty acid oxidation — the liver burns more fat, reducing liver fat stores

If these mechanisms shift the body's energy sourcing toward fat metabolism, the proportion of weight lost from lean tissue could theoretically be lower with retatrutide than with drugs that lack glucagon activity.

Important caveats

This is a hypothesis based on glucagon physiology, not confirmed clinical data. The Phase 2 DXA sub-study published in June 2025 is the first direct measurement of this question. The Phase 2 sub-study was conducted in T2D patients, and body composition responses may differ in non-diabetic obesity populations. Larger Phase 3 data will be needed to draw definitive conclusions.

Preserving Muscle During Weight Loss

The muscle-preservation protocol

For older adults, clinicians often target the higher end of this protein range (around 1.4–1.6 g/kg/day), because age-related muscle loss (sarcopenia) compounds the risk during a caloric deficit.

Why resistance training is non-negotiable

Resistance exercise (weight lifting, bodyweight movements, resistance bands) is the single most effective intervention for preserving muscle during weight loss — it signals the body that muscle tissue is needed, encouraging it to preferentially draw energy from fat stores. The advisory is explicit that nutrition cannot substitute for the training stimulus: increased protein intake alone, it states, "is likely inadequate to support the preservation of muscle mass in the absence of structured resistance/strength training." Protein supplies the building blocks; the training is what tells the body to keep the muscle.

Hitting the protein target is harder on retatrutide

How much of the loss is muscle?

What We Do Not Know Yet

• Phase 3 body composition data has not been published. The Phase 2 sub-study provides initial signal, but larger datasets are needed.
• Long-term effects on lean mass beyond 36 weeks are unknown. Weight loss drugs are intended for chronic use, and the body composition trajectory over years has not been characterized for retatrutide.
• Whether the glucagon hypothesis holds up in practice — whether retatrutide actually produces a better fat-to-lean-mass loss ratio than existing drugs in head-to-head comparison.
• Bone mineral density effects — significant weight loss can reduce bone density, and no retatrutide-specific bone data has been published.
• Effects in older adults — the T2D Phase 2 sub-study may not reflect outcomes in elderly patients, where sarcopenia is a greater concern.

Frequently Asked Questions

Sources

• Phase 2 Body Composition Sub-Study. The Lancet Diabetes & Endocrinology. June 2025. (NCT04867785 sub-study)
• Coskun, T., Wu, Q., Schloot, N.C., Haupt, A., Milicevic, Z., Khouli, C., Harris, C. (2025). Effects of retatrutide on body composition in people with type 2 diabetes: a substudy of a phase 2, double-blind, parallel-group, placebo-controlled, randomised trial. The Lancet Diabetes & Endocrinology. DOI: 10.1016/S2213-8587(25)00092-0
• Rosenstock, J., et al. (2023). Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes. The Lancet. DOI: 10.1016/S0140-6736(23)01053-X
• Wilding, J.P.H., et al. (2021). Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). New England Journal of Medicine. DOI: 10.1056/NEJMoa2032183
• Mozaffarian, D., et al. (2025). Nutritional priorities to support GLP-1 therapy for obesity: a joint advisory from the ACLM, ASN, OMA, and TOS. Obesity Pillars. PMC12264624
• ClinicalTrials.gov: NCT04867785