Does Retatrutide Cause Muscle Loss?

All weight loss — from drugs, surgery, caloric restriction, or any other method — involves some degree of lean body mass (muscle) loss alongside fat loss. This is a fundamental physiological reality, not a deficiency of any particular drug. The clinical question is not whether muscle is lost, but how much muscle is lost relative to fat, and whether the ratio can be improved.

For GLP-1 class drugs broadly, studies suggest that approximately 25-40% of total weight lost is lean mass, with the remainder being fat mass. This ratio matters because excessive lean mass loss can reduce metabolic rate, impair physical function, and undermine long-term health outcomes.

Retatrutide's first published body composition data — a Phase 2 sub-study using DXA scans — was reported in The Lancet Diabetes & Endocrinology in June 2025. This page covers what that data shows, what it means, and what we still do not know.

Retatrutide is an investigational drug that has not been approved by the FDA.

The Phase 2 Body Composition Sub-Study

Published in The Lancet Diabetes & Endocrinology (June 2025), this sub-study provides the first DXA body composition data for retatrutide in patients with type 2 diabetes.

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

Of the 281 participants in the main Phase 2 trial, 189 enrolled in the body composition sub-study; 155 had a baseline DXA scan and 103 completed treatment plus both baseline and week-36 scans. Per-arm enrollment:

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

Coskun and colleagues published the results in The Lancet Diabetes & Endocrinology (2025). At 36 weeks, fat mass reduction was dose-dependent:

Total lean mass decreased by up to 12.5% in the 8-mg group, and android visceral fat decreased by up to 31.4% with the 12-mg dose. The fat loss index — the share of total weight loss attributable to fat rather than lean tissue — ranged from 62.0% to 69.3% across the effective retatrutide doses. By difference, approximately 30.7% to 38.0% of weight lost was lean mass — comparable to what has been reported for semaglutide and tirzepatide rather than dramatically better or worse.

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."

Safety within the sub-study: adverse events were similar between groups, with gastrointestinal events the most frequently reported. Serious adverse events occurred in 7% of the placebo arm, 6% at retatrutide 0.5 mg, 0% at 4 mg, 9% at 8 mg, and 3% at 12 mg — and no deaths were reported.

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

Regardless of which weight loss drug is used, the standard clinical recommendations for preserving lean mass are:

Resistance training

Resistance exercise (weight lifting, bodyweight exercises, resistance bands) is the single most effective intervention for preserving muscle during weight loss. It signals the body that muscle tissue is needed, encouraging the body to preferentially break down fat for energy.

• Frequency: At least 2-3 sessions per week targeting major muscle groups
• Intensity: Progressive overload — gradually increasing weight or resistance over time
• Evidence: Studies in bariatric surgery patients and GLP-1 drug users consistently show that resistance training preserves lean mass during weight loss

Adequate protein intake

Protein provides the amino acids necessary for muscle maintenance and repair. During weight loss, protein requirements increase because the body is breaking down tissue.

• General recommendation: 1.2-1.6g of protein per kilogram of body weight per day during active weight loss
• Higher end for older adults: 1.4-1.6g/kg/day, given age-related muscle loss risk
• Practical note: As appetite decreases on GLP-1 drugs, people often eat less protein. Deliberate attention to protein-rich foods or supplementation may be necessary

Maintaining physical activity

Beyond resistance training specifically, overall physical activity — walking, cardio, daily movement — supports metabolic health and provides stimulus for the body to maintain functional muscle mass.

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
• ClinicalTrials.gov: NCT04867785