Every conversation about triple agonists eventually arrives at the same question: when someone loses 28% of their body weight, how much of that is fat and how much is muscle? For retatrutide, the honest answer is that the retatrutide results on body composition currently rest on a single published imaging substudy — and only 103 participants in it completed both scans.

That number matters more than it sounds, because retatrutide is the first weight-loss agent in late-stage development that deliberately activates the glucagon receptor alongside GLP-1 and GIP, and glucagon is the one arm of the three where a muscle question is biologically plausible rather than hypothetical hand-waving.

Why the glucagon arm raises the lean-mass question at all

Semaglutide (Ozempic/Wegovy) hits one receptor. Tirzepatide (Mounjaro/Zepbound) hits two. Retatrutide adds glucagon, which is part of why it produces the largest average reductions yet reported for an incretin-class drug — see what retatrutide is for the mechanism in plain terms.

The trade-off discussed in the published literature is that glucagon influences amino-acid handling. Reviews of incretin and multi-agonist effects on body composition note that chronic glucagon receptor activation has been hypothesised to affect circulating amino acids and, by extension, muscle protein balance. That is a mechanistic concern raised in the literature, not a finding — nobody has demonstrated disproportionate muscle wasting with retatrutide.

What the one published DXA substudy actually measured

The evidence base is a prespecified substudy of the phase 2 type 2 diabetes trial, published in The Lancet Diabetes & Endocrinology. Design details worth knowing before quoting any number from it:

  • It ran across 42 US medical centres, in people with type 2 diabetes — not the general obesity population of TRIUMPH-1.
  • The endpoint was percent change in total fat mass by DXA from baseline to week 36 — not 80 or 104 weeks.
  • 189 participants were enrolled to the substudy; 103 completed treatment plus both the baseline and week-36 DXA scans. That leaves roughly 30 people per randomised arm at enrolment, and fewer with paired imaging.

The fat-mass results at week 36 were dose-dependent overall: 4.9% (0.5 mg), 15.2% (4 mg), 26.1% (8 mg) and 23.2% (12 mg), against 4.5% for placebo and 2.6% for dulaglutide 1.5 mg. Note that the 12 mg estimate sat slightly below the 8 mg estimate — the sort of ordering that can appear when arms hold about thirty people each, and a reason to treat these as indicative rather than definitive.

On the muscle question, the substudy's conclusion was reassuring but bounded: retatrutide significantly improved total fat-mass reduction versus both placebo and dulaglutide, and the proportion of lean mass lost relative to total weight lost was comparable to other obesity pharmacotherapies. In other words, the ratio did not look worse. Since the headline phase 3 numbers are far larger — up to 28.3% average weight loss at 12 mg over 80 weeks in TRIUMPH-1 — a stable ratio still means more absolute tissue of every kind leaves the body. That follows from arithmetic, not from any lean-mass measurement made in TRIUMPH.

The phase 3 gap: thousands of participants, ~100 scans

Here is the part that gets missed in the weekly flow of clinical trial headlines. The TRIUMPH registrational programme — described in a 2026 design paper in Diabetes, Obesity and Metabolism covering obesity, obstructive sleep apnoea and knee osteoarthritis — is large, but imaging body composition is not the point of it. The trial documentation describes a DXA substudy in the order of 100 participants within TRIUMPH-3 (severe obesity plus established cardiovascular disease), assessing percent and absolute change in total body fat and lean mass.

So across a phase 3 programme enrolling thousands, the prospectively scanned population for body composition remains small, and it sits in a cardiovascular-disease cohort rather than the general obesity population most readers are asking about. Until those data are reported, "does retatrutide spare muscle?" is answered by a 103-scan phase 2 substudy in type 2 diabetes at 36 weeks. Anyone claiming more precision than that is going beyond the published record.

Lilly's EASD 2026 programme in Milan (28 September – 2 October) includes a retatrutide symposium on 30 September presenting phase 3 TRIUMPH-2 data in obesity with type 2 diabetes — up to 20.8% average weight loss and up to 1.6 percentage points of HbA1c reduction at 80 weeks, per Lilly. What that session says about body composition specifically has not been announced.

Where this leaves Australian readers

Nothing about the lean-mass question is Australia-specific, but the timing is: retatrutide is still investigational and not TGA-approved, with Lilly's first regulatory filing signalled for the first quarter of 2027. That means the body-composition evidence base will very likely be argued over well before anyone here can be prescribed it — useful context on our Australia page, and worth reading alongside the known safety and tolerability picture, where gastrointestinal effects and dose-related discontinuation are the better-documented issues.

Join the free updates list and we'll send the body-composition readouts as they're published, not as they're paraphrased.

FAQ

Does retatrutide cause muscle loss?

Some lean mass is lost, as with any substantial weight reduction. In the only published DXA substudy — phase 2, in type 2 diabetes, at week 36 — the proportion of lean mass lost relative to total weight loss was comparable to other obesity pharmacotherapies. Larger and longer phase 3 body-composition data have not yet been reported.

How much fat did retatrutide remove in trials?

In the phase 2 body composition substudy, total fat mass fell by 26.1% at 8 mg and 23.2% at 12 mg at week 36, versus 4.5% with placebo and 2.6% with dulaglutide 1.5 mg. Arm sizes were roughly 30 people, so these are indicative figures from a small substudy.

Is retatrutide approved yet?

No. Retatrutide remains investigational worldwide. Lilly has said it plans to submit its first regulatory filing in the first quarter of 2027, and it is not TGA-approved in Australia.

How is body composition measured in retatrutide trials?

By DXA scanning at set timepoints. The published phase 2 substudy scanned at baseline and week 36, with 103 participants completing both scans, and trial documentation for the phase 3 TRIUMPH programme describes a DXA substudy of about 100 participants within TRIUMPH-3.

Sources

This article is general information only and is not medical advice. Retatrutide is an investigational medicine and is not approved by the TGA or any other regulator. retatrutide.net.au is an independent information site and is not affiliated with, endorsed by or sponsored by Eli Lilly and Company. Speak with a qualified health professional about your own circumstances.