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Retatrutide Body Composition: What the DXA Data Shows

Published October 19, 2026 · 7 min read

Scope: Retatrutide is an investigational compound and is not approved by the FDA or any other regulator. This article summarizes published and sponsor-reported research for education only. It is not advice about any person's use of any substance. Compound Brief is published by the team behind DAAVY Research, a research-supply company; DAAVY is not discussed further in this article.

"Weight loss" is one number. Body composition is the question behind it: how much of the change was fat, how much was lean tissue, and what the instruments used to measure it can and cannot say. This article looks at what has actually been reported for retatrutide, what has not, and why the answers are less settled than headlines suggest.

Why body composition is a separate question

A scale reports total mass. It cannot say what that mass is made of. When a trial reports that participants lost a given percentage of body weight, a reader still does not know how that split between fat mass, lean mass (muscle, organs, water, connective tissue) and bone. Regulators and clinicians care about the split because lean tissue loss is linked, in the wider weight-loss literature, to concerns about physical function. Whether any specific compound changes that split is an empirical question that needs a measured answer.

How DXA works, and what it can't see

Most body-composition substudies in drug trials use dual-energy X-ray absorptiometry (DXA). Two X-ray beams of different energies pass through the body and the instrument infers fat, lean and bone mineral from how much each beam is absorbed. It is fast, widely available and reasonably reproducible, which is why it is the default.

It also has well-known limits. DXA reports "lean mass," which is not the same as skeletal muscle: it includes body water, organs and other soft tissue. Shifts in hydration can move the lean number without any change in muscle. DXA also says nothing about muscle strength or quality, and it cannot tell where fat sits except in rough regional terms. Trial DXA substudies are usually run on a subset of participants, so they are smaller than the main trial and are not always powered to compare groups on body composition. These caveats apply to every compound measured this way, not just retatrutide.

The phase 2 type 2 diabetes substudy

The main peer-reviewed body-composition report for retatrutide is a substudy of a phase 2, double-blind, placebo-controlled trial in adults with type 2 diabetes, published in The Lancet Diabetes & Endocrinology in 2025 (sources 1 and 2). The parent trial enrolled 281 adults at 42 US centres. The DXA substudy included 189 of them, split across placebo (29), dulaglutide 1.5 mg (34), and retatrutide at 0.5 mg (32), 4 mg (31), 8 mg (33) and 12 mg (30). Participants had HbA1c of 7.0-10.5% and BMI of 25-50 kg/m². The treatment period for the primary endpoint was 36 weeks (source 2). The authors' affiliation shown on the journal index entry is Eli Lilly and Company (source 1).

A published summary of the paper reports these changes in fat mass at week 36, as percent change from baseline (source 2):

GroupFat mass change (DXA)
Placebo-4.5%
Dulaglutide 1.5 mg-2.6%
Retatrutide 0.5 mg-4.9%
Retatrutide 4 mg-15.2%
Retatrutide 8 mg-26.1%
Retatrutide 12 mg-23.2%

Two features of this table are worth pausing on. First, the placebo group lost 4.5% of its fat mass, so the fair comparison is against placebo, not against zero. Second, the 12 mg group shows a smaller reduction than the 8 mg group. That is a reminder that small arms (about 30 people each) produce noisy averages, and that the dose-response curve in a 36-week phase 2 study should not be over-read.

What about lean mass?

This is the part that gets quoted most and is hardest to verify. The authors' stated conclusion, as described in secondary coverage, is that the weight-loss effect did not come with disproportionate lean mass loss (sources 2 and 3). The same coverage describes a "fat loss index," the share of lost weight that was fat, with the remainder being lean tissue (source 3). We have not been able to check the specific lean-mass figures against the full paper text, so this article does not repeat them. Readers who need the exact numbers should go to the paper itself (source 1).

How to read any such result: "expected proportions" depends on the reference you expect. In weight-loss research, a commonly discussed rule of thumb is that a meaningful minority of weight lost can be lean tissue; whether a given result is better or worse than that depends on the comparator, the population and the timepoint. A lean-mass percentage change is also not a muscle change. The reduction in "lean" can include water and organ tissue as body mass falls, and the paper is not a measure of strength or function.

What the phase 3 announcements say (and don't)

The sponsor's May 21, 2026 TRIUMPH-1 announcement reported 2,339 randomized participants and, at 80 weeks on the efficacy estimand, mean body weight reductions of 19.0% (4 mg), 25.9% (9 mg) and 28.3% (12 mg) versus 2.2% with placebo (source 4). It also reported a waist circumference reduction of 24.1 cm in the 12 mg group versus 3.6 cm with placebo. The release does not report lean mass or any DXA data, and neither does the sponsor's ADA 2026 congress page for the trial (source 5). As of this writing, we did not find a published Phase 3 body-composition analysis. That is not evidence that none exists, only that we did not find one. For how to read the two ways of counting results, see our explainer on endpoints and estimands, and for the topline numbers see Retatrutide Trial Results Explained.

Waist circumference is sometimes treated as a proxy for fat distribution. It is a useful trial measure, but it is not a substitute for imaging, and it does not answer the lean-mass question.

Safety and limitations in the same data

Body-composition findings are only meaningful alongside tolerability. In TRIUMPH-1 as announced, nausea ranged from 28.6% to 42.4% across retatrutide doses versus 14.8% with placebo; dysesthesia (an abnormal skin sensation) occurred in 5.1% (4 mg), 12.3% (9 mg) and 12.5% (12 mg) versus 0.9% with placebo; and discontinuation because of adverse events was 4.1%, 6.9% and 11.3% by dose versus 4.9% with placebo (source 4). In the phase 2 diabetes substudy, adverse events were reported as mostly gastrointestinal and similar in frequency across groups, with serious adverse events in 0% to 9% of the arms (source 2).

Limitations of the body-composition evidence as it stands:

The short version

Retatrutide-treated groups in a small phase 2 diabetes substudy showed larger reductions in DXA fat mass than placebo or dulaglutide, and the authors concluded that lean mass loss was not disproportionate. The lean-mass details are available to us only in secondary summaries, the study is small and short, and the Phase 3 announcements so far do not report body composition. The most honest reading is "promising signal, incomplete evidence." For a general framework on weighing results like these, see How to Read a Clinical Trial Result. If peer-reviewed Phase 3 body-composition data appear, this article will be updated.

Sources

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