Retatrutide and Body Composition: What Has Research Measured?
A lower number on the scale is only the beginning of the story. When we read a retatrutide study, the next question is: what actually changed?
Was it body fat? Lean tissue? Fat stored in the liver? Those questions sound similar, but they require different measurements. Understanding the difference makes retatrutide research much easier to follow—and helps you spot when a headline promises more than a study can tell us.
The short answer: Retatrutide research has measured body weight, DXA-estimated fat and lean mass, and liver fat using MRI. These studies provide evidence of fat reduction, but they do not establish that all weight lost is fat or that muscle is fully preserved. See the primary sources below.
If you are new to the compound itself, our retatrutide triple-agonist overview explains the three receptor pathways. Here, we will stay focused on what researchers measured and how to read the results.
Weight loss and body composition answer different questions
Think of body weight as the total on a receipt. Body composition asks what makes up that total.
A scale can document a change in overall mass. It cannot tell you how much came from fat, lean tissue, or water. That is why a weight-loss percentage should never quietly become a “fat-loss percentage” as it moves from a research paper into an online article.
| Measurement | What it helps answer | What it cannot establish alone |
|---|---|---|
| Body weight | How much did total weight change? | The split between fat and lean tissue. |
| DXA scan | How did estimated fat mass and lean soft tissue change? | Muscle strength or physical performance. |
| Liver MRI–PDFF | How did the fat fraction within the liver change? | Total-body fat loss or reversal of liver scarring. |
For context, the 2023 phase 2 obesity trial reported an average body-weight reduction of 24.2% at 48 weeks in the 12 mg trial group, compared with 2.1% with placebo. That is a body-weight result; the number itself does not tell us the fat-to-lean split. [1]
Dose labels in this article identify study groups. They are not dosing recommendations.
What the retatrutide body-composition substudy found
A 2025 phase 2 substudy used dual-energy X-ray absorptiometry (DXA) in adults with type 2 diabetes. The primary endpoint was percentage change in total fat mass at 36 weeks. [2]
Total fat mass fell by an estimated 26.1% in the pooled 8 mg group and 23.2% in the 12 mg group, compared with 4.5% with placebo. These are relative changes in fat mass—not percentage points of body-fat percentage, and not changes in total body weight.
Lean mass also declined. The authors interpreted the proportion of weight lost as lean mass as comparable with other obesity treatments. That does not mean zero lean-tissue loss or prove muscle preservation.
Of 189 substudy participants, 103 completed treatment with baseline and week-36 scans. Consider this incomplete follow-up when interpreting the findings. [2]
Explore Retatrutide research details
Does lean-mass loss mean muscle loss?
This is where we need to slow down a little. Lean mass and skeletal muscle are not interchangeable terms. DXA estimates lean soft tissue; it does not isolate muscle proteins or directly measure how strong someone is.
Hydration can influence the result. In a separate methods study, changes in hydration affected DXA lean-tissue measurements. That is one reason researchers need consistent scanning conditions when they compare measurements over time. [3]
This cuts both ways. It would be too strong to label every kilogram of DXA lean-mass change as a kilogram of muscle lost. It would also be too strong to dismiss a lean-mass decline as “just water.”
For a clearer picture, we would want muscle-specific measurements alongside strength and physical-function testing. A scan and a strength test answer related but different questions. Neither should be made to stand in for the other.
What about retatrutide and liver fat?
A separate 2024 substudy examined 98 participants with metabolic dysfunction-associated steatotic liver disease, or MASLD, and at least 10% liver fat at baseline. Researchers used MRI proton density fat fraction, or MRI–PDFF, to quantify liver fat. [4]
At 24 weeks, the average relative liver-fat reduction was 82.4% in the 12 mg trial group, compared with a 0.3% increase with placebo.
That is a striking liver-fat result. It does not mean participants lost 82.4% of their total body fat. It also does not, by itself, establish that liver fibrosis—scarring—was reversed. The measurement concerns fat within the liver, not every aspect of liver health.
A simple way to read research percentages
Before comparing two numbers, ask: “A percentage of what?”
Here is a hypothetical example, unrelated to any individual trial participant. If fat mass decreases from 40 kg to 30 kg, that is a 25% reduction in fat mass. You still need total body weight at both time points to calculate body-fat percentage.
Now imagine a liver-fat fraction decreasing from 20% to 5%. That is a reduction of 15 percentage points, or a 75% relative reduction. Both descriptions are mathematically correct, but they express different things.
Keeping the starting measurement, units, and time point visible prevents a surprisingly large amount of confusion.
How newer retatrutide trials fit into the picture
Retatrutide research has moved beyond the early phase 2 trials. In September 2026, Lilly reported phase 3 TRIUMPH-2 results in adults with type 2 diabetes and overweight or obesity, with findings also published in The Lancet. [5]
For this article’s question, the reading rule stays the same: a newer body-weight result cannot supply a fat-to-lean breakdown unless body composition was measured and reported. We should evaluate the endpoint actually presented, rather than assume a larger weight change tells us more about muscle.
Retatrutide remains investigational as of this article’s evidence check. Clinical-trial findings concern the study medicine used under trial conditions; they do not demonstrate equivalent outcomes from commercially sold research materials. [5]
What we look for when reading the next study
For anyone following this field, the useful question is becoming more specific: how does the composition of weight change relate to health and function over time?
We look for the population studied, the measurement method, the comparison group, and the number of people who completed follow-up. We also look for results reported in both kilograms and percentages, and whether muscle function was actually tested.
Longer follow-up and direct comparisons would help answer questions that cross-trial headlines cannot settle. A study in adults with type 2 diabetes should not automatically become a prediction for athletes, older adults, or everyone interested in body composition.
Common questions about retatrutide and body composition
Does retatrutide preserve muscle?
The evidence discussed here does not establish complete muscle preservation. DXA lean mass and muscle function need to be interpreted separately. Be cautious with any claim that turns a favorable fat-loss result into a guarantee about muscle.
Is liver-fat reduction the same as losing belly fat?
No. Fat within the liver, visceral fat around abdominal organs, and subcutaneous fat beneath the skin are different compartments. A result for one cannot simply be relabeled as a result for another.
Can we compare retatrutide with semaglutide or tirzepatide using headline percentages?
Not reliably for body composition. Populations, trial lengths, measurement methods, and analyses may differ. A useful comparison needs compatible endpoints and study designs, ideally a direct randomized comparison.
Want to keep exploring the compound and its research context? Our dedicated page brings together Retatrutide information and links to published studies.
Sources and reading notes
This article discusses selected primary publications and a dated sponsor update. It is an educational overview, not a systematic review. The retatrutide trials discussed were sponsored by Eli Lilly; sponsorship is relevant context when assessing the evidence.
- Jastreboff AM, et al. Phase 2 retatrutide obesity trial. New England Journal of Medicine. 2023;389:514–526. DOI: 10.1056/NEJMoa2301972.
- Retatrutide body-composition substudy in people with type 2 diabetes. The Lancet Diabetes & Endocrinology. 2025;13:674–684. DOI: 10.1016/S2213-8587(25)00092-0.
- Toomey CM, McCormack WG, Jakeman P. Hydration and DXA lean-tissue measurement. European Journal of Applied Physiology. 2017;117.
- Sanyal AJ, et al. Retatrutide liver-fat substudy in MASLD. Nature Medicine. 2024;30:2037–2048. DOI: 10.1038/s41591-024-03018-2.
- Eli Lilly and Company. TRIUMPH-2 phase 3 results update. September 29, 2026. Sponsor news release; included for current trial context.
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