Retatrutide has produced substantial weight loss in human trials, and a smaller study found reductions in body fat. That does not mean all the lost weight was fat or that muscle and strength were preserved. The body-composition report followed a subset of adults with type 2 diabetes; the newer obesity trials answer different questions. Some lost weight was lean tissue, but a scan's lean-mass result is not a direct measurement of muscle strength. Body-composition report; 2026 obesity trial.
Why are people interested in retatrutide?
Retatrutide is an experimental peptide medicine that acts at three hormone receptors: GIP, GLP-1 and glucagon. Researchers are studying its effects on body weight, blood sugar and obesity-related conditions. The interest comes from sizeable average changes in randomized trials, rather than evidence that having three targets automatically makes it better or safer. Trial results are group estimates, not a forecast for an individual. TRIUMPH-1 primary abstract; TRIUMPH-2 primary abstract.
The fat-and-lean report belongs to a smaller diabetes trial
Parent trial: 281 adults with type 2 diabetes
The phase 2 trial randomly assigned adults aged 18–75 with type 2 diabetes and a body mass index of 25–50 to retatrutide, placebo or dulaglutide, another diabetes medicine. Participants and investigators were masked to assignment. Its main outcome was change in longer-term blood sugar (HbA1c) at 24 weeks; weight change at 36 weeks was a secondary outcome. Eli Lilly sponsored the study. Parent trial; official study record and results.
Within that trial: a body-composition substudy
This was a subset of the same study, not an independent repeat of its findings. Researchers used dual-energy X-ray absorptiometry, usually called DXA, to assess body composition. The prespecified primary substudy outcome was percentage change in total fat mass at 36 weeks. The posted analysis plan treats total lean-mass change and fat-to-lean ratios as exploratory outcomes. Published substudy abstract; analysis plan, Appendix 7.
- 189 people enrolled in the substudy.
- 155 people had a baseline DXA scan.
- 103 people completed treatment and both baseline and week 36 scans.
Those counts matter. The published abstract describes an analysis using available, nonmissing scans while participants remained on treatment. Its estimates are not simply a result observed in all 189 people, nor should every estimate be described as a simple average of the 103 completers. Missing measurements and treatment discontinuation limit interpretation. Analysis population and participant flow.
Estimated mean reductions in total fat mass ranged from 4.9% to 26.1% across retatrutide groups, compared with 4.5% with placebo and 2.6% with dulaglutide. These percentages use starting fat mass as the denominator. The lowest study group did not show a statistically significant advantage over placebo on this endpoint (p=0.83); higher groups did. A favorable result in some groups does not make every comparison positive. Primary fat-mass findings and uncertainty.
How to read a fat-loss or muscle-preservation claim
- Percentage change in body weight
- The denominator is starting body weight. A reported 25% reduction is not a claim that 25% of starting body fat disappeared, or that 25% of participants responded.
- Percentage change in total fat mass
- The denominator is starting fat mass. The substudy's 26.1% fat-mass estimate is not 26.1% total weight loss, and it is not the share of lost weight that was fat.
- Lean-mass change or its share of weight loss
- This is a different body-composition measurement or ratio. It cannot be calculated by subtracting a weight percentage from a fat percentage with a different denominator, population or timepoint. It does not directly measure muscle strength.
The substudy authors report that the proportion of lean-mass loss relative to weight loss was similar to other obesity treatments. Similar proportions do not mean no lean tissue was lost. Nor is that statement a randomized head-to-head comparison with semaglutide or tirzepatide: the trial's comparators were placebo and dulaglutide. Substudy interpretation and actual comparators.
For example, a headline saying “26.1% fat loss” cannot answer “How much muscle would I lose?” The starting quantity is different, and the selected primary abstract does not supply a verified numerical lean-mass result or establish preserved strength. We have not filled that gap with a calculation from separate trial headlines. The posted plan's scan endpoints support a body-composition question; they do not turn a lean-tissue estimate into a direct test of muscle performance. Planned DXA endpoints, sections 7.7.1–7.7.3.
Newer trials report weight loss in different populations
Two phase 3 reports published in September 2026 add larger, longer weight-loss comparisons. They do not make the earlier DXA subset a larger body-composition study. The following figures are estimated average changes in starting body weight at 80 weeks, not fat or muscle percentages.
- TRIUMPH-1: 2,339 adults with obesity, without diabetes. The highest study group had a 25.0% mean weight reduction versus 3.9% with placebo. This randomized, double-blind trial also evaluated knee-pain and sleep-apnea outcomes in specified subgroups. Those clinical outcomes are separate from a DXA lean-mass or muscle-preservation result. TRIUMPH-1 abstract; trial record.
- TRIUMPH-2: 1,152 adults with overweight or obesity and type 2 diabetes. The highest study group had an 18.8% mean weight reduction versus 5.1% with placebo. The main analysis included all randomized participants with specified missing-data methods. Its population and analysis differ from the older on-treatment DXA analysis. TRIUMPH-2 abstract; trial record.
These are two separate placebo-controlled studies, not a randomized comparison between people with and without diabetes. Do not subtract their percentages to predict an individual's response or rank retatrutide against a different medicine. Both were funded by Eli Lilly. The semaglutide–tirzepatide comparison describes a separate direct trial; it cannot supply a retatrutide comparison.
Side effects and unanswered safety questions
Stomach-and-bowel adverse effects are important. In the older diabetes parent trial, nausea, diarrhoea, vomiting or constipation occurred in 67 of 190 retatrutide recipients (35%), versus 6 of 45 placebo recipients (13%). That pooled count describes this study, not a universal risk rate or the newer trials. Parent-trial adverse events.
In TRIUMPH-2, diarrhoea and nausea were more frequent with retatrutide than placebo. Low blood pressure and altered or unpleasant skin sensation, called dysesthesia, were also more frequent. In the two higher study groups, permanent discontinuation because of adverse events or death occurred in 12% and 8%, versus 5% with placebo. The authors reported deaths in all treatment categories; investigators judged them unrelated to study treatment; that judgment does not establish safety for every person or rule out uncommon harms. TRIUMPH-2 safety findings and denominators.
Weight reduction alone does not settle whether a particular person would maintain strength, avoid nutritional problems, tolerate treatment or gain a durable health benefit. The selected body-composition evidence cannot resolve those individual questions. Discuss current approved options and your own health circumstances with a qualified clinician rather than using a trial percentage as a treatment plan.
Retatrutide remains investigational in the United States
At our November 6, 2026 check, FDA states that retatrutide is not a component of an FDA-approved drug and cannot be used in compounding under federal law. Products marketed for “research” or as compounded retatrutide are not made approved or equivalent to trial material by citing these studies. FDA warns about products of unknown quality sold for human use under research labels. Current FDA statement.
What can you reasonably take from these findings?
The human evidence supports further investigation of retatrutide and documents weight and fat reductions in the studied groups. It does not establish that all lost weight is fat, that lean tissue is untouched, or that muscle strength is preserved. Before accepting a claim, identify the study, diabetes status, measured quantity, timepoint, comparator and missing-data limits. A question about your own function or treatment needs evidence beyond a headline percentage.
Sources10 linked sources
- Rosenstock and colleagues. Retatrutide in people with type 2 diabetes: phase 2 trial. The Lancet, 2023. Primary indexed abstract.
- Coskun and colleagues. Effects of retatrutide on body composition in people with type 2 diabetes. The Lancet Diabetes & Endocrinology, 2025. Primary indexed abstract.
- NCT04867785: official parent-trial results, participant flow and posted documents. Protocol, amendment c, August 29, 2022. Statistical analysis plan, version 3, December 13, 2022; Appendix 7, pages 50–51.
- TRIUMPH-1: Retatrutide, a Triple Hormone Receptor Agonist, for Treatment of Obesity. New England Journal of Medicine, September 2026. Primary indexed abstract. Official trial record.
- TRIUMPH-2: Retatrutide in adults with obesity and type 2 diabetes. The Lancet, September 2026. Primary indexed abstract. Official trial record.
- FDA’s Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. Current retatrutide and research-product sections.
Selected-source check: November 6, 2026. We read the primary indexed abstracts, current official trial records, relevant posted protocol and analysis-plan sections, and the FDA statement. Publisher full articles and supplements were inaccessible. The two newer registry records did not yet display posted results at this check; the September 2026 journal abstracts are separate publication evidence. Planned analyses are not proof of precisely how the published analysis was implemented. This is not a systematic literature review or clinician review. Additional sources retained in the retatrutide profile remain available.