Retatrutide Muscle Loss: What the DXA Study Found

By The RX Index Editorial Team · Last verified October 2026

Retatrutide reduces lean mass as well as fat, but lean mass isn't all muscle. In a 36-week study of adults with type 2 diabetes, about 35% of the measured fat-plus-lean tissue lost was lean mass. That does not prove muscle protection or predict your own loss. Retatrutide is not FDA-approved. (Study; FDA)

That 35% is a pooled lean-mass figure, not a personal forecast of retatrutide muscle loss. You'll also see 19%, 25%, and 40% online. The table below separates real study numbers from claims we couldn't trace in the sources checked.

Retatrutide muscle-loss numbers you'll see online, checked

The numbers below come from different studies, different math, or claims with no source we could find. As of October 15, 2026, the published whole-body DXA evidence we found comes from one Phase 2 substudy. DXA stands for dual-energy X-ray absorptiometry: a scan that estimates fat, bone mineral, and lean tissue. Here's how the numbers floating around stack up against it.

Sources: Coskun et al., full Phase 2 report, 2025; SURMOUNT-1 body-composition report, 2025; TRIUMPH-1, NEJM, Sept. 29, 2026. Checked October 15, 2026.

Does retatrutide cause muscle loss?

Some lean mass was lost on retatrutide, as is common during substantial weight loss. In the DXA study, the pooled lean share was about 35% of fat-plus-lean tissue lost. The researchers called that share similar to other obesity treatments; they did not establish that retatrutide preserves more muscle. (Study)

Set bone mineral aside for a moment, and think of the remaining tissue as two buckets.

• Fat mass is the bucket you're trying to empty.
• Lean mass on this scan is everything that isn't fat or bone mineral: muscle, yes, but also water, organs, skin, and connective tissue.

A lower lean-mass result can reflect changes in muscle, water, and other lean tissues. The total lean-mass number can't tell you how much came from each.

So "35% lean" does not mean "35% muscle." This study did not measure skeletal muscle separately or test strength. (Study limitations)

What the retatrutide DXA study actually measured

The study enrolled 189 adults with type 2 diabetes from a larger Phase 2 trial; 155 had a starting DXA scan. Only 103 finished treatment with both a starting and a qualifying 36-week scan. The main goal was to measure fat loss, and the 4, 8, and 12 mg retatrutide groups cut fat much more than placebo or dulaglutide (Trulicity).

Source: Coskun et al., PubMed abstract. Doses are study groups, not dosing advice. Percent changes are from each group's starting fat mass. “Different” means statistically different from placebo. The analysis used available on-treatment data and estimated missing week-36 values; it was not limited to the 103 treatment completers.

On the lean side, the largest estimated group loss was 6.5 kg (about 14 pounds), in the pooled 8 mg group. Across the 4, 8, and 12 mg groups combined, the pooled lean share was about 35%. To picture that ratio, 35% of 20 pounds is about 7 pounds. That is a math illustration, not a prediction for someone losing 20 pounds. (Full study)

The exact calculation matters: the researchers divided fat loss by fat-plus-lean loss for each group, then pooled those results with larger groups carrying more weight. That gave 64.6% fat and, by subtraction, 35.4% lean. It was not a measurement of each person's muscle loss.

Four limits matter, so they sit right here instead of at the bottom:

• It was small. Only 103 people completed treatment with both qualifying scans.
• Everyone had type 2 diabetes. People without diabetes may respond differently.
• It was 36 weeks. TRIUMPH-1 ran 80 weeks and produced bigger weight loss, but its weight-loss results do not supply a lean-mass result. (TRIUMPH-1)
• Lilly ran it. Lilly funded the study, and all seven authors were Lilly employees and shareholders. That doesn't make the numbers wrong. Independent replication would strengthen the finding. (Funding and disclosures)

Choosing an available GLP-1 is a separate question. Which one fits you depends on your insurance, your budget, whether you want a shot or a pill, and your state. The RX Index's Find My GLP-1 Path tool helps you compare paths before you choose.

Why a percentage can go up while your lean mass goes down

Percent numbers trip people up because each one divides by something different. The same change can look like 5%, 30%, or "better than before," depending on the math. That's why one study can produce three headlines that all sound different.

Here's a made-up example. These aren't retatrutide results; they're teaching numbers. Say someone goes from 200 to 180 pounds, and their measured lean mass goes from 120 to 114 pounds.

Lean mass went down. The lean share of their body went up, because total body weight fell faster than lean mass. And none of those numbers tells you whether they got weaker.

The retatrutide study's 35% asks a similar question to the middle row, but uses fat-plus-lean loss rather than the change on a body-weight scale. Check which total a percentage is divided by before comparing it with another study.

Can your smart scale tell you if you're losing muscle?

Not reliably. A 2021 study tested three popular smart scales against DXA in about 50 people each. The scales got body weight right, but their body-composition estimates were off. (JMIR mHealth and uHealth, 2021)

Those scales send a tiny current through your feet and estimate body composition. Water shifts the estimate. That study didn't test every scale sold today, but one scary "muscle" reading on a home scale isn't proof of muscle loss. Watch the trend over weeks, and pay more attention to what your body can do. (Scale study)

Is retatrutide worse for muscle than Zepbound or Wegovy?

The body-composition studies below did not compare retatrutide with Zepbound or Wegovy head to head. Read side by side, retatrutide's 35% lean share falls between tirzepatide's (Zepbound, about 25–26%) and semaglutide's (Wegovy injection, about 39–40%). Larger total weight loss can still mean more pounds of lean mass lost, even when the share is similar.

Different people, doses, lengths, and math. Read across, not as a ranking. Full methods and sources: how lean-mass studies differ by drug.

Why two STEP 1 figures? Using the appendix's estimated fat and lean changes and rounding to one decimal place, [5.26 ÷ (8.36 + 5.26)] × 100 = 38.6% in the analysis that counts outcomes regardless of stopping treatment. The analysis estimating continued treatment gives [6.92 ÷ (10.40 + 6.92)] × 100 = 40.0%. SURMOUNT-1's abstract rounds its split to 75% fat / 25% lean; the results text reports 74% / 26%. Neither difference changes these into muscle-only measurements.

Two things jump out. First, these studies don't establish a muscle-preservation winner. Second, look at the last row. The review reported a lower lean-loss share with strength training added to lifestyle treatment. That supports strength work as part of a care plan—not a claim that it beats every drug difference in this table. (Review)

If you were holding out for retatrutide because you heard it spares muscle, the muscle data alone don't give you a reason to wait. Zepbound, Wegovy, and Foundayo are approved now, and each has body-composition evidence tied to a specific study. For help comparing available paths by your goals, budget, insurance, and situation, use Find My GLP-1 Path.

See which GLP-1 path fits you

Does the glucagon part of retatrutide break down muscle?

The specific effect of retatrutide's glucagon action on muscle has not been settled. Retatrutide works on three hormone signals: GLP-1, GIP, and glucagon. Researchers flagged that steady glucagon signaling can lower amino acids, the building blocks of protein, in the blood, which in theory could slow muscle repair. The DXA study reported a lean share similar to other treatments, but it wasn't built to isolate glucagon's role. (Study)

Two facts help. In receptor testing, retatrutide had about 0.3 times the potency of natural glucagon at the glucagon receptor, according to the Phase 2 trial paper. That laboratory number does not tell you how much muscle someone will lose. And the DXA study's authors raised the amino-acid concern themselves, then reported a lean share similar to other weight-loss treatments.

The other side of the myth is just as unproven: the reviewed retatrutide studies do not show that its glucagon action protects muscle. For more on how the third hormone changes things, see triple versus dual agonist weight loss.

How do you keep muscle while losing weight on a GLP-1?

We found no published trial proving a retatrutide-specific muscle-protection plan. The best guidance here comes from a 2025 joint advisory from four obesity and nutrition groups for people on GLP-1 medicines: eat enough protein, and work toward strength training at least three times a week, tailored to your ability. Protein without strength work is unlikely to be enough. (Joint advisory)

Protein: ask for a number that fits you

The advisory says targets of 1.2–1.6 grams of protein per kilogram of body weight per day have been proposed during active weight loss. It also warns that using your full current weight can overshoot what you need, and experts don't agree on which weight to use. (Mozaffarian et al., American Journal of Clinical Nutrition, 2025)

So don't just multiply your scale weight. Ask your clinician or a dietitian for a daily target, and which weight they used to set it. If you have kidney disease, your care team needs to set this; needs can change, especially with dialysis. (NIDDK)

Strength training: three times a week, starting where you are

The same advisory recommends strength training at least three times a week, plus at least 150 minutes of moderate cardio each week, tailored to your fitness and physical ability. It doesn't mean a heavy barbell on day one. Chair squats, bands, and wall push-ups are examples of strength work; choose safe starting movements and build from there. (Joint advisory)

The strength-training row in the table above is the most hopeful number on this page. And there's proof the fat-versus-lean split can change. In the Phase 2 BELIEVE trial, adding an experimental drug called bimagrumab to semaglutide sharply cut lean loss. It isn't approved; see bimagrumab explained.

For the full step-by-step plan, see protecting muscle during GLP-1 weight loss and how to get enough protein on a GLP-1.

Track what your body can do, not just what the scan says

A scan measures tissue. It doesn't measure strength. The advisory lists simple checks a clinician can use: standing up from a chair, climbing stairs, and a timed get-up-and-walk test. (Joint advisory)

Here's the honest limit: no web page, including this one, can tell you how much muscle you'll lose. Your starting muscle, age, food, and training all change it. What you can do is notice changes early and bring them to someone who can check.

Copy this card into your phone notes before your next visit. It's for a conversation, not a test.

My muscle and strength check-in What worries me: a scan or scale number / I feel weaker / I'm struggling to eat enough / a claim I read online What I've noticed: ________ When it started: ________ What my report calls the number, and its units (lean mass, muscle, fat-free mass): ________ / I don't know Questions for my clinician: 1. How can we check my strength and how much I'm eating, instead of guessing from a percentage? 2. Would a body-composition scan change my care? If so, which one, and compared to what? 3. What protein target and strength-training starting point fit my health? 4. What change should make me call you, and when should we look at this again?

Who should be most careful?

If you start with less muscle, you have less to spare. That often means older adults, people who already struggle with stairs or getting out of a chair, and anyone losing weight very fast while barely eating. That's a reason to have a clinician build the plan with you, not a reason to panic. (Joint advisory)

When does this need a clinician instead of another search?

Feeling weak doesn't always mean you lost muscle. But ongoing or localized weakness with no clear cause needs a medical check, not a supplement. Sudden weakness on one side of the body is an emergency. (MedlinePlus; CDC)

For sudden weakness on one side of the body, trouble speaking, or a face that droops, call 911—even if it goes away. (CDC stroke warning signs)

If you're under 18, pregnant or planning a pregnancy, have type 1 diabetes, or have a history of an eating disorder, start with your own doctor before any weight-loss medicine. Protein targets and weight goals need a plan built for you.

Will the Phase 3 trials tell us more?

TRIUMPH-3 was designed to measure fat and lean mass in a DXA substudy of about 100 people. The big TRIUMPH-1 paper, published September 29, 2026, reported weight, knee pain, and sleep apnea results; its paper and supplement did not report DXA fat-and-lean results. We found no numerical TRIUMPH-3 DXA results in the sources checked on October 15, 2026.

That planned scan matters because TRIUMPH-3 covers adults with severe obesity and established cardiovascular disease, with or without type 2 diabetes, over 80 weeks. It could extend the evidence beyond the 36-week diabetes study, but it is not a study of otherwise healthy athletes. (TRIUMPH design paper; TRIUMPH-1, NEJM; Lilly Medical, last reviewed July 23, 2026)

When results come, look for three things: who was scanned, how much lean and fat they actually lost, and whether anyone tested strength separately. A weight-loss headline or a "physical function" questionnaire score isn't the same as measured muscle. For the full weight data, see retatrutide weight-loss results.

If you're using a product sold as retatrutide

The retatrutide trial numbers came from people given study drug at known doses, with doctors checking in. A vial bought online may not contain what the label says. The trial cannot establish what is in that vial or what effect it will have. (Study; FDA)

Here's where things stand:

• It isn't approved anywhere. Lilly says no retatrutide medicine has been approved for human use by any regulator in the world, and it can't be sold to consumers. (Lilly, August 12, 2026)
• It can't be compounded. The FDA says retatrutide cannot be used in compounding under federal law. It has warned companies selling it as "for research purposes" or "not for human consumption," and says those products are of unknown quality. (FDA, checked October 15, 2026)

No judgment here. If you're using it and feel weaker, can't eat, or just want out, tell a clinician what you took. Our retatrutide side effects guide has the warning signs and a note you can bring to the visit.

Can you get retatrutide now?

Not as a regular prescription. Retatrutide isn't FDA-approved. Lilly plans to submit it for U.S. regulatory review in the first quarter of 2027; that is a filing plan, not an approval date. Clinical trials and a narrow, doctor-requested expanded-access program are possible pre-approval paths. (Lilly's filing plan; expanded-access record)

Lilly's sponsor-posted expanded-access record, last updated August 6, 2026, now lists criteria. These include being 18 or older, having a body mass index (BMI) of at least 35 despite taking and tolerating the required weight-management treatment at its highest available dose, and having at least two serious or life-threatening obesity-related complications already under standard care. The person must be unable to join a retatrutide or comparable trial, and all standard options, including bariatric surgery, must have been discussed. A treating doctor must check all criteria, including reasons retatrutide would be unsafe, and request access; supply, location, and individual review still matter.

Read our retatrutide availability guide for the details, or search open studies on Lilly's official trial finder. If you'd rather start an approved medicine now, see legal retatrutide alternatives.

What we actually verified

We read the primary sources for the study numbers and guidance used here, recalculated the examples, and kept measured results, study plans, and open questions separate. Here's exactly what we checked, and what we couldn't.

Last verified October 2026 — sources checked October 15, 2026:

• The full retatrutide DXA paper: its methods, fat-loss-index formula, pooled and group results, missing-data handling, funding, and disclosures. The 35.4% lean figure is the complement of its 64.6% pooled fat-loss index—not a number taken on trust from a news summary.
• The STEP 1 appendix, SURMOUNT-1 report, ATTAIN-1 paper, and the original abstract of the 2026 lifestyle review, for the comparison table. We also checked the BELIEVE report for the bimagrumab statement.
• The TRIUMPH-1 paper and supplement, the original TRIUMPH design paper, and Lilly's TRIUMPH-3 summary. The design documents a DXA substudy; the reviewed results did not supply a numerical Phase 3 fat-and-lean result.
• The FDA's current unapproved-drug page, the approved products' labeling, Lilly's retatrutide statements, and its sponsor-posted expanded-access record.
• The joint nutrition advisory, kidney-disease guidance, smart-scale study, strength-training trial, weakness guidance, and CDC stroke advice. We also checked the linked RX Index pages and the tool's live description.

What we could not establish: an exact retatrutide muscle-loss percentage, a personal forecast, or a muscle-preservation winner from these separate trials. We did not test a product or run a body scan.

The RX Index is the independent GLP-1 decision resource that scores telehealth providers and treatment paths on clinical legitimacy, care quality, transparency, access, and cost, so readers can choose the path that fits their situation. Read how The RX Index evaluates evidence and treatment paths.

Frequently asked questions

Does retatrutide cause more muscle loss than Ozempic?

The reviewed body-composition studies do not answer that head to head. Retatrutide's pooled lean share was about 35% in adults with type 2 diabetes over 36 weeks; STEP 1's roughly 39–40% came from semaglutide 2.4 mg injection, the Wegovy dose, in adults without diabetes over 68 weeks—not an Ozempic study. The studies differ too much to name a winner. (Retatrutide; STEP 1 appendix)

What percentage of retatrutide weight loss is muscle?

There is no verified muscle-only percentage in the DXA study. Its pooled lean share was about 35% of fat-plus-lean tissue lost. Lean mass includes water and organs, so you cannot turn that number into a muscle-loss percentage. (Study)

Will the Phase 3 trials report muscle data?

TRIUMPH-1's paper and supplement did not report DXA fat-and-lean results. TRIUMPH-3 has a planned DXA substudy of about 100 people; we found no numerical results from it in the sources checked on October 15, 2026. A DXA result would still not, by itself, be a strength test. (TRIUMPH-1; TRIUMPH design)

Can you build muscle on retatrutide?

The retatrutide DXA study did not test muscle building. A separate 12-week trial found that adults could gain lean mass and strength while losing weight with resistance training and a planned diet, but it did not test retatrutide. That supports a training discussion with your care team—not a promise about this drug. (Strength-training trial)

How can I tell if I'm losing fat or muscle?

The scale alone can't split them, and one home smart-scale estimate does not prove muscle loss. Track what you can do, like stairs, getting out of a chair, and your usual lifts, and bring any change to your clinician. They can decide whether a scan or a strength check would help. (Scale study; Joint advisory)

Is retatrutide muscle-sparing?

Not based on the evidence reviewed. The study's authors said its lean-mass share was similar to other obesity treatments, not lower. Claims that it protects muscle go beyond the data. (Study)

Does the glucagon action of retatrutide break down muscle?

It's a reasonable question that hasn't been settled. Researchers noted that glucagon signaling can lower amino acids in the blood, but the DXA study did not isolate how much its glucagon action contributed to lean-mass change. It does not prove that this action protects or damages muscle. (Study)

Is compounded retatrutide the same as the study drug?

You cannot assume it is. The FDA says retatrutide can't be used in compounding under federal law, and products sold for "research use" are of unknown quality. A label doesn't prove it's the drug that was tested. (FDA)

Still not sure which GLP-1 path fits you? Take The RX Index's free Find My GLP-1 Path quiz — about 2 minutes, no signup.

Sources

Sources checked October 15, 2026. Publication and source-update dates below are separate from that check date.

1. Coskun T, et al. Effects of retatrutide on body composition in people with type 2 diabetes. Lancet Diabetes & Endocrinology. 2025;13(8):674–684. Published online June 30, 2025. PubMed · Original paper
2. Jastreboff AM, et al. Triple–hormone-receptor agonist retatrutide for obesity — a phase 2 trial. NEJM. 2023. Link
3. Jastreboff AM, et al. Retatrutide, a triple hormone receptor agonist, for treatment of obesity (TRIUMPH-1). NEJM. Published September 29, 2026. Main paper and supplementary appendix. Link
4. Giblin K, et al. Retatrutide for the treatment of obesity, obstructive sleep apnea and knee osteoarthritis: rationale and design of the TRIUMPH registrational clinical trials. Diabetes, Obesity and Metabolism. 2026;28(1):83–93. First published online October 15, 2025. Link
5. Lilly Medical. Results of retatrutide from TRIUMPH-3. Last reviewed July 23, 2026. Link
6. U.S. FDA. FDA's concerns with unapproved GLP-1 drugs used for weight loss. Link
7. Eli Lilly. Lilly calls on online platforms, payment companies and regulators to shut down the illegal retatrutide black market. August 12, 2026. Link
8. Eli Lilly. Lilly's triple agonist, retatrutide, successful in two additional Phase 3 obesity trials. July 23, 2026. Includes the Q1 2027 regulatory-submission plan. Link
9. Eli Lilly, sponsor-posted record on ClinicalTrials.gov. Pre-approval expanded access of retatrutide (LY3437943), NCT07629401. First posted June 5, 2026; last update posted August 6, 2026. Study record · Record data · Lilly's expanded-access policy
10. Mozaffarian D, et al. Nutritional priorities to support GLP-1 therapy for obesity: a joint advisory. American Journal of Clinical Nutrition. 2025;122:344–367. Journal version · Full text of the joint advisory, co-published in American Journal of Lifestyle Medicine
11. Frija-Masson J, et al. Accuracy of smart scales on weight and body composition. JMIR mHealth and uHealth. 2021;9(4):e22487. Link
12. MedlinePlus. Weakness. Reviewed July 23, 2024. Link
13. CDC. Signs and symptoms of stroke. May 19, 2026. Link
14. NIDDK. Healthy eating for adults with chronic kidney disease. Link
15. Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). NEJM. 2021. Supplementary appendix, DXA tables on printed pages 20 and 24. Original paper · Appendix copy
16. Look M, et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes, Obesity and Metabolism. 2025. Link
17. ATTAIN-1 investigators. Orforglipron Phase 3 obesity trial, including the 171-participant DXA subgroup. NEJM. 2025. Original paper
18. Eisa N, Barood O. Lean mass changes with incretin therapy versus lifestyle intervention: a systematic review and meta-analysis of randomised controlled trials. Diabetes, Obesity and Metabolism. 2026;28(6):4818–4827. Published online March 24, 2026. Original abstract used for the reported lifestyle estimates. Link
19. Heymsfield SB, et al. BELIEVE randomized Phase 2 trial report: bimagrumab and semaglutide for obesity. Nature Medicine. 2026;32:869–882. Published March 2, 2026. Link
20. Keenan SJ, et al. Intermittent fasting and continuous energy restriction result in similar changes in body composition and muscle strength when combined with a 12 week resistance training program. European Journal of Nutrition. 2022. Link
21. Approved-product labeling: Zepbound, revised August 2026; Wegovy, 2026 labeling; Foundayo, revised July 2026. Used for product and approval status, not as a source for the cross-trial lean-mass percentages.
22. The RX Index. Find My GLP-1 Path and GLP-1 lean mass loss by drug. Used for the tool's stated features and related reading; the comparison figures above were checked against the original research.

1. Eli Lilly. Clinical development pipeline, data as of August 5, 2026. Bimagrumab obesity program listed under Phase 2. Link