Retatrutide Muscle Loss: What the Scans Show and How to Keep Lean Mass

Yes, some retatrutide muscle loss is expected, because any big weight loss takes lean tissue with it. The only published body-scan study so far, a Phase 2 substudy in people with type 2 diabetes, found fat mass fell 23.2% on 12 mg over 36 weeks, and the Lilly authors reported that the share of weight lost as lean mass was similar to other obesity treatments. What nobody has published is a Phase 3 scan (TRIUMPH-3 has a scan substudy that hasn't reported), a strength test or a clean kilogram figure for muscle.

Retatrutide's own scan data is thin, so this page leans on tirzepatide and semaglutide scans where it has to, and labels them as class evidence each time.

Retatrutide muscle loss in the only DXA study so far

It's a substudy of Lilly's Phase 2 type 2 diabetes trial, published in The Lancet Diabetes & Endocrinology in 2025, and it used dual-energy X-ray absorptiometry (DXA), the scan that splits your body into fat, lean tissue and bone. Of the 189 people enrolled, 155 had a baseline scan and 103 finished treatment with scans at both ends.

Over 36 weeks, fat mass fell 4.9% on 0.5 mg, 15.2% on 4 mg, 26.1% on 8 mg and 23.2% on 12 mg, against 4.5% on placebo and 2.6% on dulaglutide, an older GLP-1 drug. For scale, the whole 12 mg arm of the main trial lost 16.94% of body weight by week 36, according to the Lancet trial paper. Fat fell faster than the scale did, so the non-fat part of the body fell more slowly by that rough comparison.

On lean mass, the open abstract gives only the authors' conclusion: the proportion of lean mass loss to weight loss "was similar to other obesity treatments." It adds that the findings "could provide reassurance that a greater proportion of lean mass is not lost with retatrutide despite the overall increased weight loss." The per-dose lean figures sit in the paywalled full text, and we couldn't verify them. Everyone in the substudy had type 2 diabetes, and every author is a Lilly employee. For what the same scans say about fat, see retatrutide belly fat.

As of October 31, 2026, none of the 33 retatrutide records on ClinicalTrials.gov lists lean mass, DXA or muscle as an outcome, TRIUMPH-1 to TRIUMPH-5 included. But the TRIUMPH program's design paper in Diabetes, Obesity and Metabolism says TRIUMPH-3, the trial in severe obesity with cardiovascular disease, includes a DXA substudy of about 100 people measuring fat and lean mass. Lilly's July 23, 2026 topline didn't report it.

Lean mass isn't the same as muscle

A DXA "lean mass" number counts muscle plus other non-fat tissue. The 2025 joint nutrition advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association and The Obesity Society puts muscle at about half of lean body mass. So when a study says someone lost 6 kg of lean mass, the muscle part is closer to 3 kg.

The other way to read a scan is by share. In tirzepatide's SURMOUNT-1 DXA substudy, people on the drug started with an average of 46.6 kg of fat and 50.8 kg of lean mass, and lost 15.9 kg and 5.6 kg by week 72. Worked through, fat went from 47.8% to 40.4% of their fat-plus-lean tissue, so lean mass shrank in kilograms but grew as a share. That's tirzepatide data, not retatrutide.

Retatrutide muscle loss compared with tirzepatide and semaglutide

These are different trials in different people over different lengths of time, so read across rows with care. The tirzepatide and semaglutide rows are class evidence. For the whole-drug comparison, see retatrutide vs tirzepatide.

Sources: Lancet Diabetes & Endocrinology 2025 abstract; Diabetes, Obesity and Metabolism 2025; STEP 1 results on ClinicalTrials.gov. The STEP 1 share is 5.8 kg divided by the 15.1 kg of fat plus lean lost.

In SURMOUNT-1 the lean share barely moved between drug and placebo (26% and 25%), and on tirzepatide the fat share held at 74% to 76% across age groups, including people 65 and older. The SURMOUNT-1 authors note that many diet studies report the same split, about 75% fat and 25% lean.

Does reta eat muscle through the glucagon receptor?

This is the worry specific to retatrutide, because the glucagon receptor is the one tirzepatide and semaglutide don't hit (how retatrutide works covers all three receptors). One of glucagon's jobs is telling the liver to break down amino acids, the building blocks of protein, into urea. A 2020 Molecular Metabolism paper saw urea output rise within minutes when glucagon was added to human liver cells in a lab dish. Liver cells in a dish say nothing direct about muscle in people.

What retatrutide data exists doesn't show extra loss so far. Beyond the scan study's "similar" verdict, a 2026 blood-sample analysis of both Phase 2 trials, in The Journal of Clinical Endocrinology & Metabolism, found retatrutide shifted branched-chain amino acids in a direction the authors link to better metabolic health. It didn't measure muscle, and Lilly scientists led it too.

Our read: the glucagon effect hasn't shown up as extra lean loss in the one study that looked, but 103 people over 36 weeks is too small to rule out a modest difference. TRIUMPH-3's substudy has no tirzepatide arm, and TRIUMPH-5, the head-to-head trial against tirzepatide, doesn't list body composition among its outcomes, so a direct comparison may wait.

How much muscle could you lose at Phase 3 weight loss?

The bigger worry with retatrutide is how much weight goes overall. In TRIUMPH-1, people on 12 mg lost an average of 28.3% of their body weight (31.9 kg) over 80 weeks on Lilly's efficacy estimand, and 62.5% lost 25% or more. On the treatment-regimen estimand used in the NEJM paper, which counts everyone randomized, the 12 mg average was 25.0%. The more you lose, the more lean tissue tends to go with it: in SURMOUNT-1's tirzepatide group, bigger weight losses meant more fat mass and more lean mass lost.

Scale matters in the other trials too. In TRANSCEND-T2D-2, reported October 28, 2026, people with type 2 diabetes on 12 mg lost 20.9% (21.5 kg) against 9.4% (9.5 kg) on Ozempic 2 mg over 80 weeks, on the efficacy estimand. Lilly reported no scan results, but if the lean share is similar, more kilograms off means more lean kilograms off.

This is only arithmetic. It applies TRIUMPH-1's 28.3% average to four starting weights, then takes the lean share seen with tirzepatide (26%) and with semaglutide (38%).

With muscle at roughly half of lean mass, the TRIUMPH-1 row means something like 4 to 6 kg of muscle. No retatrutide trial has measured any of these numbers, and the 28.3% is an average for people who started near 112.7 kg, so treat the other rows as scaling, not a forecast.

Mass and function are separate questions. In TRIUMPH-4, people with knee osteoarthritis on 12 mg improved their WOMAC physical function score by 4.2 points from a baseline of 5.8, against 2.1 on placebo, per Lilly's efficacy estimand. WOMAC is a questionnaire: people reported that daily movement got easier, whatever lean tissue went. Weight results by dose are on retatrutide results.

How to keep muscle on retatrutide

No retatrutide trial has tested a muscle-sparing plan. The best-supported advice comes from the 2025 joint advisory for people on GLP-1 drugs, which is class guidance, not retatrutide data.

Lift at least 3 times a week

The advisory recommends regular strength training at least 3 times weekly plus at least 150 minutes of moderate aerobic exercise to protect muscle and bone. The CDC's floor for all adults is 2 days of muscle-strengthening a week. The advisory is blunt that more protein on its own probably won't preserve muscle without structured training.

Eat enough protein, on purpose

The advisory lists 0.8 g per kg a day as the general adult allowance, 1.2 to 1.6 g per kg proposed during active weight loss, and 1.2 to 1.7 g per kg during strength training. At 100 kg, 1.2 to 1.6 g per kg works out to 120 to 160 g a day, which is a lot when retatrutide has shut your appetite down. The advisory also warns that actual body weight in obesity can overstate needs, and that intake shouldn't stay above 2 g per kg for long. Ask a doctor or dietitian which weight to use.

If you're struggling to eat enough, or feel drained at the gym, retatrutide fatigue covers the energy side.

Dose pace and dose accuracy

The advisory names how hard you cut calories and how fast the weight comes off as factors in lean loss, and says some people may benefit from a slower dose schedule. Trial doses went up in 4-week steps, shown in the retatrutide dosage guide; the pace is a call for you and your doctor. Dose accuracy counts here too. Lilly's October 2026 FAQ says illicit products can hold too much or too little active ingredient, or the wrong ingredient entirely. Australia's TGA reported on September 14, 2026 that a vial sold as retatrutide held no retatrutide, but did hold undeclared semaglutide at about 8 times the level in approved semaglutide products. If you're looking at retatrutide for sale, ask for a batch-specific third-party test report whose batch number matches the vial.

Track strength and body composition

The advisory suggests checking muscle strength, function and body composition before starting. A DXA scan or a training log tells you more than the scale does, and more than photos, which before and after pictures explains.

Can you build muscle on reta?

No trial has tested it. Every group in the table above with published lean-mass numbers, placebo included, lost lean mass on average, and averages hide people who did better or worse. Individual reports online are single cases without scans or a comparison group, so they can't tell you how often it happens. Retatrutide benefits sorts the muscle claims by evidence.

A stalled scale during a hard calorie deficit could mean muscle gain or just a plateau, and only a scan can tell them apart. If the pain you're feeling is on your skin rather than in the muscle, TRIUMPH-1 found burning or oversensitive skin (dysesthesia) in 12.5% of people on 12 mg against 0.9% on placebo, and retatrutide skin sensitivity covers it.

Our read: every group on this page with published lean-mass numbers lost lean mass on average, so muscle you gain on retatrutide would be training and protein pulling against that trend. Nobody has measured how often people manage it.

No study has tested tesamorelin or MOTS-c, both pitched online for muscle, during retatrutide weight loss; those pages cover what each has shown alone.

Who loses the most, and when to see a doctor

The advisory flags a higher risk of muscle loss with older age, perimenopause or menopause, lower testosterone, sitting most of the day, and no strength training. Lean loss matters more when you start with less to spare. For every other side effect on the list, see retatrutide side effects, and for the wider risk picture, is retatrutide safe.

Where things stand as of October 31, 2026: retatrutide isn't approved anywhere. Lilly last said, on September 29, 2026, that it plans to submit retatrutide to the FDA in the first quarter of 2027 as a Biologics License Application (BLA). On October 2, 2026, the FDA decided that retatrutide is a drug, not a biological product, so an application would need to be filed under section 505 of the Federal Food, Drug, and Cosmetic Act. Lilly's appeal on that question is pending in the Seventh Circuit, and we found no ruling as of October 31, 2026. If you're planning a long course, a baseline DXA scan now gives you something real to compare against later.