Key Takeaway
What the published retatrutide trials do and do not say about lean mass, how they compare with the semaglutide and tirzepatide body-composition substudies, and the general evidence on protecting muscle during weight loss. Retatrutide is investigational and not available by prescription.
Like other incretin-based medications, retatrutide produces some lean mass loss alongside fat loss. Whether its glucagon receptor component preserves muscle better than single- or dual-agonist drugs is a hypothesis, not an established finding: the only published retatrutide body-composition data (a type 2 diabetes phase 2 substudy, 2025) concluded that the proportion of lean mass loss to weight loss was similar to other obesity treatments. This page lays out that data, the comparators, and what remains unknown.
Regulatory status as of September 2026
Retatrutide is an investigational drug. As of September 5, 2026 it is not approved by the FDA for any use, and Eli Lilly has said it plans to submit for approval by the first quarter of 2027 following the TRIUMPH phase 3 program (toplines: TRIUMPH-4 on December 11, 2025; TRIUMPH-1 on May 21, 2026; TRIUMPH-2 and TRIUMPH-3 on July 23, 2026). The FDA states that retatrutide "cannot be used in compounding under federal law" and has issued warning letters to companies selling it, including under "Research Use Only" labels (for example, the March 31, 2026 letter to Gram Peptides). FormBlends does not prescribe, dispense or sell retatrutide, and nothing on this page is dosing or treatment guidance. It is a summary of published trial data.
The Lean Mass Problem in Weight Loss
Every form of weight loss, whether through diet, exercise, surgery, or medication, results in some loss of lean body mass alongside fat. The best-documented recent figure comes from the SURMOUNT-1 DXA substudy of tirzepatide (Look et al., Diabetes, Obesity and Metabolism 2025, n=160): roughly 75% of the weight lost was fat and 25% lean mass at week 72. Correction, September 2026: an earlier version of this paragraph presented 75/25 as the ratio for "conventional dieting" and added a claim about bariatric surgery; neither was sourced and the surgery claim has been removed. The goal of obesity treatment is to shift the ratio toward fat.
Lean mass includes muscle, bone, water, and organ tissue. When people talk about "muscle loss" with weight loss medications, they're usually referring to the lean mass component measured by DEXA scans in clinical trials. Not all lean mass loss is muscle. Some is water bound to glycogen stores, some is the structural tissue that supported a larger body frame, and some is genuinely functional skeletal muscle. The clinical significance depends heavily on which compartments are losing mass.
What Semaglutide and Tirzepatide Showed
To understand where retatrutide fits, it helps to look at the body composition data from its predecessors. In the STEP 1 trial, semaglutide 2.4 mg produced a mean 14.9% total body weight loss over 68 weeks. A DXA substudy of STEP 1 found that fat mass fell more than lean mass, but the published abstract does not state what share of the lost weight was lean tissue. Correction, September 2026: an earlier version said "roughly 40% of the weight lost was lean mass" against a "historical 25% benchmark"; the 40% was a derived figure and the benchmark had no source, so both have been removed.
Check your GLP-1 eligibility
Use our free BMI Calculator to see if you may qualify for provider-reviewed GLP-1 therapy.
Try the BMI Calculator →View data table
| Category | Mean Body Weight Loss (%) | Detail |
|---|---|---|
| Placebo | 2 | ~2% weight loss |
| 4 mg | 17 | ~17% at 48 weeks |
| 8 mg | 22 | ~22% at 48 weeks |
| 12 mg | 24 | ~24% at 48 weeks |
Tirzepatide produced larger weight loss in the SURMOUNT trials. At the 15 mg dose, participants lost a mean 20.9% of body weight at week 72 (Zepbound label, revised 08/2026; Jastreboff et al., NEJM 2022[2]). In the DXA substudy (Look et al. 2025, n=160), fat mass fell 33.9% and lean mass 10.9%, so about 25% of the weight lost was lean. Correction, September 2026: an earlier version quoted 22.5% weight loss and a lean share of "25 to 33% depending on the sub-study"; 22.5% is a different statistical estimand from the label figure, and no substudy reports 33%. Whether GIP activity protects lean mass is not established.
These numbers sparked an important conversation in obesity medicine about whether medications that produce dramatic weight loss might leave patients metabolically weaker despite being lighter.
Retatrutide's Theoretical Advantage
Retatrutide adds a third receptor to the equation: the glucagon receptor. This is where the body composition story gets genuinely compelling. Glucagon has well-established effects on energy metabolism that differ fundamentally from GLP-1 and GIP.
Glucagon stimulates lipolysis, the breakdown of stored fat into free fatty acids for energy use, and increases energy expenditure. Its primary metabolic targets are adipose tissue and the liver rather than skeletal muscle. Correction, September 2026: an earlier version added that animal studies show glucagon agonism "preferentially" spares lean tissue; no specific study was cited and the sentence has been removed.
The theory is that by activating the glucagon receptor alongside GLP-1 and GIP, retatrutide might draw more heavily on fat stores during a caloric deficit. If true, that would show up as a more favorable fat-to-lean ratio in DXA data. As of September 2026, the one published retatrutide DXA substudy did not find that (see below).
What the Phase 2 Trial Actually Showed
The phase 2 obesity trial of retatrutide (Jastreboff et al., New England Journal of Medicine 2023[3]; 338 participants, 48 weeks) reported mean weight change of 8.7% (1 mg), 17.1% (4 mg), 22.8% (8 mg) and 24.2% (12 mg) versus 2.1% on placebo. The 24.2% figure was the largest reported for an obesity medication at the time; the phase 3 TRIUMPH-1 (28.3% at 80 weeks on 12 mg, May 21, 2026) and TRIUMPH-4 (28.7% at 68 weeks, December 11, 2025) toplines have since exceeded it.
Correction, September 2026: an earlier version of this paragraph said the obesity phase 2 trial's DXA substudy showed lean mass loss "in the range of 25-30% of total weight lost." The NEJM obesity paper reports no body-composition data, and that range could not be traced to any publication. The only published retatrutide DXA data come from the type 2 diabetes phase 2 trial (Lancet Diabetes & Endocrinology, August 2025; 189 enrolled, 103 with both scans). At week 36, fat mass fell 4.9% (0.5 mg), 15.2% (4 mg), 26.1% (8 mg) and 23.2% (12 mg), versus 2.6% on dulaglutide and 4.5% on placebo. The authors' conclusion, quoted exactly: "The proportion of lean mass loss to weight loss was similar to other obesity treatments." No percentage for the lean share is given in the abstract.
The phase 3 TRIUMPH program has now reported toplines: TRIUMPH-4 (knee osteoarthritis, December 11, 2025: 28.7% on 12 mg and 26.4% on 9 mg versus 2.1%), TRIUMPH-1 (obesity, May 21, 2026: 19.0%, 25.9% and 28.3% on 4, 9 and 12 mg versus 2.2% at 80 weeks; 45.3% of the 12 mg group lost 30% or more), and TRIUMPH-2 and TRIUMPH-3 (July 23, 2026: 12.7% to 20.8% in type 2 diabetes and 21.6% to 22.6% in cardiovascular disease). None of these announcements reported body-composition results. Correction, September 2026: an earlier version stated Lilly "has included DEXA endpoints" in phase 3; we could not source that and have removed it. Until peer-reviewed body-composition data appear, any claim of superior lean mass preservation remains theoretical.
Practical Strategies to Protect Muscle
The strategies below are general weight-loss guidance, not instructions for taking retatrutide, which is not available by prescription. People losing weight by any method can influence how much lean mass they keep.
Resistance training. Progressive resistance exercise is the standard recommendation for preserving muscle during weight loss. Correction, September 2026: an earlier version stated that "studies consistently show" strength training two to four times a week preserves lean mass specifically during GLP-1 therapy; no GLP-1-specific trial was cited, so the sentence has been reworded as general guidance. Bodyweight exercises, resistance bands, or free weights performed consistently all count.
Protein intake. Appetite suppression on incretin drugs can make it hard to eat enough, and low-protein convenience foods make it worse. Prioritizing protein at each meal is standard advice during weight loss. Correction, September 2026: specific gram-per-kilogram targets that used to appear here were unsourced and have been removed; a registered dietitian can set an individual target.
Don't let calories drop too low. Very low intake accelerates lean mass loss. If appetite suppression from any prescribed weight-loss medication leaves you eating very little, that is a conversation for the prescriber of that medication. Retatrutide is investigational and has no prescriber outside a clinical trial.
Consider creatine supplementation. Creatine monohydrate is one of the most studied supplements in exercise science and has strong evidence supporting its role in muscle preservation and function. Taking 3-5 grams daily alongside resistance training may provide additional lean mass protection during weight loss. It's safe, inexpensive, and widely available.
Sleep and recovery matter. Muscle repair happens largely during sleep. Correction, September 2026: an earlier sentence claiming sleep-deprived people lose more lean mass during caloric restriction was unsourced and has been removed; adequate sleep remains a reasonable part of any body-composition plan.
The Long View on Body Composition
The muscle loss conversation often misses an important nuance. Carrying 250 or 300 pounds requires more muscle mass just to move through daily life. When someone loses 50 or 60 pounds, they simply need less structural muscle to support their frame. Some of the measured lean mass loss is the body naturally right-sizing itself for a lighter body. This isn't pathological. It's adaptive.
What matters clinically is functional capacity. Can you walk farther, climb stairs more easily, carry groceries without strain? If your strength and endurance are improving even as the scale and DEXA show some lean mass reduction, you're likely in good shape. The patients who need to worry are those who lose weight without exercising and notice declining physical function, weakness, or difficulty with daily activities.
Retatrutide's triple-agonist mechanism gives it a theoretical edge in body composition that the published data have not yet demonstrated. For anyone losing weight by any method, the biggest determinant of how much muscle you keep remains what you do in the gym and the kitchen.
Medical References
- Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021;384(11):989-1002. [PubMed | ClinicalTrials.gov | DOI]
- Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387(3):205-216. [PubMed | ClinicalTrials.gov | DOI]
- Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. 2023;389(6):514-526. [PubMed | ClinicalTrials.gov | DOI]
Related guides
- Does Retatrutide Cause Muscle Loss? Lean Mass Data and What to Do About It
- Retatrutide Before and After: Clinical Trial Weight Loss Data
- Retatrutide and Muscle Loss: What to Expect and How to Manage
- Muscle Loss Prevention on Retatrutide
- Retatrutide and Cardiovascular Risk Factors: Trial Data
- Retatrutide Patient Satisfaction Trial Data
Side-by-side: the published DXA numbers for semaglutide, tirzepatide and retatrutide
As of September 5, 2026, these are the body-composition substudies that have been published for the three drugs. They differ in population, duration and dose, so the table is a reference, not a head-to-head comparison.
| Drug and trial | Population, duration | Published body-composition result |
|---|---|---|
| Semaglutide 2.4 mg, STEP 1 DXA substudy | Obesity, 68 weeks | Fat mass fell more than lean mass; the abstract does not report the lean share of weight lost |
| Tirzepatide, SURMOUNT-1 DXA substudy (Look et al., Diabetes Obes Metab 2025, n=160) | Obesity, 72 weeks | Body weight -21.3%, fat mass -33.9%, lean mass -10.9%; about 75% of weight lost was fat and 25% lean |
| Retatrutide, type 2 diabetes phase 2 DXA substudy (Lancet Diabetes Endocrinol, Aug 2025; 103 with both scans) | Type 2 diabetes, 36 weeks | Fat mass -26.1% (8 mg), -23.2% (12 mg) vs -4.5% placebo; "proportion of lean mass loss to weight loss was similar to other obesity treatments" |
| Retatrutide, TRIUMPH phase 3 toplines (Dec 2025 to Jul 2026) | Obesity, T2D, CVD, knee OA; 68 to 80 weeks | No body-composition results released |
Phase 3 toplines to date and what they did not report
| Trial (announcement date) | Population, duration | Weight change reported | Body composition or thyroid data |
|---|---|---|---|
| TRIUMPH-4 (December 11, 2025) | Obesity with knee osteoarthritis, 68 weeks | -28.7% (12 mg), -26.4% (9 mg) vs -2.1% placebo | None released |
| TRIUMPH-1 (May 21, 2026) | Obesity, n=2,339, 80 weeks | -19.0% (4 mg), -25.9% (9 mg), -28.3% (12 mg) vs -2.2%; discontinuation for adverse events 11.3% vs 4.9% | None released |
| TRIUMPH-2 (July 23, 2026) | Type 2 diabetes | -12.7%, -19.1%, -20.8% vs -4.0% | None released |
| TRIUMPH-3 (July 23, 2026) | Cardiovascular disease | -21.6%, -22.6% vs -3.2% | None released |
Sources: Lilly press release via PR Newswire (May 21, 2026) and Pharmaceutical Executive reports (December 11, 2025 and July 23, 2026), seen September 5, 2026. Lilly's investor pages could not be fetched directly. The most common adverse events at 12 mg in TRIUMPH-1 were nausea (42.4%), vomiting (25.3%) and dysesthesia (12.5%).
FAQ
Is retatrutide FDA approved or available by prescription?
No. As of September 5, 2026 retatrutide is investigational. Lilly has completed several TRIUMPH phase 3 trials and has said it plans to submit for approval by the first quarter of 2027 (Lilly via PharmExec, July 23, 2026). The FDA states retatrutide cannot be used in compounding under federal law and has sent warning letters to sellers marketing it as a research chemical. FormBlends does not offer it.
Does retatrutide preserve muscle better than semaglutide or tirzepatide?
Not shown. The only published retatrutide body-composition data, a 36-week DXA substudy in type 2 diabetes (Lancet Diabetes and Endocrinology, August 2025), found fat mass fell up to 26.1% and concluded the proportion of lean mass loss to weight loss was similar to other obesity treatments. The phase 3 toplines released through July 2026 did not report body composition. The glucagon-receptor advantage remains a hypothesis.
How much lean mass is lost on tirzepatide, the closest approved comparator?
In the SURMOUNT-1 DXA substudy (Look et al., Diabetes Obes Metab 2025, 160 participants), tirzepatide reduced body weight by 21.3%, fat mass by 33.9% and lean mass by 10.9% at week 72. About three quarters of the weight lost was fat and one quarter lean tissue, a ratio the authors describe as similar to other weight-loss methods. That is the benchmark retatrutide's eventual data will be compared against.
Sources
- Jastreboff AM et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity, a Phase 2 Trial. N Engl J Med 2023;389:514-526. https://europepmc.org/article/MED/37366315
- Coskun T et al. Effects of retatrutide on body composition in people with type 2 diabetes: a substudy of a phase 2, double-blind, parallel-group, placebo-controlled, randomised trial. Lancet Diabetes Endocrinol 2025;13:674-684. https://europepmc.org/article/MED/40609566
- Look M et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes Obes Metab 2025;27:2720-2729. https://europepmc.org/article/MED/39996356
- Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med 2022;387:205-216. https://europepmc.org/article/MED/35658024
- Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med 2021;384:989-1002. https://europepmc.org/article/MED/33567185
- Eli Lilly and Company via PR Newswire. Lilly's triple agonist retatrutide delivered powerful weight loss in pivotal phase 3 obesity trial (TRIUMPH-1), May 21, 2026. https://www.prnewswire.com/news-releases/lillys-triple-agonist-retatrutide-delivered-powerful-weight-loss-in-pivotal-phase-3-obesity-trial-302778859.html
- Pharmaceutical Executive. Retatrutide delivers up to 22% weight loss in two phase III trials (TRIUMPH-2 and TRIUMPH-3), July 23, 2026. https://www.pharmexec.com/view/retatrutide-delivers-22-weight-loss-two-phase-iii-trials
- Pharmaceutical Executive. Lilly's retatrutide posts positive topline results in phase III TRIUMPH-4 trial, December 11, 2025. https://www.pharmexec.com/view/lilly-retatrutide-positive-topline-results-successful-phase-iii-trial
- U.S. Food and Drug Administration. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss, content current as of September 1, 2026. https://www.fda.gov/drugs/postmarket-drug-safety-information-patients-and-providers/fdas-concerns-unapproved-glp-1-drugs-used-weight-loss
- U.S. Food and Drug Administration. Warning letter to Gram Peptides, March 31, 2026. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/warning-letters/gram-peptides-721806-03312026
See your options in about 2 minutes
Take the free quiz and see what fits you. Quick, private, and no commitment to continue.
See my options →