The question is not unreasonable. GLP-1 medications produce significant weight loss. Weight loss, by definition, involves losing some muscle. So the concern that active people have when considering Mounjaro or Wegovy is legitimate: what happens to the muscle you have spent months or years building?
The short answer, supported by DXA body composition data from clinical trials, is that you can maintain and build muscle on GLP-1 medications, but it requires deliberate effort. The medications do not protect muscle automatically. Resistance training and adequate protein intake do. What the data clarifies is what the lean mass picture actually looks like, where the differences between tirzepatide and semaglutide sit, and what the practical training and nutrition response should be.
Resistance training is the most evidence-supported intervention for preserving and building lean mass during GLP-1 treatment. The signal to preserve muscle comes from progressive mechanical loading, not from the medication itself.
What “lean mass loss” actually means on GLP-1s
A common misconception is that lean mass loss on GLP-1 medications is unusual or a drug-specific side effect. It is neither. Every significant weight loss intervention, whether dietary restriction, bariatric surgery, or medication, produces a combination of fat and lean mass loss.
The physiological reason is straightforward: a lighter body requires less structural support, so some lean tissue is released as part of the adaptive remodeling that accompanies major weight reduction.
The SURMOUNT-1 DXA substudy, published in Diabetes, Obesity and Metabolism in February 2025 (Look et al.), assessed body composition in 124 tirzepatide-treated participants using dual-energy X-ray absorptiometry (DXA) at baseline and at 72 weeks. The data produced a clear picture: total weight fell 21.3%, fat mass fell 33.9%, and lean mass fell 10.9%. Of the total weight lost, approximately 75% was fat mass, and 25% was lean mass. These proportions held consistently across subgroups by age, sex, and degree of weight loss.
The 25% lean fraction is consistent with what is seen across other weight loss modalities, including bariatric surgery and calorie-restricted diet. Tirzepatide does not produce disproportionate lean loss. The fat-to-lean ratio of weight lost is comparable to other established approaches.
SURMOUNT-1 DXA body composition data at 72 weeks. Fat mass fell 33.9% while lean mass fell 10.9%. The ratio of fat to lean loss is comparable to other weight loss interventions, including bariatric surgery.
The tirzepatide versus semaglutide lean mass difference
For active people choosing between the two medications, the body composition difference matters. The data here is more nuanced than is often reported.
Proportional lean loss: tirzepatide is better
The STEP-1 DXA data for semaglutide, as reported in a 2025 PMC case series, showed lean mass accounting for approximately 40% of total weight lost, compared to 25-26% with tirzepatide in SURMOUNT-1. On a proportional basis, tirzepatide produces a more favorable fat-to-lean loss ratio. This is attributed to its dual GIP and GLP-1 receptor activation, with GIP signaling believed to have a muscle-protective effect.
Absolute lean loss: the picture is more complicated
A large real-world analysis published on medRxiv in April 2026 analyzed body composition data from 670,422 first-episode GLP-1 patients across 12 months, including 7,965 with paired pre- and post-treatment measurements. The findings showed tirzepatide was associated with greater relative lean body mass loss than semaglutide at every measured time point, with excess LBM losses of 1.1%, 1.5%, 1.3%, and 2.0% at 3, 6, 9, and 12 months, respectively.
The explanation is arithmetic rather than pharmacological. Tirzepatide drives approximately 47% more total weight loss than semaglutide (20.2% vs 13.7% in SURMOUNT-5). When the same proportion of lean mass is lost from a larger total weight loss, the absolute amount of lean mass lost is greater. The real-world analysis also identified a “Depletive GLP-1 metabotype” (greater than 20% total body weight loss with greater than 5% LBM loss) in 10.3% of tirzepatide users versus 6.7% of semaglutide users. For highly active people who are already lean, this distinction is worth understanding before starting treatment.
What This Means for Training
The lean mass data does not mean GLP-1 medications should be avoided by active people. It means the protective work that resistance training and protein do becomes more important, not less, during treatment.
Resistance training: the evidence-supported intervention
Resistance training provides the primary stimulus for muscle protein synthesis. It signals the body to retain and build muscle regardless of whether a calorie deficit is present. A 2025 systematic review published in PMC confirmed that tirzepatide promotes substantial weight loss primarily through fat mass reduction while preserving lean mass, and that nutrition and physical activity can mitigate lean mass loss further.
For GLP-1 users who are currently training, the practical guidance is simple: maintain your resistance training frequency and do not reduce volume significantly as a result of lower energy intake. Two to three sessions per week of compound resistance training (squats, deadlifts, rows, pressing movements) provides the mechanical signal that tells your body to retain muscle even during a calorie deficit.
For people who have not been resistance training and are starting GLP-1 treatment, beginning a structured strength routine at the same time is one of the highest-value changes you can make. You will not only reduce the proportion of lean mass lost during treatment, but you will also likely end up with better muscle quality and functional strength than before treatment, because the fat you lose improves the ratio of muscle to total body mass.
Protein: the numerical target that matters
The American Journal of Clinical Nutrition recommends 1.2 to 1.6g of protein per kilogram of body weight per day during active weight loss to preserve lean mass. For people who are resistance training, the evidence points to 1.6 to 2.2g/kg/day as a more appropriate target, with some research supporting up to 2.4g/kg during aggressive calorie restriction. For a 90kg active person, this means 144 to 198g of protein per day.
On GLP-1 treatment, where appetite is substantially reduced, hitting protein targets requires deliberate planning. The practical approach is to build meals around protein-first: eat the protein portion of every meal before anything else, so that if appetite cuts off early, you have still hit the most nutritionally critical element.
High-protein foods that are easy to eat in smaller portions are particularly valuable: Greek yogurt, eggs, cottage cheese, canned fish, lentils, and protein shakes for days when solid food appetite is very low.
Can you actually build muscle on GLP-1 medications?
Yes, if you are resistance training and eating sufficient protein. The studies on which this question depends are not large-scale randomized controlled trials (those have not been conducted), but the mechanistic case is sound and consistent with what we know about muscle protein synthesis.
A 2025 case series published in PMC (SAGE Open Medicine) reported three patients who prioritized lean tissue preservation strategies during semaglutide or tirzepatide treatment and achieved lean mass preservation significantly better than trial benchmarks: one patient lost only 8% of total weight as lean mass, versus the trial average of 26% to 40%.
The calorie deficit required for weight loss does make hypertrophy (net muscle gain) more difficult. To build new muscle, you generally need either a calorie surplus or at minimum maintenance calories, neither of which applies during active weight loss.
The achievable target on GLP-1 treatment is lean mass preservation rather than net muscle gain. That means ending treatment with the same absolute amount of muscle you started with, even though you weigh less overall.
Given that this also means your body fat percentage has fallen significantly, the practical outcome is a meaningfully better physique and better health markers than before.
Retatrutide and the next generation of body composition data
The triple agonist retatrutide (GLP-1, GIP, and glucagon receptor activation) is currently in phase 3 trials in the UK, with the first trial completing in December 2025. Phase 2 data showed 24.2% weight loss at 48 weeks (12mg dose), with lean mass loss proportions similar to tirzepatide and semaglutide in the same-class phase 2 trial.
The glucagon receptor component of retatrutide is specifically relevant to body composition because glucagon stimulates energy expenditure and fat oxidation, theoretically improving the fat-to-lean ratio of weight loss.
Early phase 2 data showed 26.1% total body fat mass lost in one trial of people with type 2 diabetes, with muscle loss proportions similar to other GLP-1 medications. Voy’s retatrutide body composition analysis covers where triple agonists fit relative to existing GLP-1 medications for active people.
Whether retatrutide will prove meaningfully better for lean mass preservation than tirzepatide remains to be determined by the phase 3 data, the glucagon mechanism is promising in theory, but the existing data does not yet demonstrate a clear advantage over tirzepatide’s dual-agonist lean mass performance.
The practical protocol for active people on GLP-1s
This is the evidence-supported approach for active people starting Mounjaro or Wegovy:
Training
Maintain resistance training frequency: At minimum two sessions per week. Three to four, if possible. Compound lifts that load the largest muscle groups (squats, deadlifts, horizontal and vertical rows, pressing movements).
Do not dramatically reduce training volume: Lower energy intake does not mean lower training should follow automatically. Many people find that training performance is relatively well maintained on GLP-1 treatment, particularly after the initial adjustment period.
Prioritize recovery: Adequate sleep and protein are the two variables that most affect muscle protein synthesis. Both are within your control.
Nutrition
Protein target: 1.6 to 2.2g/kg/day: For a 90kg person, this is 144 to 198g. Build every meal around a protein anchor first.
Do not over-restrict calories: GLP-1 medications already reduce intake significantly. Adding aggressive additional restriction accelerates lean mass loss without improving fat loss proportionally. Let the medication do the work.
Protein shakes are a legitimate tool: On days when appetite is very low, a protein shake is one of the most efficient ways to hit protein targets without requiring the appetite for a full meal.
Monitoring
DEXA or body composition tracking: The scale alone does not distinguish fat from muscle. If body composition matters to you, a DEXA scan at baseline and periodically through treatment gives the actual picture. Many gyms and health centers offer this at a low cost.
Performance as a proxy: If your strength training performance (loads, reps) is maintained or improving, your muscle is largely intact. If it drops significantly despite adequate protein and recovery, it is worth reviewing your nutrition targets.
Frequently asked questions
Do GLP-1 medications cause muscle loss?
Some lean mass loss occurs with any significant weight loss, including on GLP-1 medications. In the SURMOUNT-1 DXA substudy, 25% of weight lost on tirzepatide was lean mass and 75% was fat.
This proportion is consistent with other weight loss interventions, including bariatric surgery and calorie restriction. GLP-1 medications do not cause disproportionate lean mass loss. Resistance training and adequate protein intake reduce the proportion of lean mass lost.
Is Mounjaro or Wegovy better for muscle preservation?
Tirzepatide (Mounjaro) produces a lower proportional lean mass loss than semaglutide (Wegovy): approximately 25-26% of weight lost as lean mass versus approximately 40% with semaglutide.
However, because tirzepatide drives greater total weight loss, the absolute amount of lean mass lost may be higher for some users.
A large 2026 real-world analysis confirmed greater relative LBM loss with tirzepatide at all measured time points. For active people who are already lean and concerned about absolute lean mass, this nuance is worth factoring into the choice between medications with a prescriber.
Can you build muscle while on Mounjaro?
Muscle hypertrophy (net muscle gain) is difficult during a calorie deficit, and GLP-1 treatment typically involves a calorie deficit. The achievable target is lean mass preservation: ending treatment with the same absolute muscle mass you started with, even though total body weight is lower.
With resistance training at two or more sessions per week and protein at 1.6 to 2.2g/kg/day, this is achievable. After treatment, or during a maintenance phase, building muscle follows normal principles without the complication of a significant calorie deficit.
What protein intake is needed on GLP-1 medications?
During active weight loss, the American Journal of Clinical Nutrition recommends 1.2 to 1.6g of protein per kg body weight per day. For active people doing resistance training, the evidence supports 1.6 to 2.2g/kg/day.
The practical challenge of GLP-1 treatment is that reduced appetite can make hitting protein targets difficult. Building meals protein-first and using protein shakes on low-appetite days are the most effective strategies.
How does retatrutide compare to Mounjaro for lean mass?
Phase 2 retatrutide data show lean mass loss proportions similar to tirzepatide and semaglutide, despite greater total weight loss. The glucagon receptor activation in retatrutide theoretically supports preferential fat oxidation, but head-to-head body composition data comparing retatrutide directly to tirzepatide have not yet been published.
Phase 3 TRIUMPH trial data, expected to be completed in 2026, will provide more detailed body composition outcomes. Voy has published a review of the current retatrutide body composition data at the link above.
This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting any weight loss treatment. Individual results may vary. Treatment is subject to clinical suitability assessment.
References
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. pmc.ncbi.nlm.nih.gov/articles/PMC11965027/
PMC case series. Preservation of lean soft tissue during weight loss induced by GLP-1 and GLP-1/GIP receptor agonists. SAGE Open Medicine, 2025. pmc.ncbi.nlm.nih.gov/articles/PMC12536186/
Hidalgo Ramos et al. Effects of Tirzepatide on Skeletal Muscle Mass in Adults: A Systematic Review. PMC, 2025. pmc.ncbi.nlm.nih.gov/articles/PMC12394919/
Venkatakrishnan AJ et al. Greater lean-body-mass decline with tirzepatide than semaglutide in routine care. medRxiv, April 2026. medrxiv.org/content/10.64898/2026.04.11.26350687v1.full
Aronne LJ et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity. SURMOUNT-5. New England Journal of Medicine, 2025.
Leidy HJ et al. The role of protein in weight loss and maintenance. American Journal of Clinical Nutrition, 2015. ajcn.nutrition.org/article/S0002-9165(23)27427-4/fulltext
Jastreboff AM et al. Retatrutide, a GIP, GLP-1, and Glucagon Receptor Agonist, for Obesity. Phase 2 trial. New England Journal of Medicine, 2023.
Voy. Retatrutide fat loss vs muscle loss: what the data shows. joinvoy.com/blog/retatrutide-fat-loss-vs-muscle-loss
