Losing weight on a GLP-1? The muscle you keep matters as much as the pounds you lose
The newest weight-loss medications work. The body-composition data from their trials also shows a cost that is easy to miss on a bathroom scale, and the research on how to limit it is getting clearer.
Key takeaways
- In clinical trial body-composition substudies, roughly 25% to 40% of total weight lost on semaglutide or tirzepatide came from lean mass.
- A 2025 joint advisory from four nutrition and obesity medicine societies recommends 1.2 to 1.6 grams of protein per kilogram of body weight per day during active weight loss on these medications.
- Resistance training, rather than aerobic exercise alone, is the intervention most consistently linked to preserving lean mass during weight loss.
- Appetite suppression makes eating enough protein harder, which is why dietitian guidance is increasingly treated as part of treatment rather than an add-on.
GLP-1 medications such as semaglutide and the dual-action tirzepatide have changed what is possible in weight loss. Trial participants routinely lose 15% or more of their body weight. But the scale reports total weight, and total weight includes muscle, organ and bone tissue as well as fat. The body-composition data from those trials has prompted a practical question that patients, clinicians and dietitians are now asking more often: how do you lose the fat while keeping the muscle?
What the trial data shows
Several large trials included substudies that measured body composition with DXA scans. In the tirzepatide SURMOUNT-1 substudy, about 25% of weight lost was lean mass, a ratio similar to the placebo group. STEP 1 body-composition data for semaglutide showed a higher proportion, near 40% in some analyses. Reviews of the evidence generally put the lean-mass share of GLP-1 weight loss in a range of roughly 25% to 40%, with differences driven by population, measurement timing and method.
Share of weight lost that was lean mass
Body-composition substudies in GLP-1 trials (approximate)
Some context matters. Rapid weight loss of any kind tends to include lean tissue, and with semaglutide the proportion of lean mass relative to total body weight actually rose, because fat fell faster. Lean mass is also not identical to skeletal muscle. Still, for older adults, who already lose muscle with age, and for anyone whose strength and balance are borderline, losing a meaningful amount of lean tissue is not a trivial side effect.
Protein: the 2025 guidance
In 2025, a joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association and The Obesity Society recommended protein intake of 1.2 to 1.6 grams per kilogram of body weight per day during active weight loss with these medications. That is well above the general adult recommendation of 0.8 grams per kilogram.
The difficulty is that GLP-1 medications reduce appetite, sometimes sharply. Patients who were eating too much before treatment can find themselves eating too little of the right things during it. Practical strategies dietitians commonly use include building each meal around a protein source first, spreading protein across the day rather than loading it into dinner, and using higher-protein foods that are easy to eat when appetite is low. Individual targets should be set with a clinician or registered dietitian, particularly for people with kidney disease or other conditions that affect protein needs.
Strength training: the other half
The exercise evidence points in a consistent direction. Reviews of GLP-1 therapy and exercise conclude that resistance training, more than aerobic exercise alone, is what attenuates lean-mass loss during weight-loss programs. Common recommendations involve training the major muscle groups at least two to three times a week, starting early in treatment rather than after significant weight has already come off. Small published case series have even documented patients who gained lean mass while losing weight when structured training and protein targets were combined with medication, though case series are a very low level of evidence.
Why the support around the medication matters
None of this argues against the medications. It argues for treating them as one part of a plan. That is the model several telehealth programs now emphasize: medication alongside nutrition coaching, protein targets and strength guidance, with a clinician reviewing health history. Elevation Health, a program founded by registered dietitians Lindsay Gayman and Meredith Cross, is built around that approach; its dietitian-led GLP-1 weight loss program pairs medication access with coaching, and a short GLP-1 assessment helps people judge whether the approach fits them.
Questions worth asking your clinician
- What protein target makes sense for me, given my weight, age and health history?
- Should I have a body-composition measurement at the start of treatment?
- What strength-training routine is realistic for my fitness and any joint problems?
- How will we adjust if I am losing weight faster than planned or feel weak?
- What is the plan for maintaining muscle and weight if I reduce or stop the medication?
This article is general health information, not medical advice. GLP-1 medications are prescription drugs; decisions about them, and about protein and exercise targets, should be made with a licensed clinician. Elevation Health is a marketing client of the agency that publishes Action Global News; see the disclosure above.
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Frequently asked
Do GLP-1 medications cause muscle loss?
Some lean-mass loss is a known effect of the weight loss these medications produce. In trial body-composition substudies, roughly 25% to 40% of total weight lost was lean mass, depending on the drug, population and method.
How much protein should I eat on a GLP-1?
A 2025 joint advisory from four nutrition and obesity medicine societies recommends 1.2 to 1.6 grams of protein per kilogram of body weight per day during active weight loss. Individual needs vary, so confirm a target with your clinician or dietitian.
What exercise helps preserve muscle on a GLP-1?
Resistance training is the intervention most consistently linked to preserving lean mass during weight loss, typically working the major muscle groups at least two to three times a week.
Is tirzepatide better than semaglutide for keeping muscle?
Trial data suggests a somewhat lower lean-mass share with tirzepatide, but the trials were not head-to-head and used different populations and methods, so they are not directly comparable. Discuss options with a clinician.