Key takeaways:
Up to 45% of GLP-1 weight loss comes from lean mass. But it’s important to remember that lean mass doesn’t just mean muscle — it also includes organs, total body water, and connective tissue.
Studies suggest physical function doesn’t decline in people taking GLP-1s, despite a loss of lean mass. Patients who lost 20% or more of their body weight have tested the same or better on strength and function measures.
It’s still important to monitor older adults, postmenopausal women, and anyone who starts with low muscle mass. For those patients, protein and resistance training belong in the plan from day one.
Your patient is 64 years old, has lost 38 lbs taking a GLP-1, and is concerned about muscle loss. They read something about it online.
The short answer for your patient is yes, they likely have had some muscle loss. But whether it matters in any meaningful way depends on the patient.
Here we’ll review the evidence on how much muscle your patients can expect to lose while taking a GLP-1, which patients to watch out for, and what you can do to help them maintain muscle mass.
1. There’s inevitable lean mass decrease with any weight loss
It’s important to keep in mind that all weight loss comes with some decrease in lean mass.
A 2006 systematic review looked into how much lean mass was lost with various weight-loss methods, including low calorie diets and bariatric surgery. Here’s what that study showed, alongside what we know about lean-mass loss with GLP-1s:
Method of weight loss | Share of weight lost as lean mass |
|---|---|
Low-calorie diet | About 14% |
Very low-calorie diet | About 23% |
Bariatric surgery | 18%-31% |
About 45% | |
About 25% |
Three things to keep in mind:
Some fat loss shows up as lean-mass loss: Adipose tissue is only about 80% to 85% triglyceride. The other 15% to 20% is water and protein, and that counts as lean mass on a scan.
A smaller body needs less muscle. Carrying 40 fewer lbs is less work, so your muscle adapts down accordingly.
Muscle gives up water before it gives up protein. Most of the early “muscle” loss on a scan is actually just intracellular water.
2. Why the numbers overstate the issue
Lean body mass loss is measured with DXA scans. And DXA scans can’t actually measure muscle — they only measure fat-free mass, which is a much broader category.
Skeletal muscle accounts for about 40% of fat-free mass in women (and about 50% in men). The rest comes from your organs, total body water, and connective tissue.
In other words, a 10 kg drop in lean mass means a loss of only 4 kg of muscle in the average woman. So when you read a study that notes substantial lean-mass weight loss from GLP-1s, remember that about half (or less) of that lean mass is actually skeletal muscle loss.
3. Don’t forget strength and function
At the end of the day, mass is what gets measured, but function is what matters. And there’s no evidence that function takes a hit while losing weight with GLP-1s. In fact, all evidence points to function being unchanged or improving for patients taking GLP-1s.
Here’s where the evidence stands:
In a cagrilintide-semaglutide study presented this year, measured muscle strength did not drop relative to placebo.
In the same study, patient-reported physical function actually improved versus placebo.
Despite absolute lean mass decreasing while taking GLP-1 medications, the proportion of lean tissue actually increased.
4. Who’s actually at risk
Muscle and bone loss accelerate after age 40, and after age 60 bone resorption outpaces formation. So while some lean-mass loss in younger patients is usually nothing to worry about, it’s a different story in older adults. It’s a good idea to keep a closer eye on sarcopenia in these patients:
Adults over 60, especially postmenopausal women
Anyone with low baseline function
Patients who are frail or losing weight unusually fast
Patients already at risk for falls or fracture
Patients who can’t or won’t eat enough protein
5. What to do about it
While the headlines make muscle loss seem like a bigger deal than it actually is, it’s still worth telling your patients about strategies to help them retain as much muscle as possible:
Protein intake: Target 1.2 to 1.6 g/kg/day, roughly double the recommended dietary allowance, or a flat 80 g to 120 g per day if that’s an easier number to share with a patient. Recommend patients spreading it across the day. Protein shakes are always a good suggestion in the weeks after a dose increase, when appetite is low.
Resistance training: Patients should try for two to three sessions a week covering all major muscle groups. Remind patients that resistance training doesn’t necessarily mean hitting the weights at the gym. Body weight exercises like squats, lunges, and push-ups are just as good.
Monitoring: Get in the habit of measuring strength and repeating that measurement over future visits. Grip strength or chair-stands are easy to do in the clinic. The trend is more important than any single value.
For your patients: GLP-1s, How Much Protein You Need, and the Best Ways to Get It
Frequently asked questions
Not routinely. DXA reports fat-free mass, which is the measurement causing the confusion in the first place (fat-free mass is not the same as muscle mass, so it’s not clear how much muscle is actually lost when a DXA scan shows a drop in lean mass). A baseline grip strength or chair-stand test is faster, cheaper, and closer to what you actually care about.
The trial data suggest that lean-mass loss is higher with semaglutide, but the studies that demonstrated these findings enrolled different patients, ran different lengths, and used different scanners. There’s no head-to-head comparison, so the honest answer is that nobody knows.
There are several drugs in various phases of study aimed at preventing muscle loss. In phase 2 trials, adding an activin receptor antibody to semaglutide to promote muscle growth kept lean mass loss to 2.9%, versus 7.4% on semaglutide alone. Adding an anti-myostatin antibody had similar results. None of these drugs are approved, and none has shown a benefit in how patients actually function.
Not routinely. DXA reports fat-free mass, which is the measurement causing the confusion in the first place (fat-free mass is not the same as muscle mass, so it’s not clear how much muscle is actually lost when a DXA scan shows a drop in lean mass). A baseline grip strength or chair-stand test is faster, cheaper, and closer to what you actually care about.
The trial data suggest that lean-mass loss is higher with semaglutide, but the studies that demonstrated these findings enrolled different patients, ran different lengths, and used different scanners. There’s no head-to-head comparison, so the honest answer is that nobody knows.
There are several drugs in various phases of study aimed at preventing muscle loss. In phase 2 trials, adding an activin receptor antibody to semaglutide to promote muscle growth kept lean mass loss to 2.9%, versus 7.4% on semaglutide alone. Adding an anti-myostatin antibody had similar results. None of these drugs are approved, and none has shown a benefit in how patients actually function.
The bottom line
When patients ask about muscle loss with GLP-1s, there’s a lot that you can say to reassure them. All weight loss leads to some muscle loss, the scan behind the alarming statistic doesn’t actually measure muscle, and despite any lean mass losses, physical function will probably improve rather than decline.
The only thing that should give you pause is your patients’ age. Older patients, postmenopausal women, and anyone who was already short on muscle is at higher risk. For those patients, increased protein intake, resistance training, and a solid monitoring plan belong in the plan from the first visit.
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References
Chaston, T. B., et al. (2006). Changes in fat-free mass during significant weight loss: a systematic review. International Journal of Obesity.
Heymsfield, S. D., et al. (2026). Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: A randomized phase 2 trial. Nature Medicine.
Look, M., et al. (2025). Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes, Obesity and Metabolism.
Mozaffarian, D., et al. (2025). Nutritional priorities to support GLP-1 therapy for obesity: a joint Advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society. The American Journal of Clinical Nutrition.
Neeland, I. J., et al. (2024). Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes, Obesity and Metabolism.
Pharmaceutical Technology. (2026). ECO 2026: REDEFINE-1 demonstrates CagriSema’s improvement in body composition.
Regeneron. (2025). Results from Phase 2 COURAGE trial demonstrating potential to improve quality of GLP-1 receptor agonist-induced weight loss by preserving lean mass, presented at EASD.
Shaw, M. L. (2026). GLP-1 therapies in 2026: Beyond blood sugar and the scale. AJMC.
Tinsley, G. M., et al. (2024). Fundamental body composition principles provide context for fat-free and skeletal muscle loss with GLP-1 RA treatments. Journal of the Endocrine Society.
