Am I Losing Muscle on a GLP-1? What the Studies Actually Measured
Week 2 of Ask the Research — one question, one paper, every week.
The short answer: less than you've been told, and not in the way you think. The figure everyone quotes — that a quarter to a third of the weight you lose is muscle — is measuring lean mass, which is not the same thing as muscle. Correct for that and actual muscle is closer to a fifth. And that fifth isn't fixed, because the modelling behind it assumes nobody is strength training.
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Where the number comes from
A review out of the University of Milan pooled the body composition data from the major GLP-1 trials. Publicly funded, no drug company money, which is rarer than it should be.
Trial | Medication | Lean tissue, as a share of weight lost |
SURMOUNT-1 | tirzepatide (Zepbound) | about 25% |
STEP-1 | semaglutide (Wegovy) | about 30% |
SUSTAIN-8 | semaglutide | about 43% |
That spread should already tell you the measurement is softer than the headline. A separate joint advisory from four professional societies puts STEP-1 a little higher again, at 38% — same scan substudy, different denominator.
Lean mass is not muscle
This is the part almost nobody says out loud.
Fat-free mass is every non-fat molecule in your body — roughly 74% water, 19% protein, 6.5% mineral. Skeletal muscle is about half of it. The rest is organ tissue, bone, connective tissue, and the non-fat portion of fat tissue itself.
The advisory does the arithmetic out loud: muscle is about half of lean body mass, so a 38% lean-mass loss works out to roughly 20% of total weight lost being actual muscle.
Not a third. A fifth.
And one more piece nobody mentions: fat tissue is itself 15–20% non-fat material. So losing fat obligatorily costs you some lean mass. That isn't damage. It's arithmetic.
What the trials didn't measure
Where function was checked — mobility, physical performance — it generally held up. One analysis found muscle quality preserved or even improved while volume fell.
But most of these studies never measured strength, walking speed, or physical performance at all. They weighed muscle. They didn't test it. The review's authors write that how often this leads to clinically meaningful muscle wasting "remains to be determined."
So the honest position is this: the number is smaller than you were told, and we know less about what it means than anyone implies.
The seven words that change the number
The modelling behind the muscle figures puts loss at 10–15% of total weight lost in women and 20–25% in men — followed by a phrase I would underline:
"in the absence of structured strength training."
That describes a default. Not a destiny.
What I'd actually do
Stop tracking this on a body composition scale. Track it on your stairs, your grocery bags, and getting up off the floor without using your hands. Those tell you about function — which is the thing the trials mostly failed to measure.
Strength training is the part that isn't optional. The advisory is blunt about it: protein alone is "likely inadequate to support the preservation of muscle mass in the absence of structured resistance/strength training." At least 3 sessions a week, plus 150 minutes of moderate activity. Brisk walking counts.
If three sessions a week sounds impossible right now, start smaller. Once an hour, stand up and walk for five minutes. In a study of nearly 20,000 people published this year, the people doing exactly that reported less fatigue by the end — not more. Fatigue is the reason most people tell me they can't train, so that's the place to start.
Pay closer attention if more than one of these is you: older age, rapid weight loss, low protein intake, being sedentary, or kidney disease. The review flags all five as raising the risk. If two or three describe you, it's worth a conversation with someone who knows your history.
One honest caveat
The Milan paper is a narrative review — it summarises other people's studies rather than running a new one, which puts it lower on the evidence ladder than a randomised trial. I'd rather tell you what kind of evidence you're standing on than let it sound sturdier than it is.
Next week
How much protein do you actually need? Real numbers, and what it looks like on a plate.
If you want help with the training side
This is what I do. I'm a registered dietitian and a certified diabetes care and education specialist — and a personal trainer. Which means the strength training half of this isn't something I recommend and then hand off. It's the part I coach people through.
Sources
Rossi G, Bucciarelli L, Mananguite C-L, Giovarelli M, Fiorina P. Muscle loss and GLP-1R agonists use. Acta Diabetologica. 2025;63(2):333–342. Open access.
Mozaffarian D, Agarwal M, Aggarwal M, et al. 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. Obesity Pillars. 2025;15:100181. Open access.
Prado CM, et al. Fundamentals of body composition for the clinician. Journal of the Endocrine Society. 2024;8(11):bvae164.
Diaz KM, Murdock ME, Serafini MA, et al. Evaluating movement breaks as a public health strategy to mitigate the harms of prolonged sitting. British Journal of Sports Medicine. 2026;60:1208–1217.
Not medical advice. Your plan is yours and your care team's.
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