The short answer
Some, yes, but less than the headlines suggest. GLP-1 labels state that these drugs lower weight with more fat loss than lean loss. In body-scan studies, lean mass made up about 38% of the weight lost on semaglutide and about 25% on tirzepatide, and lean mass is only partly muscle. Protein at every meal and strength training at least three times a week are the two levers that protect what you keep.
At GLP-1 SOS this question usually comes from women in their fifties and sixties who are thrilled with the scale and worried about their arms, their knees and the stairs. That worry is reasonable. Any fast weight loss takes some muscle with the fat, whether it comes from a medication, surgery or a strict diet. The useful questions are how much, for whom, and what changes the answer.
At a glance
Three things the body scans show
1
Mostly fat
In the trial scans, about 62% to 75% of the weight lost was fat. The rest was lean mass.
2
Lean is not all muscle
Lean mass also counts water, organs and bone. Muscle is roughly half of it.
3
You can tilt the ratio
Protein and strength training are the levers the 2025 nutrition advisory puts first.
What the body scans actually found
Both large trials ran DXA body scans on a subgroup. In STEP 1, semaglutide 2.4 mg, the 2025 joint nutrition advisory reports that of an average 13.6 kg (about 30 pounds) lost, 8.3 kg was fat and 5.3 kg was lean mass: 62% fat, 38% lean. In the SURMOUNT-1 tirzepatide substudy of 160 adults, weight fell 21.3% over 72 weeks, fat mass 33.9% and lean mass 10.9%. About 75% of the weight lost was fat and 25% lean, the same split seen on placebo, and it held across most subgroups, including sex and age.
Lean mass is not the same as muscle. On a DXA scan it counts muscle, but also organs, bone, fluids and the water inside fat tissue. The advisory estimates that muscle is about half of lean mass, so roughly 20% of the weight lost in STEP 1 was muscle. A 2024 review notes that published lean-mass shares run from about 15% or less to as much as 60% of weight lost, depending on the study, the population and the scan method.
Lab note
The scale counts kilos. Your legs count muscle. Protect the second.
There is a hopeful side. The same review found that, on MRI, the drop in muscle volume on these drugs looked roughly in line with the weight lost and with age, and that fat stored inside muscle fell, which can mean better muscle quality per pound. The Wegovy and Zepbound labels both say the drugs lower weight with greater fat loss than lean loss. The risk is not equal for everyone, though: the advisory names older age, menopause, low protein and no strength training as the factors that tip losses toward muscle.
Eating is the quiet driver. A GLP-1 cuts appetite, and protein is usually the first thing to fall off a small plate, which leaves muscle with less to rebuild from. Our page on how much protein to eat on a GLP-1 covers the grams; this page is about the muscle those grams protect.
From the labels and trials
The numbers that matter
25%
share of weight lost that was lean mass on tirzepatide in the SURMOUNT-1 body-scan substudy
38%
share of weight lost that was lean mass on semaglutide 2.4 mg in STEP 1, per the 2025 nutrition advisory
3 times
a week: the minimum strength-training frequency the 2025 advisory recommends alongside a GLP-1
Sources: Diabetes, Obesity and Metabolism, 2025; American Journal of Clinical Nutrition, 2025; NEJM, 2021.
What to do
- Lift something, three times a week. The 2025 advisory recommends strength training at least three times weekly plus at least 150 minutes of moderate aerobic activity. Bands, light weights, sit-to-stands from a chair and wall push-ups all count. If cramps or sore joints hold you back, see muscle cramps and joint pain on a GLP-1.
- Put protein first on every plate. Eat the protein before the rest while the stomach still has room. The advisory discusses 1.2 to 1.6 grams per kilogram a day during weight loss, or 80 to 120 grams a day; your prescriber can set your number.
- Spread it out. Four or five small protein portions beat one large dinner on a slowed stomach. Our list of protein sources and shakes for small appetites helps on low days.
- Keep your gut moving so you can keep eating. Constipation and that full, bloated feeling crowd out meals and protein with them. GLP-1 SOS Daily Gut Relief is not a muscle product; it is built for that comfort side, with magnesium, ginger and peppermint in 2 capsules with dinner to support regular bowel movements and digestive comfort.*
- Track strength, not just weight. Once a month, note how many times you can rise from a chair in 30 seconds, or the weight you carry for groceries. If those numbers hold or improve while the scale drops, the loss is mostly the right kind.
- Ask about pace and a body scan. The advisory suggests assessing muscle strength and body composition when starting a GLP-1. If you are over 65, past menopause or already frail, ask your prescriber whether a slower dose schedule or a referral to a dietitian or physical therapist makes sense.
*GLP-1 SOS Daily Gut Relief supports regular bowel movements and digestive comfort. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.
Two kinds
Expected change, or worth a visit
Usually expected
- Some loss of lean mass with the fat
- Clothes looser everywhere, arms included
- Strength steady or rising if you train
- Tired days in dose-increase weeks
Raise it with your doctor
- Trouble rising from a chair or climbing stairs
- A fall, or feeling unsteady
- Grip or carrying getting weaker
- Eating so little that protein is near zero
- Weight falling much faster than planned
Some lean loss is expected. Getting weaker is not.
When to call your doctor
Call your prescriber soon if:
- You find it hard to get up from a chair, climb stairs or carry what you used to
- You have fallen or feel unsteady on your feet
- You are eating so little that most days include almost no protein
- You are losing weight much faster than your plan
- You are vomiting, cannot keep liquids down or feel faint when you stand
- You have severe belly pain that does not go away, or pain in the upper right belly with fever
Muscle is built slowly and lost quietly. Strength checks catch it early.
From the people behind the Lab
You do not have to figure this out alone.
GLP-1 SOS is the daily companion made for life on a GLP-1. Plain answers like this one, a support team that replies to real questions from real people, and daily formulas like GLP-1 SOS Daily Gut Relief, designed around the side effects that come with the medication. The GLP-1 Lab is our free education arm: no login, no paywall, no hype. Side effects are part of the road. Suffering through them is not.
Questions people also ask
Does Ozempic or Wegovy make you lose muscle?
Some lean mass, yes. In the STEP 1 body-scan substudy of semaglutide 2.4 mg, the drug in Wegovy and Ozempic, about 38% of the weight lost was lean mass and 62% was fat, per the 2025 nutrition advisory. Lean mass includes water and organs, so the muscle share was smaller, roughly 20% by the advisory’s estimate. The Wegovy label says fat loss exceeds lean loss.
How much muscle do you lose on Zepbound or Mounjaro?
In the SURMOUNT-1 body-scan substudy, adults on tirzepatide, the drug in Zepbound and Mounjaro, lost 21.3% of body weight over 72 weeks; about 75% of that was fat and 25% lean mass. The split was the same on placebo and consistent across most subgroups, including sex and age. Lean mass is not all muscle, so true muscle loss was below 25% of the total.
How can I keep muscle on a GLP-1?
Two levers matter most. Strength training at least three times a week, plus 150 minutes of moderate activity, is what the 2025 advisory recommends. And protein at every meal, eaten first: the advisory discusses 1.2 to 1.6 grams per kilogram a day during weight loss. Our page on how much protein to eat on a GLP-1 helps you plan it.
Is muscle loss on a GLP-1 worse for women over 50?
The risk can be higher. The 2025 advisory lists older age, perimenopause or menopause, lower testosterone, low protein and no strength training as factors that raise the risk of muscle loss and sarcopenia. In the SURMOUNT-1 substudy, the fat-to-lean split held across age groups, but starting with less muscle means each kilogram matters more. Strength training is the best protection.
Is losing lean mass always a bad sign?
Not always. Lean mass includes organs, fluids and the water inside fat tissue, so some of it falls naturally as fat shrinks. A 2024 review found MRI muscle changes on GLP-1 drugs roughly in line with the weight lost and with age, with less fat stored inside muscle. What matters most is function: if your strength holds, the loss is mostly the right kind.
Sources (6)
- Wegovy (semaglutide) injection and tablets, Prescribing Information. Novo Nordisk / FDA, revised June 2026.
- Zepbound (tirzepatide) injection, Prescribing Information. Eli Lilly, revised August 2026.
- Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). New England Journal of Medicine, 2021.
- Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes, Obesity and Metabolism, 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. American Journal of Clinical Nutrition, 2025.
- Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes, Obesity and Metabolism, 2024.
Reviewed by the GLP-1 SOS editorial team · Education, not medical advice · Talk to your prescriber about your dose and your symptoms.
