Yes, some of the weight people lose on a GLP-1 medication is lean mass, not fat. In trials, most of the loss was still fat, and the two habits with the best support for protecting muscle are eating enough protein and doing regular strength training.
Do you lose muscle on a GLP-1?
You can. The best-known numbers come from a body-composition substudy of the STEP 1 trial of semaglutide, in which 140 participants had DEXA scans at the start and after 68 weeks. People taking semaglutide in that group lost an average of 8.36 kg of fat and 5.26 kg of lean body mass, about 13.6 kg (30 lb) combined. That works out to roughly 61% fat and 39% lean. (The figures are in the trial's supplementary appendix.)
Lean body mass is not the same as muscle. It also includes water, organs, and other non-fat tissue. A 2025 joint advisory from four obesity and nutrition societies estimates that muscle is about half of lean mass, so roughly 20% of the weight lost in STEP 1 was muscle.
A body-composition analysis of the SURMOUNT-1 trial of tirzepatide found the same pattern. Lean mass fell along with fat, and most of the loss was fat. The fat-to-lean split was about the same as in the placebo group, who lost weight through lifestyle changes alone.
Most of the weight lost was fat. The joint advisory notes that the loss of fat, lean mass, and muscle on GLP-1s tracks with how much weight comes off and looks similar to bariatric surgery and very low-calorie diets.
The amount varies from person to person. According to the advisory, how far calories drop, how fast weight comes off, and whether someone does strength training all affect how much lean mass is lost.
Why does muscle loss matter?
Muscle is what lets you climb stairs, carry groceries, and catch yourself when you trip. The joint advisory links low muscle and bone mass to physical disability, falls and fractures, and lower quality of life. The advisory adds that the risk is greater at older ages and for people who get too little protein or physical activity. The advisory flags bone loss as a related concern, and the habits below help protect both.
Muscle also matters for maintenance. Our guide on what happens when you stop a GLP-1 explains why that phase needs its own plan.
How do you protect muscle on a GLP-1?
Eat your protein first
When a GLP-1 cuts your appetite, protein tends to drop along with everything else. The joint advisory, from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society, notes that targets of about 1.2 to 1.6 grams of protein per kilogram of body weight per day have been proposed during active weight loss. Because that math can overshoot at higher body weights, it also offers a simpler range of about 80 to 120 grams a day. The American Association of Clinical Endocrinology (AACE) gives a similar target of 1.2 to 1.5 g/kg a day in its nutrition guide for patients on obesity medications.
These are general figures, not a personal plan. Your clinician decides your plan, based on your size, kidney health, and other conditions. When your appetite is low, these approaches can help:
- Eat the protein on your plate before anything else.
- Try smaller, more frequent meals if a full plate feels like too much.
- Keep easy options on hand, such as Greek yogurt, cottage cheese, eggs, fish, poultry, tofu, beans, and lentils.
- Use a protein shake on days when solid food doesn't appeal.
Add strength training, not only cardio
The advisory says protein alone is likely not enough to preserve muscle without structured strength training. Its authors recommend strength training at least three times a week plus at least 150 minutes of moderate aerobic exercise. AACE also pairs protein with regular resistance training.
You don't need a gym. Bodyweight moves, bands, and dumbbells all count. Work the major muscle groups until the last few reps feel hard, and add weight or reps over time. If you're new to exercise or have joint, heart, or other health concerns, ask your clinician before you start.
Make each meal count
When you eat less, each meal has to carry more nutrition. The advisory recommends nutrient-dense, minimally processed foods such as vegetables, fruits, whole grains, legumes, nuts, and seeds. It also says supplements such as vitamin D, calcium, or B12 can be considered for people at risk of low levels. Ask your clinician before you add any. Keep up your fluids, too. The advisory notes that dehydration from nausea and vomiting can affect kidney function.
Ask your clinician to track more than the scale
Follow-up visits are where body composition gets attention. A clinician may ask about your energy, your strength, and how much you're eating, and may order labs when your history calls for them. Our post on blood work before starting a GLP-1 explains what clinicians often check. Our guide to clinician follow-up in cash-pay GLP-1 care covers what ongoing check-ins look like.
When should you talk to your clinician about muscle loss?
Bring it up at your next visit, or sooner, if any of these apply:
- You feel weaker, or stairs and carrying groceries are getting harder.
- Nausea or low appetite keeps you eating far less than you planned.
- You're losing weight faster than expected.
- You feel dizzy or unusually tired.
If you can't keep fluids down, contact your clinician promptly. The advisory lists fatigue beyond what you'd expect and muscle weakness among the signs of a possible nutrient shortfall.
FAQ
Is muscle loss on Ozempic or Wegovy a reason to avoid GLP-1s?
Not by itself. Some lean mass loss comes with most large weight loss, and in STEP 1 most of the weight lost was fat. Whether a GLP-1 fits you is a decision to make with a licensed clinician.
How much protein should I eat on a GLP-1?
Guideline groups cite about 1.2 to 1.6 g/kg a day (joint advisory) or 1.2 to 1.5 g/kg (AACE) during active weight loss. Your clinician sets your target.
Can I build muscle while losing weight on a GLP-1?
For most people, the realistic goal is to keep as much lean mass as they can. The joint advisory calls structured strength training, paired with enough protein, a well-established way to help preserve lean mass during weight loss.
Do I need a body-composition scan?
Not always. The advisory describes several ways to monitor muscle, including bioelectrical impedance (BIA) in the clinic and DEXA scans. Ask your clinician what makes sense for you.
The bottom line
Some lean mass loss is normal with large weight loss, including on a GLP-1. Enough protein, regular strength training, and steady follow-up are the best-supported ways to keep most of the loss coming from fat.
If you're exploring GLP-1 care, you can check your eligibility or browse options by goal. YourHealthRx connects you with independent licensed clinical partners. A licensed clinician reviews your history and decides what's appropriate. A prescription is not guaranteed.
This article is for educational purposes only and is not medical advice. Talk with a licensed healthcare provider before starting, changing, or stopping any medication, diet, or exercise program.
Sources
- Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021;384:989–1002. Body-composition data: Supplementary Appendix, Table S5. PubMed
- 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 (Silver Spring). 2025;33(8):1475–1503. Co-published in The American Journal of Clinical Nutrition, Obesity Pillars, and American Journal of Lifestyle Medicine. Free full text (PMC) · PubMed
- Look M, et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes Obes Metab. 2025. PubMed
- American Association of Clinical Endocrinology. Nutritional Considerations for Patients on Obesity Medications. Based on the 2025 AACE Consensus Statement on the evaluation and treatment of adults with obesity.