Muscle loss on GLP-1 medicines: what DXA studies found, lean mass vs muscle, who is at risk, expert protein and strength-training advice and how to monitor.
Fact-checked against FDA labels and official sources; not yet reviewed by a licensed clinician. This page is education, not medical advice. Talk to your doctor before starting, stopping or changing any treatment.
Key takeaways
- In the STEP 1 DXA substudy, 62% of the weight lost on semaglutide was fat and 38% lean mass; about half of lean mass is muscle.[1]
- Across GLP-1 studies, lean mass made up from about 15% or less to 40-60% of weight lost; lean mass also includes organs and water.[2]
- The Wegovy and Zepbound labels state these medicines lower weight with greater fat loss than lean loss.[3]
- Experts advise strength training at least 3 times a week plus 150 minutes of moderate aerobic exercise during GLP-1 treatment, with adequate protein.[1]
- In a randomized trial, exercise plus liraglutide kept off 9.5 kg more than placebo after a year, versus 6.8 kg with the drug alone.[7]
In short: When people lose a lot of weight, with or without medicine, part of what they lose is lean mass, which includes muscle. On GLP-1 medicines, studies report that lean mass made up anywhere from about 15% or less to 40-60% of the weight lost, and both the Wegovy and Zepbound labels state that these drugs lower weight with more fat loss than lean loss. Experts from four obesity and nutrition societies recommend regular strength training, enough protein and monitoring to protect muscle during treatment (checked on 2026-10-06).
This guide explains what the body-composition studies actually measured, why “lean mass” is not the same as muscle, who may be at higher risk, what the expert advice says about protein and exercise, and what to ask your care team about monitoring. It is information only, not a personal plan: protein needs, exercise and medicine doses depend on your health and should be set with a licensed clinician and, ideally, a registered dietitian or physical therapist. Please get evaluated before changing your treatment or starting a new training program.
Fat, lean mass and muscle: three different things
Most trials measure body composition with a DXA scan, which splits weight into fat mass, bone and “lean mass”. Lean mass is everything that is not fat or bone: skeletal muscle, but also organs, fluids and the water held inside fat tissue. A 2024 review in Diabetes, Obesity and Metabolism stresses that a drop in lean mass does not always mean the same drop in muscle. The 2025 joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association and The Obesity Society notes that muscle is about half of lean body mass.
What the trials found
In the STEP 1 trial of semaglutide 2.4 mg (Wegovy), a DXA substudy measured where the weight came from. According to the joint advisory, of an average 13.6 kg lost, 8.3 kg (62%) was fat mass and 5.3 kg (38%) was lean body mass. Because muscle is roughly half of lean mass, the advisory estimates that about 20% of the total weight lost was muscle. In the SURMOUNT-1 trial of tirzepatide (Zepbound), total lean mass fell by 8.5 percentage points when the doses were pooled, the advisory reports.
| Item | Value |
|---|---|
| Fat mass | 62% |
| Lean body mass (muscle, organs, water) | 38% |
Average 13.6 kg lost: 8.3 kg fat and 5.3 kg lean mass. About half of lean mass is muscle.
Source: Mozaffarian D et al., joint advisory (ACLM, ASN, OMA, TOS), Obesity Pillars 2025, citing STEP 1 (checked on October 6, 2026)
Across GLP-1 studies the numbers vary a lot. The 2024 review by Neeland and colleagues found that in some studies lean mass reductions made up 40% to 60% of the total weight lost, while others showed about 15% or less. Reasons include who was studied, which drug, other illnesses and how body composition was measured. Using MRI scans, the authors concluded that the drop in muscle volume seemed in line with what is expected from aging, disease and the amount of weight lost, and that improved insulin sensitivity and less fat inside muscle may improve muscle quality. They still flagged older age and more severe disease as reasons for caution because of the risk of sarcopenia (age-related loss of muscle and strength).
Lean mass in the studies
- 62% / 38%fat vs lean share of weight lost in the STEP 1 DXA substudyJoint advisory, Obesity Pillars 2025
- ~20%estimated share of total weight lost that was muscle in STEP 1Joint advisory, Obesity Pillars 2025
- 15% to 60%range of lean-mass share of weight lost reported across GLP-1 studiesNeeland IJ et al., Diabetes Obes Metab 2024
Checked on October 6, 2026
The FDA labels agree on the direction. The Wegovy label says semaglutide lowers body weight with greater fat mass loss than lean mass loss, and the Zepbound label says the same of tirzepatide. The Foundayo label says the same for orforglipron, the daily GLP-1 pill. None of the labels lists muscle loss as a warning, but the question matters for strength, balance and metabolism, especially later in life.
Is this unique to GLP-1 medicines?
No. The joint advisory notes that losses of fat, lean mass and muscle track the amount of weight lost and are similar to those seen with other treatments that produce large weight loss, such as bariatric surgery and very low-calorie diets. It adds that how much lean mass is lost also depends on how strict the calorie cut is, how fast weight comes off, and whether the person does strength training. Low protein intake from a smaller appetite may add to the risk.
Modeling data cited by the advisory suggest that, without structured strength training, muscle may make up roughly 10-15% of total weight lost in women and 20-25% in men.
| Item | Low | High |
|---|---|---|
| Women | 10% | 15% |
| Men | 20% | 25% |
Estimates from models, not a single trial. Strength training and protein are the main tools to lower these numbers.
Source: Mozaffarian D et al., joint advisory, Obesity Pillars 2025 (modeling data) (checked on October 6, 2026)
Who may be at higher risk
The joint advisory lists factors that may raise the risk of muscle loss and sarcopenia during GLP-1 treatment: older age, the years around and after menopause, lower testosterone, a sedentary lifestyle and low protein intake. The 2024 review adds severe illness. If any of these apply to you, it is a reason to ask about a baseline check of strength and body composition before starting, not a reason on its own to avoid treatment. That decision belongs to you and your clinician.
Strength training: what the evidence and guidelines say
The U.S. Physical Activity Guidelines ask all adults for at least 150 to 300 minutes of moderate aerobic activity a week plus muscle-strengthening activity, such as lifting weights or push-ups, on at least 2 days a week. For people taking GLP-1 medicines, the joint advisory goes further: it recommends a structured exercise program aiming for strength training at least 3 times a week plus at least 150 minutes of moderate aerobic exercise weekly, adjusted to each person’s fitness.
One randomized trial shows why combining the two matters. In a 2021 New England Journal of Medicine study, 195 adults with obesity first lost an average of 13.1 kg on an 8-week low-calorie diet, then were assigned for a year to exercise, the GLP-1 medicine liraglutide, both, or placebo. Compared with placebo, extra weight loss was 4.1 kg with exercise, 6.8 kg with liraglutide and 9.5 kg with both. The combination lowered body-fat percentage by 3.9 percentage points, about twice as much as either approach alone.
The same advisory describes a randomized trial in which one year of GLP-1 therapy combined with exercise preserved bone mineral density, while GLP-1 therapy alone lowered it. Our strength training guide covers how to start safely, including with machines, bands or body weight, and walking is an easy way to build the aerobic minutes.
A sample week to discuss with your care team
- Two or three strength daysWork the major muscle groups (legs, hips, back, chest, shoulders, arms) with weights, machines, bands or body weight.
- Most days, moveBuild toward 150 minutes of brisk walking, cycling or swimming a week, in short bouts if that is easier.
- Protein at every mealSpread protein across the day so smaller appetites still reach the target your clinician or dietitian sets.
- Rest and sleepMuscles adapt between sessions; recovery days count.
- Track strength, not only weightNote the weights you lift or how easily you climb stairs; share it at check-ups.
Start at a level that feels manageable and increase slowly. If you have heart disease, joint problems, diabetes treated with insulin or a sulfonylurea, or feel dizzy or weak on your medicine, ask your clinician what is safe first. A physical therapist or certified trainer can teach technique.
Protein: the numbers experts discuss
GLP-1 medicines reduce appetite, and people often eat less protein without noticing. The joint advisory summarizes the targets that have been proposed: the general adult recommended dietary allowance is 0.8 g of protein per kilogram of body weight a day (a value under review by the National Academies); 1.2-1.7 g/kg a day has been proposed during strength training, and 1.2-1.6 g/kg a day during active weight reduction. For people with obesity it is unclear whether these targets should be based on actual, adjusted or ideal body weight, because using actual weight can overestimate needs. The advisory also says intake should not fall below 0.4-0.5 g/kg a day, and that long-term intake above 2 g/kg a day should be avoided.
| Item | Low | High |
|---|---|---|
| Proposed during strength training | 1.2 g per kg per day | 1.7 g per kg per day |
| Proposed during active weight reduction | 1.2 g per kg per day | 1.6 g per kg per day |
| Do not fall below | 0.4 g per kg per day | 0.5 g per kg per day |
For comparison, the general adult RDA is 0.8 g/kg a day. Ranges for discussion with a clinician or dietitian, not a personal target. Which body weight to use is unsettled for people with obesity; kidney disease can change protein advice.
Source: Mozaffarian D et al., joint advisory (ACLM, ASN, OMA, TOS), Obesity Pillars 2025 (checked on October 6, 2026)
People with chronic kidney disease may need different protein limits, so a personal target should come from your care team. Our protein calculator shows a general range, and the GLP-1 diet guide and high-protein diet page have food ideas for small appetites. Protein shakes can help some people reach their target when solid food is hard. The advisory also mentions micronutrient or protein supplementation “as needed”, which is a decision to make with a clinician.
Eating less: why vitamins and minerals matter too
Protein is not the only nutrient at risk. The joint advisory reports that people using GLP-1 medicines for obesity cut their calorie intake by 16% to 39%, and that such a large, rapid drop can leave too few vitamins and minerals, especially below about 1,200 calories a day for women and 1,800 for men. Digestive side effects can make absorption worse. The advisory lists signs of a true nutrient deficiency to report to a clinician: fatigue beyond what you would expect, excessive hair loss, flaky or itchy skin, muscle weakness, poor wound healing and unusual bruising.
Eating less on a GLP-1
- 16% to 39%lower calorie intake observed with GLP-1 medicinesJoint advisory, Obesity Pillars 2025
- 5% to 18%weight reduction with GLP-1 medicines in trials (somewhat less in real-world use)Joint advisory, Obesity Pillars 2025
Checked on October 6, 2026
The advisory’s answer is food quality: choosing nutrient-dense foods so a smaller amount of food still covers your needs, with a registered dietitian’s help where possible, and micronutrient or protein supplements only when a clinician finds they are needed. Signs like these can have many causes, so they are a reason to get checked, not to self-treat.
Bone health
Large weight loss can also affect bone. In the randomized trial described in the joint advisory, a year of GLP-1 therapy alone lowered bone mineral density, while GLP-1 therapy combined with exercise preserved it and also produced larger reductions in abdominal fat and inflammation. That is one more reason the advisory recommends pairing GLP-1 treatment with a structured exercise program that includes strength training. If you have osteoporosis, low bone density or a history of fractures, ask your clinician whether a bone scan or other monitoring makes sense.
How muscle can be monitored
| Method | What it measures | Notes from the joint advisory |
|---|---|---|
| Bioelectrical impedance (BIA) | Estimated muscle and fat | Practical, low cost, can be repeated in clinic; not for people with pacemakers or defibrillators |
| Air displacement plethysmography | Body composition | An option when BIA cannot be used; needs calibration and close-fitting clothing |
| DXA scan | Fat, lean mass and bone | Considered a gold standard; more costly; could be considered every one or two years |
| Strength and function tests | Muscle function | Sit-to-stand, stair climb and timed up-and-go help in older adults; handgrip is less sensitive to change |
What about medicines to protect muscle?
The 2024 review notes that several drugs designed to maintain or build muscle alongside GLP-1 therapy are under development. None is FDA approved for this purpose as of October 2026; any such product is investigational and available only in clinical trials. Be wary of “muscle-sparing” peptides or supplements sold online: InstaTuck does not link to them. Our emerging medications page tracks investigational weight treatments with their status badges.
After stopping the medicine
The joint advisory lists weight regain after stopping as a common challenge, and nutritional priorities after stopping as an area that needs more research. Keeping up strength training and protein through and after treatment is one of the few steps under your control. See keeping weight off after a GLP-1 and our guide to body recomposition, which explains how fat loss and muscle gain can happen together.
Where this fits
Muscle is one part of the bigger safety picture. Our GLP-1 side effects guide covers digestive effects and label warnings; the Wegovy and Zepbound pages cover each medicine; and the weight-loss injections hub has every injectable option. If preserving muscle is your main goal, our preserve muscle goal page brings the pieces together.
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- Should we measure my strength or body composition before I start and during treatment?
- What daily protein target is right for me, given my kidneys and other conditions?
- Which strength exercises are safe for me, and can you refer me to a physical therapist or trainer?
- If I lose weight quickly or eat very little, should we slow the dose increases?
- What signs of weakness or falls should make me call you?
Frequently asked questions
Do GLP-1 medicines cause muscle loss?
Any large weight loss includes some lean mass, and that is true on GLP-1 medicines too. The labels say fat loss is greater than lean loss. How much lean mass is lost varies between studies and people.
How much of the weight lost on Wegovy is muscle?
In the STEP 1 DXA substudy, 38% of the weight lost was lean mass. Because muscle is roughly half of lean mass, experts estimate that about 20% of the total loss was muscle.
Is lean mass the same as muscle?
No. Lean mass includes muscle, organs, fluids and the water held in fat tissue. A drop in lean mass does not mean the same drop in muscle.
How much protein do I need on a GLP-1?
Experts discuss 1.2-1.6 g per kilogram a day during active weight loss, but which body weight to use is unsettled, and kidney disease changes the advice. Ask your clinician or a registered dietitian to set your target.
How often should I strength train?
The joint advisory suggests aiming for at least 3 strength sessions a week plus 150 minutes of moderate aerobic activity, adjusted to your fitness. The general U.S. guideline is at least 2 strength days a week.
Who is most at risk of losing muscle?
Older adults, people around or after menopause, people with low testosterone, sedentary people and people eating little protein, according to the joint advisory. Severe illness also raises concern.
Can I check my muscle mass?
Yes. Bioelectrical impedance is practical in clinics, DXA scans are the gold standard, and strength tests such as sit-to-stand show function. Ask your care team which fits you.
Are there drugs that prevent muscle loss on GLP-1s?
Some are being studied, but none is FDA approved for this purpose as of October 2026. Avoid online "muscle-sparing" peptides; they are not approved medicines.
References
- Mozaffarian D, Agarwal M, Aggarwal M, et al.. Nutritional priorities to support GLP-1 therapy for obesity: a joint advisory from ACLM, ASN, OMA and TOS. Obesity Pillars, 2025. doi:10.1016/j.obpill.2025.100181 · PMID 40673264 (accessed October 7, 2026) Guideline
- Neeland IJ, Linge J, Birkenfeld AL. Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes, Obesity and Metabolism, 2024. doi:10.1111/dom.15728 · PMID 38937282 (accessed October 6, 2026) Review
- WEGOVY (semaglutide) injection and tablets: prescribing information. Novo Nordisk, via DailyMed, 2026. (accessed October 7, 2026) Drug label
- ZEPBOUND (tirzepatide) injection: prescribing information. Eli Lilly and Company, via DailyMed, 2026. (accessed October 7, 2026) Drug label
- FOUNDAYO (orforglipron) tablets: prescribing information. Eli Lilly and Company, via DailyMed, 2026. (accessed October 7, 2026) Drug label
- Physical Activity Guidelines for Americans, 2nd edition: top 10 things to know. U.S. Department of Health and Human Services (ODPHP). (accessed October 7, 2026) Guideline
- Lundgren JR, Janus C, Jensen SBK, et al.. Healthy Weight Loss Maintenance with Exercise, Liraglutide, or Both Combined. New England Journal of Medicine, 2021. doi:10.1056/NEJMoa2028198 · PMID 33951361 · NCT04122716 (accessed October 7, 2026) Randomized trial
Facts checked on October 6, 2026
Educational information, not medical advice. Talk with a qualified healthcare professional about your own situation. In an emergency call 911.