In this article we will explore whether Mounjaro or Wegovy cause muscle loss. It’s a question many patients ask us as GLP-1 weight loss medications surge in popularity. While both drugs are highly effective at reducing body weight, emerging research shows that some of the weight lost may come from lean mass—including muscle.
That matters, because muscle is critical for metabolism, strength, and long-term weight maintenance. Here’s what the science says—and how you can protect your muscle while losing fat.
Key takeaways
- GLP-1 medications like Mounjaro (tirzepatide) and Wegovy (semaglutide) can lead to some loss of lean mass—primarily muscle—but the majority of weight lost is still fat.
- Muscle loss can account for up to 25–40% of total weight lost, depending on protein intake, activity levels, and calorie restriction.
- With the right strategies, it’s possible to improve body composition even while using GLP-1 medications.
- Protein intake, resistance training, and adequate calories are non-negotiable for preserving muscle mass.
- GLP-1s are most effective when combined with structured lifestyle changes.
Muscle matters: the untold story of GLP-1 weight loss drugs
GLP-1 receptor agonists such as Mounjaro and Wegovy have transformed the treatment landscape for obesity and metabolic disease. But beyond the dramatic weight loss headlines lies a critical question: what kind of weight is being lost? In this post we will explore whether, and how Mounjaro or Wegovy cause muscle loss as you gradually lose weight.
While reducing excess body fat is the primary goal, preserving skeletal muscle is essential for long-term health. Muscle isn’t just for strength—it supports metabolic rate, glucose regulation, mobility, and resilience against weight regain. Unfortunately, without the right interventions, some of that weight loss can come from lean tissue.
Does Mounjaro or Wegovy cause muscle loss? The research shows it’s real, but manageable
Mounjaro: Insights from the SURMOUNT Trials
In the SURMOUNT-1 trial, participants taking the highest dose of tirzepatide lost an average of 20.9% of body weight. Of that, roughly 74–82% was fat mass, while up to 25% was lean mass, which includes muscle and organ tissue.
Other studies show:
- Average loss of 1.5 kg of fat-free mass with Mounjaro, compared to 0.8 kg with semaglutide.
- Despite some muscle loss, overall body composition improved—with a higher percentage of lean mass relative to fat.
Wegovy: A slight edge in muscle preservation
Semaglutide shows a similar pattern, though some studies suggest a modest advantage in muscle retention:
- A systematic review of 1,541 patients found lean mass accounted for 0–40% of total weight lost.
- In one study on HIV-positive individuals, psoas muscle volume decreased by 9.3%, yet functional performance was preserved.
The takeaway: some muscle loss is expected—but it doesn’t always equate to reduced strength or physical capacity, especially when supportive interventions are in place.
Why Wegovy or Mounjaro cause muscle loss and how it happens
GLP-1 medications reduce hunger and slow gastric emptying, creating a caloric deficit that promotes fat loss. However, when calories drop too low—or when dietary protein and physical activity are insufficient—the body may also catabolise muscle tissue.
Rapid weight loss, in particular, increases the risk of muscle loss, especially in sedentary individuals or those following low-protein diets.
How to protect muscle while losing fat
Mounjaro or Wegovy causing muscle loss is not inevitable—it’s preventable with the right approach. Here’s how:
1. Prioritise protein intake
Adequate protein is essential to stimulate muscle protein synthesis and prevent muscle breakdown.
- Target 1.2–1.6 g/kg of body weight daily.
- Distribute protein intake evenly across meals and snacks.
- Emphasise lean, high-quality sources: poultry, fish, eggs, dairy, tofu, legumes, protein powders.
2. Incorporate resistance training
Strength training signals the body to preserve muscle during weight loss.
- Aim for 2–3 sessions per week.
- Focus on compound movements (e.g., squats, deadlifts, rows, presses).
- Progress gradually with good form—start light, stay consistent.
3. Avoid severe calorie restriction
Extremely low-calorie diets increase the proportion of lean mass lost.
- General minimums: 1,200 kcal/day for women, 1,500 kcal/day for men.
- Work with a professional to find the right deficit—slow, steady fat loss preserves muscle.
4. Track body composition
The number on the scale doesn’t tell the whole story.
- Use DEXA scans, bioelectrical impedance analysis, or smart scales to monitor fat vs. lean mass changes over time.
5. Work with a nutrition or fitness professional
Personalised guidance makes a difference. A registered dietitian or strength coach can ensure your GLP-1 weight loss plan supports muscle preservation, energy, and long-term sustainability. There’s no reason for Mounjaro or Wegovy to cause muscle loss if you are vigilant.
A weight loss professional can tailor your plan—especially if you’re using Mounjaro or Wegovy—to ensure your nutrition supports your goals. Learn more about your options on our treatments page.
Weight loss shouldn’t come at the expense of strength
Mounjaro and Wegovy are powerful tools, but they’re not stand-alone solutions. Used correctly, they can improve fat-to-muscle ratios and support healthier body composition. But it takes intention and strategy to preserve what matters most—your muscle mass, metabolism, and physical function.
At Lotus Weight Loss, we’re here to help you navigate this process with evidence-based support and compassionate care. Whether you’re starting your GLP-1 journey or reassessing your progress, we can help you optimise results without sacrificing strength or health.
Frequently asked questions on Mounjaro & Wegovy muscle loss
Does Mounjaro or Wegovy cause muscle loss?
Some lean mass is usually lost during any significant weight loss, including on GLP-1s. Across trials, about 20–40% of total weight lost is recorded as “lean mass” by DXA, but fat loss still dominates and typically outweighs lean loss by ~3 to 1 on tirzepatide. Note that DXA “lean mass” includes water/glycogen, so early drops aren’t all muscle.
How can I prevent muscle loss while taking a GLP-1 medication?
Prioritise protein + progressive strength work + sensible deficits. As a rule of thumb, aim for ~1.2–1.6 g protein/kg/day during weight loss (the UK RNI baseline is 0.75 g/kg/day), and lift at least twice weekly. This combination consistently preserves more lean mass than diet alone. Creatine may help maintain training performance/lean mass if you lift—ask your clinician if it’s appropriate.
Is muscle loss dangerous during weight loss?
Excessive lean mass loss can lower resting energy expenditure, sap strength, and make weight regain more likely. That’s why we programme resistance training and higher protein while you’re on-treatment, not after.
Do GLP-1 medications improve body composition?
Generally, yes. Fat mass falls far more than lean mass, so body-fat percentage and fat-to-lean ratios improve, especially when you train and hit protein targets. This pattern is seen with both tirzepatide (SURMOUNT-1 DXA substudy) and semaglutide (STEP-1 DXA).
Why do some sources say lean mass drops a lot?
Because DXA “lean mass” isn’t pure muscle—it includes water, organs, and other soft tissues. Early “lean” losses partly reflect glycogen + water shifts, not just muscle fibres disappearing. Focus on strength trends, protein, and training load, not scale-only metrics.
Where can I learn more about pricing and treatment plans?
Head to our medications and pricing page where we’ll outline options, eligibility and next steps for your treatment.
Useful links
- https://dom-pubs.onlinelibrary.wiley.com/doi/pdf/10.1111/dom.16275
- https://academic.oup.com/jes/article/5/Supplement_1/A16/6240360
- https://abcd.care/sites/default/files/site_uploads/Posters/Posters_ABCDOctober2021/Poster_King_366.pdf
- https://www.nejm.org/doi/pdf/10.1056/NEJMoa2032183
- https://www.thelancet.com/journals/landia/article/PIIS2213-8587%2825%2900122-6/fulltext
- https://link.springer.com/article/10.1007/s00421-017-3552-x
- https://onlinelibrary.wiley.com/doi/epdf/10.1002/jcsm.12308
- https://www.nhs.uk/live-well/exercise/physical-activity-guidelines-for-adults-aged-19-to-64/
- https://www.nutrition.org.uk/media/1z2ekndj/nutrition-requirements-update.pdf
- https://ajcn.nutrition.org/article/S0002-9165(23)27427-4/fulltext
- https://www.mdpi.com/2072-6643/17/6/1081https://www.jci.org/articles/view/114857/pdf
