GLP-1 medications such as semaglutide (Wegovy/Ozempic) and tirzepatide (Mounjaro/Zepbound) have become highly effective treatments for obesity and can lead to significant weight loss. But as their use has increased, an important question has emerged:
How much of the weight lost is muscle? Does GLP-1 weight loss mean significant muscle loss?
The answer is some lean tissue is lost during significant weight loss, but this does not necessarily mean that someone is losing the same amount of functional muscle. Recent studies show that approximately 25–39% of weight lost with GLP-1 medications can be classified as "lean mass." For semaglutide, the figure was around 35%, compared with about 25% for tirzepatide. However, similar proportions of lean mass can be lost during other lifestyle-based weight loss without GLP-1 medications.
Lean mass is not the same as muscle
Importance: measurements such as DEXA include muscle but also water, organs and other lean tissues. Therefore, a reduction in "lean mass" does not automatically mean a clinically significant loss of muscle strength or physical function.
Why does preserving muscle matter?
Muscle is much more than a component of body weight. It is essential for:
- Strength and mobility
- Balance and stability
- Getting up from a chair and climbing stairs
- Maintaining independence
- Reducing the risk of falls
- Supporting healthy ageing
This is particularly important for older adults and people who already have low muscle strength or mobility problems. The goal of successful weight loss should therefore not simply be to reduce the number on the scales. It should be to lose excess fat while maintaining muscle, strength and physical function.
Good news: muscle loss can be reduced when losing weight
The strongest message from the current research is that exercise (strength training) and nutrition (primarily protein) matter.
A 2026 meta-analysis of 20 randomised controlled trials involving more than 15,000 people found that lean mass accounted for approximately 26% of weight lost through lifestyle intervention and 25–35% with GLP-1 medications. However, when resistance training was added the proportion fell to approximately 18%. This supports an important practical message:
If you are taking a GLP-1 medication, don't just focus on losing weight; focus on maintaining strength.
Regular resistance exercise is one of the most effective ways to stimulate the body to retain muscle during weight loss. This can include:
- Weights
- Resistance bands
- Body-weight exercises
- Squats
- Lunges
- Step-ups
- Sit-to-stand exercises
Adequate protein intake is also important, particularly because GLP-1 medications can significantly reduce appetite and food intake. Current reviews recommend combining adequate protein and resistance exercise rather than relying on supplements alone.
What should you monitor?
Your weight is only one measure of health. If you are losing significant weight with a GLP-1 medication, it can be useful to pay attention to changes in:
- Strength
- Balance
- Walking ability
- Ability to get up from a chair
- Stair climbing
- General physical function
For some people, physiotherapy assessment can provide a useful baseline and help develop an appropriate strength and exercise programme.
The bottom line
The current evidence does not support the idea that GLP-1 medications simply "waste muscle." They produce substantial fat loss, accompanied by some reduction in lean tissue and this is also seen with weight loss achieved through other lifestyle changes.
However, preserving muscle should be an important part of any weight-loss programme, particularly as we get older. If you are using a GLP-1 medication, think beyond the scales:
Preserve muscle.
Maintain strength.
Stay active.
A structured resistance-training programme, appropriate nutrition and monitoring of strength and physical function can help you get the health benefits of weight loss while protecting the muscle you need for long-term mobility and independence.
References
- Eisa N, et al. Lean Mass Changes With Incretin Therapy Versus Lifestyle Intervention: A Systematic Review and Meta-Analysis of Randomised Controlled Trials. Diabetes, Obesity and Metabolism. 2026.
- Laverde LP, et al. Effect of GLP-1 receptor agonists at doses for obesity management on muscle health: systematic review and meta-analysis of randomized controlled trials. International Journal of Obesity. 2026.
- Lean Mass and Musculoskeletal Preservation in GLP-1-Based Obesity Treatment: Nutrition, Exercise, Supplementation, and Monitoring Strategies. 2026.
- Mota DM, et al. Lean Mass Loss Associated With the Use of GLP-1 and GLP-1/GIP Agonists in the Treatment of Obesity. Journal of the American College of Cardiology. 2026.