Mounjaro, Weight Loss and Muscle Strength: Why Resistance Exercise Matters

Mounjaro (tirzepatide) is increasingly being used as part of weight-management treatment, and many people experience significant weight loss while taking it.

In practice, I have recently seen several patients who are using weight-loss medications such as Mounjaro. Some have noticed changes in their strength, muscle tone or simply how their body feels as their weight has reduced. This has prompted useful conversations about how to maintain strength and physical function during weight loss.

When we lose a significant amount of weight, we do not necessarily lose body fat alone. Some lean mass can also be lost. Lean mass includes muscle, water and other non-fat soft tissues, so a reduction in lean mass does not mean that all of the weight lost is muscle.

Research into tirzepatide and body composition is still developing. In the SURMOUNT-1 body-composition substudy, approximately 75% of the weight lost was fat mass and around 25% was lean mass. However, a more recent real-world study gives a slightly different and encouraging perspective.

The 2026 study followed 52 adults with obesity who were treated with tirzepatide for at least four months. Body composition was measured using bioimpedance, while muscle function was assessed using hand-grip dynamometry. Participants lost an average of approximately 9.9 kg, with significant reductions in body fat and visceral fat. At the same time, lean mass and muscle function were largely preserved over the four-month period.

One particularly important point is that these patients were not simply taking tirzepatide on its own. They were also given structured nutritional advice and an exercise programme that combined aerobic activity with resistance training. The programme recommended at least 150 minutes of exercise per week, and protein intake was also specifically considered. This is important because it suggests that how we lose weight may matter just as much as how much weight we lose.

Why resistance exercise matters

Resistance exercise gives the muscles a reason to continue working and adapting during a period of weight loss. This does not necessarily mean heavy lifting or spending hours in the gym. Depending on your starting point, resistance exercise might include squats or sit-to-stands, lunges, step-ups, bridges, resistance bands, modified push-ups, dumbbells or weight machines.

The aim is not necessarily to build large amounts of muscle. For many people, the more important goal is to maintain strength, mobility and physical function while body weight is reducing. Walking and cardiovascular exercise are still valuable for general health, but resistance exercise provides a different stimulus that is particularly important for maintaining muscle strength.

You do not need to start with an intensive programme

If you have not exercised for some time, suddenly starting a demanding strengthening programme may not be appropriate. Exercise can be introduced gradually and progressed according to your current strength, confidence and any existing joint or muscle problems. Even relatively simple exercises can be a useful starting point.

What if pain or stiffness is stopping you from exercising?

Sometimes people know they would benefit from exercise, but back pain, knee pain, shoulder discomfort, stiffness or an old injury makes it difficult to get started.

As an osteopath, my role is not to advise patients whether they should take Mounjaro or to manage the medication itself. That should remain with the prescribing healthcare professional.

However, if musculoskeletal pain or restricted movement is making exercise difficult, osteopathic assessment and treatment may help identify contributing factors and support a gradual return to comfortable movement and strengthening.

Weight loss can be an important part of improving health, but maintaining strength, mobility and confidence in your body is also worth protecting along the way.

If pain or stiffness is making exercise difficult, an individual assessment may be helpful in finding a comfortable and realistic way to get moving again.

References

Look M, Dunn JP, Kushner RF, et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes, Obesity and Metabolism.2025;27(5):2720–2729. doi:10.1111/dom.16275.

Blanco Anesto J, Dotres Fallat KM, Nicolau J. Tirzepatide in real-world clinical practice: changes in body composition and muscle function in patients with obesity. Nutrición Hospitalaria. 2026. doi:10.20960/nh.06592.

Locatelli JC, Costa JG, Haynes A, et al. Incretin-Based Weight Loss Pharmacotherapy: Can Resistance Exercise Optimize Changes in Body Composition? Diabetes Care. 2024;47(10):1718–1730. doi:10.2337/dci23-0100.