Ozempic, Wegovy and Mounjaro: What Happens to Muscle During Weight Loss?
Updated: Oct 13
NUTRITION & HEALTH | RESEARCH REVIEW
By Rosy | Registered Associate Nutritionist (ANut) & Level 4 Cancer Exercise Rehabilitation Specialist
Updated October 2026 | Approximately 6-minute read
Weight-loss medications such as Wegovy and Mounjaro have changed the conversation around obesity treatment. Clinical trials have demonstrated substantial reductions in body weight, offering an important treatment option for people who meet the appropriate medical criteria.
But losing weight and improving body composition aren't necessarily the same thing. As a Registered Associate Nutritionist and personal trainer, I'm particularly interested in what happens beyond the number on the scales.
When someone loses weight, how much comes from body fat? What happens to their muscle mass? And how can nutrition and exercise support their long-term health?
Let's explore what the research tells us.
How do these weight-loss medications work?
Semaglutide and tirzepatide act on hormonal pathways involved in appetite regulation and blood glucose control.
Semaglutide mimics the action of glucagon-like peptide-1 (GLP-1), a hormone involved in feelings of fullness. Tirzepatide acts on both GLP-1 and glucose-dependent insulinotropic polypeptide (GIP) receptors.
These medications can reduce appetite and food intake, helping people achieve significant weight loss.
In the UK, the brand names have different licensed uses:
Ozempic (semaglutide): Licensed for type 2 diabetes, not weight management.
Wegovy (semaglutide): Licensed for weight management in eligible patients.
Mounjaro (tirzepatide): Licensed for both type 2 diabetes and weight management, subject to the relevant criteria.
These are prescription medicines, and decisions about their suitability should be made with an appropriately qualified prescriber (MHRA, 2026).
Does taking Mounjaro or Wegovy cause muscle loss?
One of the most widely discussed concerns surrounding weight-loss medication is the potential loss of muscle.
However, it's important to distinguish between muscle mass and lean body mass. Lean body mass includes skeletal muscle, but also water, organs, blood and other non-fat tissues. When research reports a reduction in lean mass, this does not mean the entire reduction represents lost muscle.
It's also normal for some lean mass to decrease during substantial weight loss, whether that weight loss is achieved through dietary changes, medication or other interventions.
The question isn't simply whether lean mass changes. We also need to consider how much fat is lost, whether muscle strength and physical function are maintained, and the person's overall health.
What does the research actually show?
A 2025 analysis of the SURMOUNT-1 trial examined changes in body composition among 160 participants who underwent DXA scans, a method used to estimate fat and lean mass.
Among those receiving tirzepatide, average body weight decreased by 21.3% over 72 weeks.
Interestingly, the proportions of fat and lean mass lost were similar in the placebo group, although participants receiving tirzepatide lost considerably more weight overall (Look et al., 2025).
This suggests that lean mass reduction accompanies significant weight loss, rather than being a problem unique to tirzepatide. A smaller body-composition analysis from the STEP 1 trial also observed reductions in both fat mass and lean mass with semaglutide, alongside an improvement in the overall ratio of lean mass to fat mass.
However, these studies have limitations. DXA does not directly measure muscle strength or function, and the body-composition findings come from smaller groups within larger trials.
We therefore shouldn't assume that every kilogram of lean mass lost translates into an equivalent loss of functional muscle.
Why preserving muscle matters
Skeletal muscle is important for considerably more than appearance.
It contributes to movement, strength, balance, physical independence and glucose metabolism. Maintaining muscle becomes increasingly important as we age because age-related changes can make rebuilding lost strength more challenging.
For someone undergoing substantial weight loss, the goal should be to improve health and body composition while maintaining as much strength and physical function as possible. That means considering more than a target weight.
Progress can also be assessed through improvements in strength, exercise capacity, everyday function and dietary quality.
How can nutrition and exercise help?
In 2025, a joint advisory from four professional organisations, including the American Society for Nutrition and The Obesity Society, highlighted the importance of integrating nutrition and lifestyle support into GLP-1-based obesity treatment (Mozaffarian et al., 2025).
Several practical considerations are particularly relevant.
1. Prioritise adequate protein
Protein provides the amino acids needed to maintain and repair body tissues, including skeletal muscle.
When appetite is significantly reduced, it can become more difficult to eat sufficient protein across the day.
Including protein-rich foods in regular meals can help support dietary adequacy. Suitable options include fish, eggs, poultry, dairy products, tofu, beans and lentils.
Protein requirements should be individualised according to factors such as body size, activity, age and medical circumstances rather than applying one target to everyone.
2. Include resistance training
Resistance training provides an important stimulus for maintaining muscle strength during weight loss.
This doesn't necessarily mean lifting heavy weights or following an advanced gym programme.
A suitable routine might include exercises such as squats, rows, presses and other movements targeting the major muscle groups, adjusted to the individual's ability.
For many adults, strengthening activities on at least two days per week provide a useful starting point. Those who are new to exercise or have existing health conditions may benefit from professional guidance.
Although resistance training has established benefits for muscle preservation during weight reduction, more research is needed to determine the most effective exercise programmes specifically for people using these medications.
3. Avoid overlooking overall nutritional adequacy
Reduced appetite can make it harder to consume enough food to meet nutritional requirements.
Alongside protein, attention should be given to fibre, fruits and vegetables, essential fats, vitamins, minerals and hydration.
Common gastrointestinal side effects, including nausea, vomiting and diarrhoea, can make eating well more challenging.
Persistent symptoms or concerns about inadequate intake should be discussed with the prescribing clinician. Significant vomiting, dehydration or severe abdominal pain require prompt medical assessment.
4. Think beyond the medication
Weight management is rarely a short-term process.
Research and clinical guidance recognise that weight regain can occur after weight-loss medication is discontinued. Establishing sustainable eating habits, physical activity routines and appropriate ongoing support therefore remains important.
None of this means medication is an inferior option or that someone should try to manage obesity without medical treatment.
Instead, medication, nutrition and physical activity can form complementary parts of an individualised treatment approach.
A more meaningful approach to weight loss
One of the limitations of focusing exclusively on body weight is that the scales tell us very little about how someone's physical health is changing.
Two people can lose the same amount of weight but experience very different changes in body composition, strength and physical fitness.
This is why I encourage a broader view of progress.
Nutrition and resistance training are not simply ways to influence the number on the scales. They can help support physical function, nutritional adequacy and long-term wellbeing throughout the weight-management process.
Whether someone is using prescribed medication or managing their weight without it, the aim should be sustainable improvements in health rather than weight loss at any cost.
Final thoughts
Weight-loss medications such as Wegovy and Mounjaro can be effective treatments for eligible people living with obesity or overweight-related health conditions.
Research shows that reductions in lean body mass can accompany significant weight loss, but this should not automatically be interpreted as equivalent muscle loss or evidence that the medication is harmful.
Maintaining adequate nutrition, including appropriate protein intake, and participating in regular resistance training are sensible strategies for supporting muscle health.
Ultimately, successful weight management should consider not only how much weight is lost, but what is maintained along the way.
If you're looking for personalised support with nutrition, strength training or body composition, ROSY offers online coaching to clients throughout the UK.
Explore our online nutrition and strength coaching.
Disclaimer: This article is intended for general educational purposes and does not constitute individual medical advice. Exercise during or after cancer treatment should take account of personal medical circumstances, treatment-related effects and any relevant recommendations from the oncology team. Specialist assessment or medical clearance may be necessary before beginning or progressing an exercise programme.
References
Look, M. et al. (2025) 'Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight', Diabetes, Obesity and Metabolism, 27(5), pp. 2720–2729. Available at: https://doi.org/10.1111/dom.16275
Medicines and Healthcare products Regulatory Agency (MHRA) (2026) GLP-1 medicines for weight loss and diabetes: what you need to know. Available at: https://www.gov.uk/government/publications/glp-1-medicines-for-weight-loss-and-diabetes-what-you-need-to-know/glp-1-medicines-for-weight-loss-and-diabetes-what-you-need-to-know
Mozaffarian, D. et al. (2025) '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', The American Journal of Clinical Nutrition, 122(1), pp. 344–367. Available at: https://doi.org/10.1016/j.ajcnut.2025.04.023
National Institute for Health and Care Excellence (NICE) (2025, updated 2026) Overweight and obesity management. NICE guideline NG246. Available at: https://www.nice.org.uk/guidance/ng246
Wilding, J.P.H. et al. (2021) 'Once-weekly semaglutide in adults with overweight or obesity', New England Journal of Medicine, 384(11), pp. 989–1002. Available at: https://doi.org/10.1056/NEJMoa2032183
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