One of the growing concerns around GLP-1 weight loss medications is muscle loss. When you lose a significant amount of weight — whether through medication, diet, or surgery — some of that weight inevitably comes from lean mass (muscle, bone, water) rather than fat alone. With GLP-1 medications producing 20%+ weight loss, the absolute amount of lean mass lost can be substantial.
What the data shows
Body composition studies from GLP-1 clinical trials suggest that approximately 20–40% of total weight lost is lean mass, with the remainder being fat mass. This proportion is broadly similar to what's seen with calorie-restricted diets — GLP-1 medications don't appear to cause disproportionate muscle loss compared to other weight loss methods, but because the total weight loss is much greater, the absolute amount of lean mass lost is also greater.1
For example, if someone loses 22kg on Mounjaro, approximately 5–9kg of that might be lean mass. For a younger, active person, this might be manageable. For an older person or someone with limited muscle mass to begin with, this could meaningfully affect strength, mobility, and metabolic health.
Why it matters
Muscle mass isn't just about strength and appearance. It plays a critical role in metabolic health — muscle is metabolically active tissue that burns calories at rest, helps regulate blood sugar, and supports joint stability. Losing too much muscle can reduce your resting metabolic rate (making it easier to regain weight), increase injury risk, and affect mobility, particularly in older adults.
How to minimise muscle loss
The evidence is clear that two strategies make the biggest difference:
1. Adequate protein intake
When you're eating less overall (which GLP-1 medications encourage), it's easy to under-eat protein. Current recommendations for people on weight loss medication suggest aiming for 1.2–1.6g of protein per kilogram of body weight per day — higher than the standard recommendation for the general population.
For a 90kg person, that's roughly 108–144g of protein daily. Prioritise protein at every meal: lean meats, fish, eggs, Greek yoghurt, legumes, and tofu are all good sources. If appetite suppression makes it hard to eat enough, protein shakes or high-protein snacks can help bridge the gap.
2. Resistance training
Resistance exercise (weight training, bodyweight exercises, resistance bands) is the single most effective intervention for preserving muscle during weight loss. Aim for at least 2–3 sessions per week targeting major muscle groups. You don't need to join a gym — bodyweight exercises like squats, lunges, push-ups, and planks are effective.
Studies consistently show that combining calorie restriction with resistance training preserves significantly more muscle mass than calorie restriction alone. This applies equally whether the calorie deficit comes from medication-assisted appetite suppression or from dieting.
What about cardio?
Cardiovascular exercise (walking, cycling, swimming) is excellent for heart health and overall fitness, but it doesn't protect muscle mass in the same way resistance training does. Ideally, combine both: resistance training to preserve muscle, and moderate cardio for cardiovascular health. If you can only do one, resistance training is more important during active weight loss.
Is this a reason not to use GLP-1 medications?
For most people, no. The health benefits of losing 20% of excess body weight far outweigh the downsides of losing some lean mass — provided you take active steps to minimise it. The key is treating GLP-1 medication as one part of a comprehensive approach that includes diet optimisation and exercise, not as a standalone solution.
For older adults, people with sarcopenia (age-related muscle loss), or those with very low baseline muscle mass, the risk-benefit balance should be discussed carefully with a healthcare professional who can monitor body composition during treatment.