GLP-1 treatments make people lose weight, often a lot of it. Appetite drops, portions shrink, the scales go down. The question I always ask is a simple one. What exactly is being lost? Fat, we hope. But part of the weight lost is lean mass too, and that is where your food and your training make the difference. The treatment itself is your doctor's call. I am not talking here about any specific medicine, dose or stopping. Follow your doctor on everything to do with the treatment.
What the studies show about lean mass
A meta-analysis published in 2025 pooled 22 randomised trials with 2,258 participants. On average, lean mass made up about 25% of the weight lost on treatment (Karakasis et al., 2025). The most potent drugs produced the most weight and fat loss, but they were also among the least effective at preserving lean mass.
The figures vary a lot from one trial to the next. A 2024 review points to studies where lean mass accounts for 40 to 60% of the weight lost, and others where it stays around 15% or less (Neeland et al., 2024). A more recent review puts it between 25 and 40% (Sanchis-Gomar et al., 2025).
Lean mass is not the same as muscle
These numbers need a bit of perspective. Lean mass is everything that is not fat. Muscle, but also organs, bone, fluids and even the water held in fat tissue (Neeland et al., 2024). Some of that drop is expected when you lose a lot of weight. The MRI studies cited in that review also suggest the remaining muscle can improve in quality, with less fat infiltrating it.
That does not mean you can ignore it. The same authors list older age among the situations where the risk of muscle loss matters more. When you lose muscle, your resting metabolism falls even further, and losing more gets harder. And with age, muscle responds less well to protein and exercise. You have to do a little more to keep it.
Strength training sends the first signal
Outside GLP-1 treatment, a trial published in the New England Journal of Medicine in older adults with obesity who were losing weight shows the effect of strength training. For the same weight loss of about 9%, lean mass fell by 2 to 3% with resistance training, alone or combined with cardio, compared with 5% with cardio alone (Villareal et al., 2017).
Strength training burns few calories, which is why it gets neglected when people want to lose weight. But it is what tells the body to keep its muscle. Two sessions a week are a minimum, three are ideal. A weight is heavy when the last two or three reps of a set become really hard. Women and men both need this signal. If you are a beginner or have a medical condition, ask your doctor what is right for you.
Protein, even when you are not hungry
On a GLP-1, appetite drops. That is how the treatment works. The risk is eating a little of everything, and so too little protein. My rule does not change, treatment or not. At least 30 g of protein in the morning and at least 30 g after 5 pm, for everyone. It is not optional, and it matters even more when appetite is low.
Across the day, aim for about 1.5 g of protein per kilo. If you have a lot of weight to lose, work it out from your target weight, not your current one. In a calorie deficit, needs go up, and the bigger the deficit, the more protein becomes the limiting factor.
When your stomach fills up quickly, the order on your plate changes everything. Start with the protein and the vegetables, and keep the carbohydrates for the end. Pick protein-dense foods. Skyr gives about 10 g of protein per 100 g for 60 calories, twice the protein of Greek yoghurt. Three eggs and 200 g of cottage cheese come to about 40 g. A 130 to 150 g portion of chicken, beef or fish gives around 30 g. If breakfast is still too light, a scoop of unflavoured protein powder can top it up, without replacing the meal.
On days when appetite is really at its lowest, essential amino acid supplements can boost a small meal without eating more. Two eggs with a dose trigger the same muscle response as a 30 g protein meal. I explain this in Your body through menopause and beyond, and it is useful well beyond menopause. Talk to your doctor before adding any supplement.
Watch more than the scales
During weight loss, I look at three things besides weight. Training performance, which should hold steady or improve. Injuries. And mood. If the weights you lift keep going down week after week, take it seriously and tell your doctor. On treatment, the pace of weight loss is theirs to manage.
What I take from it
A GLP-1 treatment helps you lose weight. What you lose depends partly on you. Leave the treatment to your doctor and take care of your side of things, protein morning and evening, and heavy weights several times a week. Muscle is kept by using it.
This article is for information only and does not replace medical advice. For anything to do with your treatment, follow your doctor.