Am I losing muscle on a GLP-1?

In DEXA sub-studies of the STEP trials, roughly 25–40% of the total weight lost on semaglutide 2.4 mg came from lean mass rather than fat. That fraction is not unique to GLP-1s — it is the typical lean-mass loss seen with any rapid weight-loss intervention, including bariatric surgery and very-low-calorie diets. What is unique is the pace: semaglutide's 15% average weight loss and tirzepatide's ~21% both happen faster than most non-surgical interventions, which raises the muscle-loss risk if intake and training don't keep up.

Muscle is metabolically expensive. When calories drop sharply and protein intake drops with them, the body preferentially breaks down muscle protein for amino acids. This is normal physiology, not a drug side effect — but on a GLP-1, appetite suppression makes it easier than you realize to under-eat protein by hundreds of grams per week.

The mitigation is well established. Meta-analyses of resistance training combined with high-protein intake during weight loss consistently show 40–70% less lean mass loss than diet alone. The International Society of Sports Nutrition recommends 1.6–2.4 g of protein per kg of body weight per day during a caloric deficit — meaningfully higher than the general RDA of 0.8 g/kg.

The trap is that the scale still moves. You can lose 20 lb on a GLP-1 with almost no resistance training or protein target, and the number looks great — but if 8 of those 20 lb are muscle, your resting metabolic rate has dropped, your regain risk is elevated, and the physique change is different from what you pictured. Track body composition, not just weight.