Evidence-graded answer
Does GLP-1 cause muscle loss?
Some lean-mass loss is expected with any weight loss — typically 25-30% of total weight lost comes from lean tissue. With adequate protein (1.2-1.6 g/kg ideal body weight) and resistance training 2-3×/week, the lean-mass fraction can be reduced to 15-20%.
DEXA substudies from STEP-1 and SURMOUNT-1 showed lean-mass loss in the same proportion seen with comparable surgical or behavioral weight loss — not a unique GLP-1 effect.
Sarcopenia risk is highest in patients >60 with already-low baseline muscle. For these patients, slower titration, higher protein targets, and supervised resistance training are higher priority.
Whey or casein protein supplements are an easy way to hit protein targets when appetite is suppressed; aim for 30-40 g protein per meal across 3 meals.