The Lean-Mass Question Is Now the Most Important Open Problem in Incretin Therapy
Roughly a quarter to two-fifths of weight lost on semaglutide or tirzepatide is lean tissue absent resistance training and adequate protein — proportionally similar to caloric restriction, but occurring at far greater total magnitude.
Because incretin therapies produce two to three times the weight loss of prior interventions, the same lean-mass fraction translates into a much larger absolute loss of muscle. In older adults, that intersects directly with sarcopenia risk.
Trials pairing incretin therapy with structured resistance training consistently show attenuated lean loss, which has pushed clinical practice toward protein targets around 1.2–1.6 g/kg and twice-weekly resistance work as standard adjuncts.
This is also where the next generation of combination therapy is aimed: myostatin and activin pathway agents are in trials specifically to preserve muscle during incretin-driven weight loss.