As semaglutide and tirzepatide reshape obesity medicine, a critical question has emerged that the weight-loss headlines routinely bury: what happens to the muscle underneath? For older adults and anyone concerned with functional independence, the answer may matter more than the number on the scale.
This narrative review in Nutrients synthesizes findings from randomized controlled trials and observational studies examining how both GLP-1 receptor agonists and the dual GIP/GLP-1 agonist tirzepatide affect skeletal muscle mass, quality, strength, and physical performance. The central finding is nuanced: lean body mass does decline during incretin-based therapy, but the loss appears proportional to total weight reduction rather than reflecting selective drug-induced muscle toxicity. Tirzepatide shows a potentially meaningful advantage in muscle composition — specifically by reducing intramuscular fat infiltration, a marker more closely tied to functional capacity than gross mass alone. Semaglutide's effects on strength and physical performance were more variable, particularly in older or frail populations where baseline muscle reserve is already compromised.
This review arrives at a pivotal moment in clinical practice. The mainstream narrative around GLP-1 drugs has focused overwhelmingly on glycemic control and cardiovascular outcomes, while muscle health — a cornerstone of healthy aging and longevity — has received far less structured attention. The distinction the authors draw between lean body mass loss and true sarcopenic myotoxicity is analytically important: DXA-measured lean mass includes water and connective tissue, not purely contractile muscle, which means raw LBM decline overstates functional muscle loss. Still, in populations already at sarcopenia risk, even proportional losses carry real consequences. The review's coverage of nutritional and resistance exercise co-interventions as mitigation strategies is clinically relevant, though the evidence base for those adjuncts remains thin. As a narrative review rather than a meta-analysis, causality cannot be established, and heterogeneity across included studies limits firm conclusions. This is an incremental but important framing contribution — positioning muscle preservation as a co-primary outcome, not an afterthought, for an entire drug class now used by tens of millions.