One of the most consistently reported findings about GLP-1-driven weight loss is that a meaningful share of the weight lost isn't fat. Multiple studies have found that roughly a quarter to a third of total weight lost on drugs like semaglutide comes from lean mass, including muscle, which has prompted a wave of research into whether that ratio can be improved by adding a second drug specifically aimed at protecting muscle.
The Biology Behind the Approach
The experimental strategy getting the most attention pairs a GLP-1 drug with a monoclonal antibody that blocks myostatin, activin A, or both. Myostatin and activin are signaling proteins that normally act as brakes on muscle growth; blocking them is a well-established way to increase muscle mass in other contexts, and the hypothesis being tested is that adding that muscle-building signal on top of a GLP-1 drug's appetite and weight effects could shift the composition of weight lost toward fat and away from lean tissue.
What Trial Data Has Shown So Far
The most detailed public data on this approach comes from Regeneron's Phase 2 COURAGE trial, which tested semaglutide alone against semaglutide combined with one or both of two antibodies, trevogrumab (which targets myostatin) and garetosmab (which targets activin A). In that trial, semaglutide alone resulted in roughly 35% of total weight loss coming from lean mass, in line with the broader pattern seen across this drug class. Adding a lower dose of trevogrumab preserved about 50.8% of lean mass relative to semaglutide alone; a higher dose preserved about 51.3%; and combining both antibodies together preserved about 80.9% of lean mass, the largest effect observed. Fat loss was also greater in the combination groups than with semaglutide alone.
The Trade-offs
That larger effect came with a cost. The dual-antibody combination group had a substantially higher rate of treatment discontinuation due to tolerability issues, about 28.3%, compared with 4.6% for semaglutide alone. Two deaths were reported in the triplet-combination group; the trial sponsor stated no causal relationship to treatment was established, but the safety picture for the strongest muscle-preservation effect was clearly less favorable than for semaglutide alone. This is a common pattern in early combination-drug research: the intervention with the biggest measured benefit isn't automatically the one that ends up viable, because tolerability and safety have to clear their own bar.
Why This Matters Beyond One Trial
Lean mass loss during GLP-1 treatment matters clinically because muscle plays a role in metabolic health, physical function, and fall risk, particularly in older adults, independent of overall body weight. If muscle-preserving combination therapies can be refined to keep the lean-mass benefit while improving tolerability, they could change how GLP-1 treatment is managed for people at higher risk of frailty or sarcopenia. That refinement work, adjusting doses, combinations, and patient selection, is exactly what earlier-phase trials like COURAGE are meant to generate data for.
What This Isn't, Yet
No myostatin- or activin-targeting antibody is FDA-approved for use alongside a GLP-1 drug for weight management, and none should be understood as an available treatment. This is Phase 2 data from a single trial program, and translating a favorable body-composition signal into an approved, well-tolerated combination therapy typically takes multiple additional trial phases and years of further study, with no guarantee of success.
The Bottom Line
Early data suggests muscle-preserving antibodies can meaningfully change how much lean mass is lost during GLP-1-driven weight loss, but the same data shows a real tolerability cost at the doses tested so far. This is an active, evolving research area worth watching rather than a near-term treatment option, and anyone concerned about muscle loss while on a GLP-1 medication should talk to their prescribing physician about currently available strategies, including resistance exercise and protein intake, which remain the best-established approaches.