During a 24-week clinical trial, patients taking the weight-loss drug tirzepatide who also received apitegromab lost 1.9 kilograms less muscle mass than those given a placebo—a finding that could reshape how we think about weight loss itself. The discovery, published in Nature Medicine by researcher Richard Pratley and colleagues, addresses a real problem: popular GLP-1 weight-loss medications like tirzepatide are remarkably effective, but they can strip away lean body mass along with fat, potentially compromising strength, metabolism, and long-term health.
The stakes matter because skeletal muscle isn't cosmetic. It's essential for maintaining strength, supporting basic physical activity, and keeping your metabolism humming. When people lose weight rapidly—which tirzepatide enables—some of that loss inevitably comes from muscle, not just fat. Apitegromab, a drug designed to block myostatin, a protein that naturally limits muscle growth, offers a potential solution to preserve the muscle you want to keep while shedding the weight you don't.
The trial included 102 adults with overweight or obesity, randomized to receive either tirzepatide plus apitegromab or tirzepatide plus placebo. Both groups achieved similar total weight loss, but the composition differed markedly. In the apitegromab group, lean mass represented only 14.6% of total weight loss, compared with 30.2% in the placebo group—meaning the drug-treated patients lost considerably more fat relative to muscle. That 1.9-kilogram difference translates to a 54.9% greater retention of lean mass for those on apitegromab.
What's equally important is what didn't happen: apitegromab proved well tolerated, with adverse event rates nearly identical between the two groups. Thirty-nine percent of apitegromab recipients experienced any adverse event, compared with 36% in the placebo group, suggesting the treatment didn't introduce new safety concerns.
The findings open a promising door, but researchers are appropriately cautious. Pratley and his team note that the trial was relatively small and skewed female—more than 80% of participants were women—limiting how broadly these results apply. The trial also excluded people with significant cardiometabolic conditions, including diabetes, which means we don't yet know how apitegromab would work in patients with more complex health profiles who might be taking tirzepatide.
Still, the principle is clear: as GLP-1 medications continue transforming weight-loss treatment, preserving muscle quality matters. Apitegromab represents one potential tool in that effort, and its success in this phase 2 trial suggests it may deserve larger, more diverse studies. For people weighing whether to pursue tirzepatide or other GLP-1 drugs—and increasingly, many are—the possibility of protecting muscle mass during weight loss could be genuinely consequential for long-term outcomes beyond the scale.