New GLP-1 Combo Cuts Muscle Loss by 60%
The most common question we get from patients starting a GLP-1 isn't about nausea — it's about muscle. Up to a quarter or more of the weight lost on semaglutide or tirzepatide can be lean mass, not fat, which is why we push strength training and protein so hard. A newly reported phase 2 trial tested a drug designed specifically to fix that trade-off, and the muscle-preservation numbers are the best we've seen yet.
The short version
The muscle-loss problem GLP-1s created
GLP-1 drugs are remarkably effective at reducing calorie intake, but the body doesn't only burn fat when it's in a sustained deficit — it burns some muscle too. Research on standard and high-dose incretin therapy has found lean mass can account for roughly a quarter to more than 40% of total weight lost, depending on the person and the degree of resistance training and protein intake alongside treatment. That's a meaningful problem: losing muscle lowers resting metabolic rate, makes weight regain easier after stopping treatment, and is a particular concern for older adults already at risk of age-related muscle loss. It's the main reason providers now pair GLP-1 therapy with strength training and roughly 1.2 to 1.6 grams of protein per kilogram of body weight daily as standard advice.
What bimagrumab actually does
Bimagrumab works on a completely different pathway than GLP-1 drugs. It blocks activin type II receptors, part of a signaling system (which also includes myostatin) that normally puts the brakes on muscle growth. Blocking that brake promotes muscle building and, in earlier research, also appears to shift metabolism toward burning fat. In the BELIEVE trial, bimagrumab was tested both on its own and in combination with semaglutide; on its own, it was associated with a lean-mass increase of about 2.5%, which is the opposite direction from what GLP-1 drugs typically produce.
The topline weight-loss numbers moved too: 15.7% total body weight loss with semaglutide alone versus 22.1% with the combination. Because more of that combined loss came from fat rather than muscle, fat made up about 93% of the weight lost in the combination group compared with roughly 72% with semaglutide alone. Researchers also reported secondary findings worth noting cautiously — including reductions in inflammatory markers and improved blood sugar control in some participants with prediabetes — though those weren't the trial's primary endpoints.
Safety and what this doesn't mean yet
The combination was generally well tolerated, with side effects consistent with each drug's known profile: bimagrumab was associated with mild-to-moderate acne and muscle spasms, semaglutide with the usual gastrointestinal symptoms and fatigue, and both groups saw temporary increases in LDL cholesterol. No deaths were reported. But this is still a phase 2 trial, and several practical questions remain open. Bimagrumab is not FDA-approved for any indication and remains investigational. It's currently delivered by infusion, which is a meaningfully bigger commitment than a weekly injection. And whether preserved lean tissue at 72 weeks translates into durable strength and function over years is a question this trial's timeframe simply can't answer — that requires longer follow-up and, eventually, a phase 3 program.
Bottom line
This is genuinely encouraging early data on one of GLP-1 therapy's biggest trade-offs, not a treatment that's available or approved yet. If you're on or considering a GLP-1 now, the proven levers for protecting muscle are still resistance training and adequate protein — not a future infusion. We'll be watching the phase 3 data closely.
Sources: Nature Medicine (BELIEVE trial, phase 2 randomized results); Pennington Biomedical Research Center (trial reporting & investigator commentary); ClinicalTrials.gov (NCT05616013); American Diabetes Association (GLP-1 lean-mass preservation research).