Bimagrumab is another entry on this site that isn't technically a peptide, it's a monoclonal antibody, but it belongs here because of how directly it connects to one of the biggest open problems in GLP-1 treatment: muscle loss.
What It Is
Bimagrumab is a fully human monoclonal antibody that blocks activin type II receptor signaling, a pathway involved in regulating muscle growth. It was originally studied for sporadic inclusion body myositis and other muscle-wasting conditions before researchers turned to it as a potential partner drug for weight-loss treatment.
The Problem It's Meant to Address
GLP-1 receptor agonists like semaglutide produce substantial weight loss, but a meaningful share of that lost weight, by some estimates up to 40%, can come from lean muscle mass rather than fat, a concern covered in more depth in this site's guide to muscle loss on GLP-1 peptides. Because activin signaling normally restrains muscle growth, blocking it with bimagrumab is designed to push the body to retain, or even build, muscle while still losing fat.
What the BELIEVE Trial Showed
A double-blind, placebo-controlled trial known as BELIEVE, led by researchers at Pennington Biomedical Research Center and published in Nature Medicine in March 2026, tested bimagrumab alone, semaglutide alone, and the combination over 72 weeks. Semaglutide alone produced 15.7% total weight loss, with about 72% of that coming from fat mass. Bimagrumab alone produced a smaller 10.8% weight loss, but it was essentially all fat mass, and lean mass actually increased slightly. The combination produced the largest effect: 22.1% total weight loss, with roughly 93% of that coming from fat mass, meaning lean mass was largely preserved compared with semaglutide alone.
Regulatory Status
Bimagrumab is investigational. It is not FDA-approved, either on its own or in combination with semaglutide, for weight loss or for any muscle-preservation indication in this context, and it is not currently available by prescription for this use. The BELIEVE trial, while a genuine randomized, placebo-controlled human study rather than an animal or preclinical experiment, is a single trial; results like these typically need to be replicated in larger, independent studies before they translate into an approved product.
Human Evidence, With Caveats
It's worth being precise about the evidence tier here: this is Phase-level human clinical trial data, not animal research, which puts it on firmer ground than many of the research-chemical peptides covered elsewhere on this site. But one published trial, however well-designed, is not the same as an FDA-reviewed efficacy and safety package, and important questions, including long-term safety, durability of the muscle-preservation effect, and how it performs across more diverse patient populations, remain open.
Talk to a Doctor
Since bimagrumab isn't an approved or available product, there's no current pathway to prescribe it; anyone concerned about muscle loss while on an approved GLP-1 drug should raise strategies like resistance training and protein intake, discussed in this site's safety coverage, with their physician.
The Bottom Line
Bimagrumab is a promising, evidence-backed research direction for a real problem in GLP-1 treatment, but it remains investigational, not an approved therapy, and the single trial published so far, while methodologically strong, is an early result rather than a final answer.