Bimagrumab and GLP-1s: The Drug Studied to Preserve Muscle During Weight Loss

Bimagrumab is an unusual entry in the weight-loss world: it’s not a GLP-1 drug at all, but an antibody being studied to solve one of the main downsides of GLP-1 treatment, the loss of muscle along with fat. It works by blocking signals that limit muscle growth, so it reduces fat while building or preserving muscle. It’s investigational (now developed by Eli Lilly) and not FDA approved. Paired with semaglutide in a trial, it drove up to about 22% weight loss with roughly 93% coming from fat. Here’s how it works and why muscle matters.

The Muscle Problem With Weight Loss

Any significant weight loss, whether from dieting, surgery, or GLP-1 drugs, typically includes losing some muscle along with fat. Usually a meaningful fraction of the weight lost is lean mass. That’s a concern because muscle supports metabolism, strength, and physical function, and losing too much can be especially problematic for older adults or anyone at risk of frailty. This has become a hot topic as GLP-1 drugs drive larger and faster weight loss than ever before.

Bimagrumab targets this directly. Rather than reducing appetite, it’s a monoclonal antibody that blocks type II activin receptors (part of the myostatin/activin signaling system that normally restrains muscle growth). Blocking that pathway shifts body composition: it reduces fat mass, including visceral fat, while increasing or preserving muscle. It works on fat and muscle tissue directly, not through the brain’s appetite centers.

How It Pairs With GLP-1s

Because bimagrumab and GLP-1 drugs work through completely different mechanisms, combining them is appealing. The GLP-1 drug drives appetite-based weight loss, and bimagrumab shapes what kind of weight is lost, steering it toward fat and away from muscle. The result is a “quality of weight loss” approach: not just losing more, but losing better.

The key evidence comes from the BELIEVE trial, published in Nature Medicine in 2026. In this study, bimagrumab combined with semaglutide produced up to about 22% weight loss over 72 weeks, and strikingly, about 93% of that weight loss came from fat (compared with roughly 72% for semaglutide alone). Bimagrumab on its own actually increased lean muscle mass by around 2.5%, and the combination limited muscle loss to far less than semaglutide alone caused.

An Unusual Development Path

Bimagrumab has a winding history. It was originally tested by Novartis for muscle-wasting conditions, where it fell short, then studied in obesity and spun out to a small biotech (Versanis) before Eli Lilly acquired it. Consider a hypothetical older patient with obesity who’s worried that aggressive weight loss will leave them weaker. A treatment that preserves or builds muscle while cutting fat addresses exactly that fear, which is why the muscle-preservation angle has generated so much interest. Bimagrumab is now being studied in combination with tirzepatide as well, and a more convenient injectable version is being explored.

What This Means for You Right Now

Bimagrumab is not available, and TrimRx does not offer it. TrimRx provides medications you can access today, including compounded semaglutide and compounded tirzepatide plus brand options like Ozempic, Wegovy, Mounjaro, and Zepbound. If you’re focused on weight loss now, those are the options to consider. It’s also worth remembering that resistance training and adequate protein intake are proven, available ways to protect muscle during weight loss right now, no investigational drug required.

Bimagrumab represents an interesting evolution toward higher-quality weight loss, but for the moment it remains in development.

Frequently Asked Questions

Is bimagrumab a GLP-1 drug?

No. Bimagrumab is a monoclonal antibody that blocks muscle-limiting signals to reduce fat and build muscle. It works through a completely different mechanism than GLP-1 drugs, which is why it’s being studied alongside them rather than as a replacement.

How does bimagrumab preserve muscle during weight loss?

It blocks activin type II receptors, part of a signaling system that normally restrains muscle growth. Blocking it shifts body composition toward fat loss and muscle preservation or gain. In its trial, combining bimagrumab with semaglutide meant about 93% of weight lost came from fat.

Is bimagrumab FDA approved or available from TrimRx?

No. Bimagrumab is investigational and not FDA approved, available only through clinical trials. TrimRx offers currently available medications like compounded semaglutide and tirzepatide and brand GLP-1 options, not bimagrumab.

To focus on treatments you can actually start with today, you can explore the options available to you now with a licensed provider.

This information is for educational purposes and is not medical advice. Bimagrumab is investigational and not FDA approved; details and timelines may change. Consult a healthcare provider before starting any medication. Individual results may vary.