Bimagrumab Plus Semaglutide: Can a Second Drug Protect Your Muscle?

Bimagrumab is a monoclonal antibody — a lab-made protein designed to attach to one specific target in the body. Its target is the activin type II receptor, a docking site on muscle cells that receives signals telling muscle to stop growing. Myostatin and activin are the main signals that use this docking site, and they act like a brake pedal on muscle tissue. Bimagrumab blocks the receptor, which lifts the brake and allows muscle to grow even when a person is eating less.

This matters because it works through a completely different pathway than a GLP-1 medication. GLP-1 receptor agonists like semaglutide (Ozempic, Wegovy) work in the brain and gut to reduce appetite and slow stomach emptying. They are excellent at creating a calorie deficit, but the body responds to any large calorie deficit the same way it always has — by breaking down some muscle along with fat. Bimagrumab does not touch appetite at all. It simply changes how the body decides which tissue to give up.

Earlier work hinted this could be useful. A 2021 phase 2 study in adults with type 2 diabetes and obesity found that bimagrumab alone cut fat mass by roughly 20% while *increasing* lean mass, which is an unusual combination for any weight-related drug. The obvious next question was whether stacking it on top of a GLP-1 would give people the appetite control of semaglutide with the body-composition profile of bimagrumab. That question became the BELIEVE trial.

If you want the background on why lean mass loss happens in the first place, our guide to [muscle preservation on a GLP-1](/blog/muscle-preservation-on-glp1-strength-training-protein-guide) walks through the physiology.

BELIEVE was a randomized phase 2 trial that enrolled 507 adults with obesity and ran for 48 weeks. Randomized means participants were assigned to their treatment by chance rather than by choice, which keeps the comparison fair. Phase 2 means the trial was designed to find the right dose and get an early read on effectiveness — not to win FDA approval, which requires larger phase 3 trials.

Participants were split across nine groups. Some received placebo. Some received bimagrumab alone by intravenous infusion at either 10 mg/kg or 30 mg/kg every 12 weeks. Some received semaglutide alone at 1.0 mg or 2.4 mg by weekly injection. The rest received combinations of the two. This nine-arm design is unusual and expensive, but it allowed researchers to separate what each drug contributed instead of guessing.

The primary interest was body composition, not just the number on the scale. Researchers used DXA scans — the same imaging used to measure bone density — to divide each person's weight change into fat mass and lean mass. This distinction is the entire point of the trial. Two people can both lose 15 kg and end up in very different metabolic positions depending on how much of that was muscle.

It is worth being clear about the limits. Forty-eight weeks is a moderate stretch, not a long-term view. Phase 2 trials are smaller than phase 3, so rarer side effects may not appear. And DXA-measured lean mass includes water and organ tissue, not just skeletal muscle, so a small part of any lean mass change reflects fluid shifts rather than true muscle.

More than most people expect, and BELIEVE puts a number on it. In this trial, semaglutide alone reduced lean mass by 5.3% to 7.9% depending on the dose and analysis method. That is consistent with what earlier research found: in the DXA sub-study of STEP 1 (the pivotal semaglutide weight-loss trial, published in the New England Journal of Medicine in 2021), roughly 39% of the total weight lost was lean mass.

That figure sounds alarming, and it deserves context. Some lean mass loss is normal and expected with *any* weight loss, including diet-only weight loss and bariatric surgery. A larger body carries more muscle simply to move itself around, and some of that extra muscle is no longer needed at a smaller size. Studies of diet-induced weight loss typically show 20% to 30% of the loss coming from lean tissue. So GLP-1s are not uniquely destructive — but they do drive faster and larger weight loss, which means the absolute amount of muscle at stake is bigger.

The concern sharpens for two groups. The first is anyone over 50, because muscle mass already declines by roughly 1% per year after age 40 — a process called sarcopenia. The second is women in perimenopause and menopause, who lose the muscle-protective effects of estrogen at the same time. Losing 7% of lean mass at 30 is recoverable. Losing it at 55, on top of age-related and hormonal decline, is a different situation. Our article on [muscle loss on a GLP-1 during menopause](/blog/muscle-loss-on-glp1-in-menopause-sarcopenia-risk) covers that overlap in detail.

Muscle is not just about appearance or strength. It is the body's largest site for glucose disposal, a major contributor to resting metabolic rate, and the strongest predictor of independence in later life.

The headline number is the one worth remembering: approximately 92% of the weight lost on the bimagrumab-semaglutide combination came from fat mass. Compare that to semaglutide alone, where a substantially larger fraction came from lean tissue.

Breaking it down by group makes the pattern clearer. Bimagrumab alone increased lean mass by 2.3% to 2.7% — participants gained muscle while still reducing fat. Semaglutide alone lost 5.3% to 7.9% of lean mass. The combinations landed at 1.1% to 2.6% lean mass loss, which is close to what you would expect from normal fluid and tissue adjustment during weight loss rather than meaningful muscle wasting.

Meanwhile, the combination did not sacrifice weight loss to get there. High-dose bimagrumab plus semaglutide produced 17.8 kg of total weight loss at 48 weeks versus 14.2 kg for semaglutide 2.4 mg alone. The combination also produced larger reductions in visceral adiposity — the deep abdominal fat that surrounds organs and drives insulin resistance, inflammation, and cardiovascular risk. Visceral fat is the fat that matters most metabolically, and it is the fat that becomes hardest to lose after menopause.

So the trial answered its question in the affirmative: it appears possible to decouple fat loss from muscle loss pharmacologically. That is a genuinely new capability. Whether it translates into better long-term health outcomes — fewer fractures, better function, less weight regain — is a question phase 2 trials cannot answer.

Bimagrumab is not a pill or a pen. It is delivered by intravenous infusion every 12 weeks, meaning a clinic visit and an IV line four times a year. That is a real barrier compared to a weekly self-injection at home, and it adds cost and scheduling burden.

Across bimagrumab studies, the most commonly reported side effects have been gastrointestinal — diarrhea and muscle spasms in particular — along with mild acne. Elevations in liver enzymes and in pancreatic enzymes have been observed in some participants, which is why anyone taking it in a trial gets regular blood monitoring. When bimagrumab is combined with semaglutide, participants also carry the familiar GLP-1 side effect profile on top: nausea, constipation, reflux, and fatigue, especially during dose escalation.

There is also a conceptual caution. Blocking myostatin signaling makes muscle bigger, but bigger muscle is not automatically stronger or more functional muscle. Some earlier myostatin-pathway drugs increased muscle size without improving physical performance in the way researchers hoped. BELIEVE reported body composition, not a comprehensive picture of strength and function over years.

Finally, timeline. Bimagrumab is not FDA approved for obesity or anything else. Phase 3 development would take years, and many phase 2 successes do not survive phase 3. Anyone reading these results should treat them as a promising signal about the future, not as a product to ask their prescriber for today. If you are currently managing GLP-1 side effects, our guide on [GLP-1 dose escalation](/blog/glp1-dose-escalation-when-to-increase-and-when-to-hold) is more immediately useful.

Nothing in BELIEVE changes what works today — it just underlines why it matters. Two interventions have consistent evidence behind them, and both are available to you this week.

The first is protein intake. During active weight loss, most clinicians working with GLP-1 patients target roughly 1.2 to 1.6 grams of protein per kilogram of body weight per day, which is higher than the standard 0.8 g/kg recommendation designed to prevent deficiency, not to preserve muscle in a deficit. Distribution matters as much as total: aiming for 25 to 30 grams of protein per meal appears to be the threshold that reliably triggers muscle protein synthesis in adults over 50. This is genuinely hard on a GLP-1, because appetite suppression makes volume eating uncomfortable. Front-loading protein at breakfast and using [high-protein smoothies on low-appetite days](/blog/glp1-smoothies-high-protein-recipes-for-low-appetite-days) are the two most practical workarounds.

The second is resistance training. Two to three sessions per week of progressive strength work is the only non-pharmacological intervention shown to meaningfully blunt lean mass loss during a calorie deficit. It does not need to be heavy or complicated — bodyweight, bands, or machines all count, as long as the load increases over time. Walking is excellent for many things, but it does not protect muscle the way loading it does.

A useful way to track whether it is working is grip strength, a cheap and validated proxy for whole-body muscle function. If your grip strength holds steady while the scale drops, you are likely losing the right tissue. Our piece on [grip strength as a muscle marker](/blog/grip-strength-on-glp1-the-muscle-marker-that-matters) explains how to measure and interpret it.

Bimagrumab represents a shift in how the obesity field thinks about its own goals. The first generation of GLP-1 drugs competed on a single metric: total percentage of body weight lost. Semaglutide delivered roughly 15%, tirzepatide roughly 20% in head-to-head data. The next generation is starting to compete on the *quality* of that weight loss.

Several programs are moving in parallel. Some are pursuing more powerful weight loss through additional hormone targets — retatrutide adds glucagon receptor activity, and survodutide pairs glucagon with GLP-1. Others are pursuing convenience, like oral GLP-1 pills. Bimagrumab and other muscle-sparing agents represent a third direction: same weight loss, better body composition. It is plausible that future obesity care combines an appetite drug with a muscle-protective drug as standard practice, the way hypertension is treated with combinations rather than one agent at maximum dose.

There are also non-drug reasons this direction matters. As GLP-1s move into longer-term and maintenance use, and into older populations, the consequences of cumulative lean mass loss become more serious. A 65-year-old who has been on a GLP-1 for five years faces a different risk profile than a 35-year-old on it for one.

For the full landscape of what is in development, see our overview of the [next-generation GLP-1 pipeline](/blog/next-gen-glp-1-pipeline-retatrutide-orforglipron-amycretin-2026) and our breakdown of the [retatrutide TRIUMPH trials](/blog/retatrutide-triple-agonist-what-the-triumph-trials-show).