TL;DR: In the phase 2 BELIEVE trial, adding bimagrumab to semaglutide produced 22.1% body-weight loss at week 72, and 92.8% of that loss was fat mass, with lean mass essentially held in place. Semaglutide alone, over the same 72 weeks, lost less weight overall and gave up a meaningfully larger lean-mass share. One more thing worth saying plainly up front: bimagrumab is not a peptide. It is a monoclonal antibody, a much larger molecule that blocks a specific muscle-wasting signal rather than working through appetite.
Our guide to preventing muscle loss on GLP-1s covers the behavioral side of this problem — protein targets, resistance training, pacing your dose. This article is the pharmacological counterpart: a drug combination designed to solve the same problem from the muscle side of the equation, not the appetite side.
The problem bimagrumab is built to solve
Body-composition substudies of the major GLP-1 trials have made one thing clear: a meaningful share of the weight lost on semaglutide or tirzepatide is not fat. Depending on the trial, somewhere between roughly a quarter and 40% of total weight lost is lean tissue — muscle, connective tissue, the metabolically active mass most people would rather keep. That is the exact gap our preventing-muscle-loss piece addresses with protein and resistance training.
Bimagrumab approaches the same gap from a different angle entirely. Instead of asking the patient to out-train and out-eat the deficit, it pairs a GLP-1 that suppresses appetite with an antibody that independently defends and builds muscle. If the mechanisms are genuinely additive rather than overlapping, you would expect a bigger total weight loss that also skews harder toward fat. That is more or less what the BELIEVE trial found.
Bimagrumab is an antibody, not a peptide
Before the trial data, the classification matters, because it changes what kind of molecule you are actually talking about.
That distinction is not pedantry. Antibodies are dosed by intravenous infusion on a weeks-long schedule, not reconstituted and injected subcutaneously the way a peptide is, and they act on a cell-surface receptor rather than on peptide-degrading enzymes. Bimagrumab is grouped in the same news cycle as semaglutide because the two are being trialed together for weight management, not because they share any chemistry.
Inside the BELIEVE trial
BELIEVE (bimagrumab plus semaglutide, published in Nature Medicine) is a phase 2, randomized, double-blind, placebo-controlled trial run across 26 sites in the United States, Australia, and New Zealand. Screening ran from November 2022 to May 2024.
Participants were randomized to placebo, bimagrumab alone (10 or 30 mg/kg intravenously every 12 weeks), semaglutide alone (1.0 or 2.4 mg weekly, subcutaneous), or combinations of the two. The core treatment period ran 48 weeks, followed by an open-label extension to week 72, which is where the headline numbers below come from.
The results: more weight lost, and it was fat
The clearest way to see what bimagrumab adds is to compare the high-dose combination against semaglutide alone, since both ran in the same trial under the same conditions.
| Arm (high dose) | Body weight loss, wk 72 | Share of loss from fat mass | Lean mass change |
|---|---|---|---|
| Bimagrumab 30 mg/kg alone | 10.8% | Effectively all fat | +2.5% (gain) |
| Semaglutide 2.4 mg alone | 15.7% | About 72% | -7.4% |
| Bimagrumab 30 mg/kg + semaglutide 2.4 mg | 22.1% | 92.8% | -2.9% |
Two things stand out. First, the combination lost substantially more total weight than either drug alone — the two mechanisms appear to stack rather than compete. Second, and more relevant to the muscle question, the combination’s lean-mass loss (-2.9%) was far smaller than semaglutide’s alone (-7.4%), even though the combination arm lost almost twice as much total weight. Bimagrumab monotherapy actually increased lean mass by 2.5%, which is the clearest signal that the ActRII-blocking mechanism is doing real, independent work on muscle rather than just riding along with the GLP-1’s appetite effect.
BELIEVE phase 2 trial, high-dose arms, week 72. Lower is better.
Total body fat mass fell 45.7% in the high-dose combination arm at week 72, compared with 27.8% for semaglutide alone and 28.5% for bimagrumab alone — meaning the combination’s fat-mass reduction was substantially larger than either monotherapy, not just a blend of the two. Visceral fat, the fat depot most tied to metabolic risk, fell by more than half in the combination group.
Why the mechanism matters here
Semaglutide’s weight loss runs almost entirely through appetite: it slows gastric emptying and acts on hunger-regulating centers in the brain, so the body simply takes in less energy and draws down whatever tissue it is going to draw down, fat and muscle alike, in whatever ratio a calorie deficit normally produces.
Bimagrumab does not touch appetite at all. Blocking ActRII shuts down the myostatin/activin A “stop growing” signal directly at the muscle, which does two things at once: it pushes existing muscle tissue toward growth, and because activin signaling also affects adipocytes, it appears to independently mobilize fat. Layer that onto semaglutide’s appetite-driven deficit and you get a deficit that is still built mostly from fat, because the muscle compartment has its own separate reason to hold its ground.
This is the same logic our GLP-1 weight-loss plateau piece touches on: lean mass is metabolically expensive, and losing a lot of it lowers the maintenance calories you are defending against. A mechanism that protects muscle independent of the calorie deficit is a genuinely different lever than protein or resistance training, not a substitute for them.
Where this sits next to semaglutide and tirzepatide
Bimagrumab is not a replacement for GLP-1 therapy; every arm in BELIEVE that included it also included semaglutide, and it was semaglutide doing the appetite-suppression work. For readers weighing this in the context of the broader incretin landscape, our explainers on what semaglutide is and what tirzepatide is cover how those drugs work on their own. If nausea or appetite suppression from the GLP-1 side is the limiting factor in your protocol, our guide to managing common GLP-1 side effects is the practical companion piece.
What phase 2 data does and doesn’t tell you
BELIEVE is a phase 2 trial. That is a real, meaningful evidence tier — randomized, double-blind, placebo-controlled, with DXA-quality body-composition measurement across 507 people — but it is one tier below the phase 3 programs that produced the semaglutide and tirzepatide approvals. Bimagrumab plus semaglutide is not an approved combination, and no dosing regimen exists outside a trial protocol. What the data supports is a mechanistic proof of concept: an antibody that blocks ActRII can add meaningfully to a GLP-1’s weight loss while shifting the composition of that loss toward fat. Whether that holds up in a larger, longer phase 3 program is the next open question.
Frequently asked questions
Is bimagrumab a peptide?
No. Bimagrumab is a monoclonal antibody — a full immunoglobulin protein built from folded heavy and light chains, roughly 100 times larger than a typical peptide. It binds activin type II receptors (ActRII) on muscle cells, which is a receptor-blocking mechanism, not the short amino-acid-chain structure that defines a peptide like semaglutide.
What did the BELIEVE trial actually show?
In 507 randomized adults with obesity, high-dose bimagrumab plus semaglutide produced 22.1% body-weight loss at week 72, versus 15.7% for semaglutide alone and 10.8% for bimagrumab alone. Of the combination’s weight loss, 92.8% was fat mass, and lean mass fell only 2.9%, compared with a 7.4% lean-mass drop on semaglutide alone.
Does bimagrumab preserve muscle better than semaglutide alone?
Yes, in this trial. Semaglutide alone reduced lean mass by 7.4% at week 72, while the bimagrumab-plus-semaglutide combination limited that drop to 2.9%, despite the combination arm losing substantially more total weight. Bimagrumab alone actually increased lean mass by 2.5%, consistent with its ActRII-blocking mechanism working independently of the GLP-1.
Is bimagrumab approved for weight loss?
Not currently. BELIEVE is a phase 2 trial, an earlier evidence stage than the phase 3 trials that supported semaglutide and tirzepatide approvals. There is no approved bimagrumab-semaglutide combination and no outside-of-trial dosing regimen.
How is bimagrumab different from the muscle-preservation strategies in your other GLP-1 guide?
Our preventing muscle loss on GLP-1s guide covers behavioral levers — protein intake and resistance training — that work by giving the body a reason to keep muscle during a calorie deficit. Bimagrumab works pharmacologically, blocking the ActRII signaling pathway that tells muscle to atrophy, independent of diet or exercise. The two approaches are not competing; they act on different parts of the same problem.
References
- Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial. Nature Medicine. 2026. doi:10.1038/s41591-026-04204-0.
- BELIEVE Trial Demonstrates Combination Therapy Cuts Body Fat While Preserving Lean Mass. American Diabetes Association 86th Scientific Sessions coverage, 2026.
- Bimagrumab-Semaglutide Combination Achieves Significant Weight Loss While Preserving Muscle Mass. Pharmacy Times. 2026.
- Results Show Bimagrumab With Semaglutide Achieve Substantial Weight Reduction. Drug Topics. 2026.
- Combination GLP-1 Therapy Shows Fat Mass Loss While Preserving Lean Mass in Adults with Obesity. Pennington Biomedical Research Center news release, 2026.
For research and educational use only. This article summarises published literature and is not medical advice or a personal dosing recommendation.