Bimagrumab: the muscle-preserving partner that could change obesity treatment
In the phase 2b BELIEVE trial, bimagrumab plus semaglutide produced 22.1% weight loss with 92.8% of it from fat — and lean-mass loss limited to 2.9%.
Direct answer
In the phase 2b BELIEVE trial, bimagrumab combined with semaglutide produced 22.1% weight loss at 72 weeks, of which 92.8% was fat mass, while lean-mass loss was limited to 2.9%. Bimagrumab alone increased lean mass by 2.5%. It is investigational and not approved for obesity.
What bimagrumab is
Bimagrumab is a monoclonal antibody that blocks the activin type II receptor (ActRII). This is the receptor through which both myostatin and activin A signal to restrain muscle growth. Blocking the receptor therefore intercepts the pathway one step further downstream than a myostatin-specific antibody does.
Interestingly, ActRII blockade appears to influence adiposity as well as muscle — which is why bimagrumab produces weight loss on its own, unlike a purely muscle-sparing agent.
The pairing
Semaglutide suppresses appetite and drives weight and fat loss. Bimagrumab blocks ActRII signalling, preserving or increasing lean mass. Combining them targets the two halves of body composition simultaneously.
The BELIEVE trial
A phase 2b randomised, double-blind, placebo-controlled trial (Heymsfield SB et al., Nature Medicine, 2026), conducted at Pennington Biomedical Research Center and other sites.
- Participants: 507 adults with obesity
- Arms: nine groups — bimagrumab 10 or 30 mg/kg IV every 12 weeks; semaglutide 1.0 or 2.4 mg weekly; combinations; and placebo
- Duration: 72 weeks
Key results:
| Treatment | Total weight loss | Lean mass change |
|---|---|---|
| Bimagrumab alone | 10.8% | +2.5% |
| Semaglutide 2.4 mg alone | 15.7% | −7.4% |
| Bimagrumab + semaglutide | 22.1% | −2.9% |
- 92.8% of weight lost was fat in the combination arm
- Total fat mass fell 45.7% with the combination versus 27.8% with semaglutide 2.4 mg
- Visceral adipose tissue fell 58.2% versus 35.8%
- hsCRP fell by up to 83%
This table is a conceptual summary of trial-arm results, not a guarantee of individual outcomes.
Safety: generally well tolerated, with mild-to-moderate acne and muscle spasms noted in bimagrumab groups — side effects the investigators say require further refinement.
Why this is scientifically interesting
Bimagrumab alone increasing lean mass by 2.5% is the striking finding. Most obesity interventions cost you lean tissue; this one added it. And the combination achieved deeper fat and visceral fat reduction than semaglutide alone while limiting lean-mass loss to a third of what semaglutide alone produced.
That said: lean body mass measured by DXA is not the same as skeletal muscle mass, and neither is the same as strength or physical function. BELIEVE was not designed to prove that patients became functionally stronger. Phase 2b evidence in 507 people is promising, not definitive.
Clinical significance
If these results hold in phase 3, the implications reach beyond obesity — into healthy ageing, sarcopenia prevention, long-term weight maintenance, and patients undergoing very large weight reductions. It would also accelerate the shift I described in the first article of this series: from weighing patients to measuring their body composition and function.
Regulatory status
Bimagrumab is not approved for obesity anywhere, including Kuwait, and is available only in clinical trials.
Frequently asked questions
What is bimagrumab?
An investigational monoclonal antibody blocking the activin type II receptor, the receptor through which myostatin and activin A limit muscle growth.
Does bimagrumab increase muscle?
In BELIEVE, bimagrumab alone increased lean body mass by 2.5% over 72 weeks. Lean body mass is not identical to skeletal muscle, and strength was not the trial's endpoint.
What did the 2026 BELIEVE trial show?
Bimagrumab plus semaglutide gave 22.1% weight loss with 92.8% from fat and only 2.9% lean-mass loss, versus 15.7% weight loss and 7.4% lean-mass loss with semaglutide alone.
Is bimagrumab approved?
No. It remains investigational for obesity as of August 2026.
What is ActRII?
The activin type II receptor — the cell-surface receptor myostatin and activin A act through to restrain muscle growth.
Can bimagrumab be combined with semaglutide?
It has been, in the phase 2b BELIEVE trial. This combination is not available outside clinical trials.
This article is educational and does not replace personalized medical advice. Contact Professor Jamal's clinic on WhatsApp +965 60621662.
Reference: Heymsfield SB et al. Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial. Nature Medicine, 2026.
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