CATEGORY: DIET & WEIGHT
The muscle mostly stayed. That’s the result that sets the BELIEVE trial apart from the usual weight-loss study. Among 507 adults with obesity, those given bimagrumab, an infusion designed to protect muscle, together with weekly semaglutide lost 17.8 kilograms, about 39 pounds, over 48 weeks, and 92% of what they lost was fat. On semaglutide alone the loss was 14.2 kilograms, and only 71% of it was fat.1
One Drug Shrinks Appetite, the Other Guards Muscle
Semaglutide and its relatives melt weight by quieting appetite, but the scale doesn’t say what kind of weight is leaving. A meaningful share is usually lean tissue. Bimagrumab comes at the body from the opposite side. It blocks the chemical signals that tell muscle to shrink, releasing a brake the body keeps on muscle growth. Lilly’s researchers expected the pairing to work like addition, with preclinical studies suggesting deeper fat loss and spared muscle, and they tested it the proper way. A randomization scheme spread the 507 volunteers across placebo, each drug alone at different doses, and the combination, with body scans separating fat from lean tissue at every checkpoint.1
What the Body Scans Separated Out
The expectation held, with one wrinkle. At 48 weeks the combination had cut fat mass by 42.2%, far beyond semaglutide’s 24.8%, while placebo barely moved. Lean mass told the real story. It fell 7.9% on semaglutide alone but only 2.3% on the combination, and men and women on bimagrumab by itself actually gained 2.7% of lean mass while still losing fat. So muscle wasn’t fully preserved by the pairing, just mostly. Carried into the extension at week 72, the combination reached 22.1% total weight loss versus 15.7% for semaglutide alone.1
Phase 2, With Real Strings Attached
This was a mid-stage trial of a drug nobody can get yet. Bimagrumab brought its own side effects, including muscle spasms, acne, diarrhea and temporary bumps in liver enzymes, and people on it alone quit the study more often than anyone else. Infusions mean a clinic visit, not a pen at home. And Eli Lilly, which funded the trial and is developing the drug, still has to prove all this again in larger phase 3 testing before regulators weigh in. What the trial changes today is smaller but real: it shows the muscle lost on GLP-1 drugs isn’t an unavoidable toll.1
Key Takeaways
- Adding the muscle-protecting infusion bimagrumab to semaglutide pushed 48-week weight loss to 17.8 kg, and 92% of it came from fat, versus 71% with semaglutide alone.
- Lean tissue still slipped slightly on the combination, so the drug softened muscle loss rather than erasing it.
- Bimagrumab remains experimental: phase 2 evidence, given by IV infusion, with its own side effects and no regulatory approval.
References
- Heymsfield SB, Aronne LJ, Montgomery P, et al. Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial. Nature Medicine. 2026;32:869–882. link