The phase 2 BELIEVE trial (NCT05616013) tested whether adding bimagrumab, a monoclonal antibody that blocks activin type II receptors on muscle and fat, changes the composition of weight lost on semaglutide. Results were published in Nature Medicine in 2026 after an initial presentation at the American Diabetes Association meeting in June 2025. The trial randomised 507 adults with overweight or obesity across 26 sites in the United States, Australia and New Zealand and ran from November 2022 to June 2025.
The design was a nine-arm factorial: three intravenous bimagrumab doses (up to 30 mg/kg, infused at weeks 4, 16, 28 and 40) crossed with three semaglutide doses (placebo, 1.0 mg or 2.4 mg weekly). The primary endpoint was change in body weight at week 48. The highest combination — bimagrumab 30 mg/kg plus semaglutide 2.4 mg — cut body weight by 17.79 kg, versus 14.24 kg for semaglutide 2.4 mg alone, 9.25 kg for bimagrumab 30 mg/kg alone and 3.31 kg for placebo.
The distinguishing result was body composition rather than total weight. In the combination arm, 92.8% of the weight lost came from fat mass, compared with 71.8% in the semaglutide-alone arm. Lean mass fell 2.9% with the combination versus 7.4% with semaglutide 2.4 mg alone, and visceral fat dropped 58.2% versus 35.8%. Bimagrumab acts independently of appetite, so its effect is mechanistically distinct from the incretin drugs it was paired with.
The findings speak to a recurring concern with GLP-1-based weight loss: a substantial share of the weight lost is lean tissue. Bimagrumab is one of several agents — alongside the SARM enobosarm, now in the phase 2b PLATEAU trial, and myostatin-pathway biologics — being tested to shift that ratio toward fat. The most frequently reported adverse events with bimagrumab in this and earlier studies were diarrhoea and muscle spasms.
BELIEVE is a phase 2 study; bimagrumab is not approved for obesity, and larger and longer trials would be needed to establish whether the muscle-sparing effect translates into functional or long-term outcomes. The compound is owned by Eli Lilly, which acquired it through its purchase of Versanis Bio.