The muscle question just became drug design: what the BELIEVE trial showed

What happened: results published in Nature Medicine in March 2026 paired semaglutide with bimagrumab — an antibody that blocks signals limiting muscle growth — in 507 adults with obesity. At 72 weeks, the highest-dose combination averaged 22.1% weight loss, and about 92% of the weight lost was fat mass. Semaglutide alone averaged 15.7% weight loss but with roughly 7.4% of lean mass lost; bimagrumab alone produced less weight loss (10.8%) while lean mass slightly increased.

What the evidence actually says

This is a phase 2 trial — a dose-finding study, not the large phase 3 programme that approval requires — and it was funded by Eli Lilly, which owns bimagrumab. Within those limits, the signal is striking: the combination lost more weight than semaglutide alone, and the composition of that loss shifted decisively toward fat. The researchers are now testing bimagrumab with tirzepatide. The practical takeaway is not about any product you can use: it is that the industry itself now treats "how much of the loss is fat?" as the question worth engineering for.

Limitations and uncertainties

Phase 2 numbers regularly soften in phase 3. The bimagrumab arms reported adverse events including acne and muscle spasms that need more study, averages hide wide individual variation, and 72 weeks says nothing about what happens after stopping. Bimagrumab is not an approved medicine anywhere.

What this means for New Zealand

Nothing in this trial is available in New Zealand, and nothing here is a reason to change treatment. Its local relevance is the principle: on today's GLP-1 medicines, a meaningful share of weight lost can be lean mass, which is why measuring body composition — rather than weight alone — is the honest way to track a result. That's a measurement question anyone can act on now: scale weight plus a periodic body-composition check tells you what a scale alone cannot. Our guides on what the trials actually measured and DEXA vs smart scales cover how.

What we still don't know

Whether the combination holds up in phase 3; its long-term safety; whether regulators will treat lean-mass preservation as an approvable claim; and pricing anywhere, let alone here. We'll report the tirzepatide combination data when it reads out.