What did the BELIEVE trial find about bimagrumab and semaglutide results in 2025?

What exactly did the BELIEVE trial test, and who was in it?

BELIEVE was a phase 2 randomised controlled trial that set out to answer a question obesity researchers have circled for years: can you get more out of semaglutide by adding something that specifically targets muscle and fat tissue biology? Bimagrumab is a monoclonal antibody that blocks activin type II receptors, the signalling pathway that ordinarily promotes fat accumulation and muscle loss. The hypothesis was that blocking those receptors while semaglutide was already suppressing appetite would shift the body's response toward burning stored fat rather than burning muscle.

The study enrolled adults with overweight or obesity, and participants received either semaglutide plus bimagrumab or semaglutide plus a placebo. The primary endpoint was not simply total weight on the scale but body composition: how much of what was lost came from fat versus lean mass. That framing is unusual in weight-loss trials, and it is one reason the results attracted attention well beyond specialist journals.

A question our prescribers hear most weeks is whether a medicine protects muscle during weight loss. BELIEVE was specifically designed to measure that, which makes its methodology meaningful even at phase 2 scale.

What did the 2025 results actually show?

The combination arm produced greater total weight loss than semaglutide plus placebo over the trial period. More striking, when researchers analysed body composition, participants on bimagrumab plus semaglutide lost a substantially larger proportion of fat mass and maintained significantly more lean mass than those on semaglutide alone. In practical terms, the scale moved further and the muscle stayed.

This matters for a few reasons. GLP-1 medicines like semaglutide work partly by reducing calorie intake; when calorie intake falls sharply, the body can draw on both fat and muscle for energy. Preserving muscle affects metabolic rate, physical function, and the likelihood of regaining weight after treatment ends. If you want to understand what semaglutide typically does in the first week of treatment, that early picture can help set realistic expectations before looking at longer-term composition data. If you want to understand what typical semaglutide weight loss looks like on its own, the evidence behind semaglutide's results gives the clinical trial picture in full.

The BELIEVE findings were reported at major obesity medicine conferences and in peer-reviewed literature during 2025. The trial was not large enough to draw conclusions about cardiovascular outcomes or long-term weight maintenance, and the follow-up period was limited. Those caveats are significant, but they are the normal constraints of phase 2 work rather than a reason to dismiss the signal.

Why does muscle preservation in weight-loss trials matter so much?

Weight lost during any calorie-restricted period is never entirely fat. Studies of GLP-1 medicines consistently show that a meaningful fraction of total weight loss comes from lean mass, including muscle. The STEP 1 trial of semaglutide 2.4mg, published in the New England Journal of Medicine, reported impressive average weight reduction but did not target muscle preservation as a primary outcome.

Losing muscle during weight loss is associated with a slower metabolism at the lower body weight, making it harder to maintain the new weight without continuing treatment or intensive lifestyle effort. Research on maintaining weight loss after stopping Wegovy consistently shows that regain is common, and the proportion of weight lost as muscle may be one of the mechanisms. If bimagrumab genuinely shifts that ratio, the clinical relevance would extend well beyond the trial period itself.

There is also a functional dimension: muscle mass matters for mobility, strength and metabolic health, particularly in older adults with obesity. A treatment that shrinks the fat depot while largely sparing muscle would represent a qualitatively different kind of weight-loss therapy.

Where does bimagrumab sit in UK clinical practice right now?

Nowhere, practically speaking. Bimagrumab has no MHRA licence in the UK, for weight management or any other indication. Phase 2 results, however promising, are not the same as the large phase 3 trials that regulators require before approval. The BELIEVE data support further investigation and those larger trials are expected, but the timeline from here to a licensed product is measured in years, not months.

The licensed options for weight management in the UK remain semaglutide, prescribed as Wegovy, and tirzepatide (Mounjaro). NICE guidance on semaglutide covers the conditions under which it is recommended, and the NHS medicines page on semaglutide provides a clear summary of how it works, its common side effects, and what to expect. For a fuller picture of how Wegovy performs in trials, the Wegovy trial results page covers the STEP programme and the more recent data on the 7.2mg dose.

If you are already on semaglutide and wondering how long Wegovy takes to work, or curious what the early weeks tend to look like, those questions are addressed in detail on this site. The BELIEVE trial is genuine scientific progress; it is also early-stage research in a field that moves quickly. The right place to weigh up current treatment options is a consultation with a prescriber who can look at your full picture. If you'd like that conversation, you can start your free consultation with our clinical team today.