Do all GLP-1 drugs cause lean mass loss?

Weight loss from any cause includes some lean tissue. The question is the proportion, which varies by compound, population, duration and measurement method.

How does bimagrumab work?

It is an antibody against the activin type II receptor, blocking myostatin and activin signalling — the pathway that limits skeletal muscle mass.

Is bimagrumab approved?

No. BELIEVE is a Phase 2 trial and the compound holds no marketing authorisation.

GLP-1 & Incretin Science

Lean Mass Loss on GLP-1 Therapy, and the BELIEVE Result

JMWritten & reviewed by Jack Muncaster · Founder, UK PeptidesLast reviewed 2026-08-234 cited sources

Weight lost on incretin therapy is not all fat. In the Phase 2 BELIEVE trial, adults on semaglutide alone lost 7.4% of lean mass; adding bimagrumab reduced that to 2.9% while producing greater total reduction of 22.1%, of which 92.8% was fat mass.

Key facts

Lean loss, semaglutide alone: 7.4%
Lean loss, plus bimagrumab: 2.9%
Total reduction, combination: 22.1%
Fat share of that loss: 92.8%
Lean fraction of total loss: ~21% → ~7%
Bimagrumab target: Activin type II receptor
Trial: BELIEVE, Phase 2, published in Nature Medicine

Why the composition of weight loss matters

A scale reports mass, not what the mass was. Skeletal muscle contributes to metabolic rate, glucose disposal, physical function and — in older populations — to the ability to remain independent. Losing a substantial fraction of it alongside fat is a materially different outcome from losing fat alone, even when the headline percentage is identical.

How much lean mass is involved

Reported estimates run as high as 40% of total weight lost coming from lean tissue, though figures vary with population, measurement method and duration. In the BELIEVE trial specifically, semaglutide alone produced 7.4% lean mass loss, corresponding to roughly 21% of total weight lost. Different studies produce different numbers, and body-composition measurement methods are not interchangeable.

What bimagrumab is

Bimagrumab is a monoclonal antibody against the activin type II receptor. Blocking that receptor interrupts myostatin and activin signalling — the pathway that restrains skeletal muscle growth. Inhibiting a brake on muscle mass is a mechanistically different intervention from anything the incretins do, which is why it combines with them rather than competing.

The BELIEVE result

Adding bimagrumab to semaglutide reduced lean mass loss from 7.4% to 2.9%, while total reduction reached 22.1% with 92.8% of it fat mass. Expressed as a fraction, the lean share of total loss fell from roughly 21% to about 7%. The combination therefore produced more total reduction and a substantially better composition — an unusual combination of outcomes.

How to read this cautiously

BELIEVE is Phase 2. Body composition is a surrogate: better composition is assumed to translate into better function and outcomes, and that assumption is reasonable but not demonstrated by this trial. A 2026 systematic review and meta-analysis in Diabetes, Obesity and Metabolism examined bimagrumab in obesity and metabolic dysfunction more broadly, and the activin/myostatin pathway as a muscle-preservation target has its own 2026 review literature.

Why this became a prominent question in 2026

As incretins moved toward 20% and beyond, the absolute quantity of tissue lost grew with them. At 5% reduction the composition question is academic; at 25% it is not. The compounds getting more effective is precisely what made this a live issue rather than a footnote.

Quick reference

	Semaglutide alone	Semaglutide + bimagrumab
Lean mass lost	7.4%	2.9%
Total weight reduction	Lower	22.1%
Fat share of loss	~79%	92.8%
Lean share of loss	~21%	~7%

Extended research context

The GLP-1 & Incretin Science deep dive

Deep dive: the two routes to a bigger effect

Every compound trying to beat GLP-1 alone has taken one of two routes. The first adds more receptors from the same hormone family — GIP in tirzepatide, GIP and glucagon in retatrutide. The second adds a non-incretin satiety hormone, which in practice means amylin: CagriSema combines cagrilintide with semaglutide, and amycretin engages both receptors from one molecule. Both routes work, because they recruit signalling pathways that do not fully overlap. Neither has escaped the constraint that binds all of them, which is that gastrointestinal tolerability worsens as effect size grows.

Deep dive: why a percentage is not a result

The most-quoted numbers in this field are the least comparable. REDEFINE 1 reported 22.7% and 20.4% for the same compound in the same trial — the first among participants who adhered to treatment, the second across everyone randomised. TRIUMPH-1 reported 28.3% in an uncomplicated obesity population while TRIUMPH-3 reported up to 22.6% in adults with established cardiovascular disease, using the same compound. Before any two figures can be compared they have to match on estimand, population, duration, comparator and whether the number is placebo-adjusted. Most published comparisons match on none of them.

Deep dive: what happens after the trial stops

Every headline figure describes weight while treatment continues. The STEP-1 extension found that a year after semaglutide was stopped, participants had given back roughly two-thirds of what they lost, moving from 17.3% mean reduction to a net 5.6% — though average weight remained below baseline and nearly half stayed at least 5% down. Meta-analysis puts regain at around 0.8 kg per month. This is why maintenance studies such as TRIUMPH-6 matter more to the field's future than another two points of peak reduction.

Research applications

▸Comparing incretin and amylin compounds on a like-for-like basis
▸Interpreting estimands, thresholds and placebo-adjusted figures in trial reports
▸Tracking the obesity pipeline across sponsors and jurisdictions
▸Understanding receptor pharmacology behind GLP-1, GIP, glucagon and amylin
▸Distinguishing licensed medicines from investigational compounds

Handling checklist

✓Identify which estimand a quoted percentage comes from before citing it
✓Check the trial population and baseline BMI against the comparison you are making
✓Confirm the duration and whether the reduction curve had plateaued
✓Read discontinuation rates alongside efficacy figures
✓Verify every NCT identifier against ClinicalTrials.gov rather than secondary reporting

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Comparing headline percentages across different trials

Fix: Population, duration, estimand and comparator all differ; the numbers are not interchangeable.

✗ Quoting the larger of two figures from the same trial

Fix: Name the estimand. Efficacy and treatment-policy answer different questions.

✗ Treating peak reduction as a durable outcome

Fix: Substantial regain follows cessation across the class; peak figures describe a maintained state.

✗ Assuming an oral route means a weaker mechanism

Fix: Route and receptor count are independent. Orforglipron is weaker because it hits one receptor, not because it is a tablet.

✗ Reading investigational compounds as available treatments

Fix: Most of this pipeline holds no authorisation anywhere; mazdutide is approved only in China.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Frequently asked questions

Do all GLP-1 drugs cause lean mass loss?: Weight loss from any cause includes some lean tissue. The question is the proportion, which varies by compound, population, duration and measurement method.
How does bimagrumab work?: It is an antibody against the activin type II receptor, blocking myostatin and activin signalling — the pathway that limits skeletal muscle mass.
Is bimagrumab approved?: No. BELIEVE is a Phase 2 trial and the compound holds no marketing authorisation.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

Written and reviewed by

Jack Muncaster · Founder, UK Peptides

Jack founded UK Peptides in Manchester after repeatedly receiving research compounds with missing or recycled paperwork. He is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate.

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