Bimagrumab inhibits multiple members of the TGF-beta superfamily, including activin A and GDF8, which regulate both muscle and fat tissue.

Study: GDF8 and activin A are the key negative regulators of muscle mass in postmenopausal females: a randomized phase I trial

The study shows that blocking two specific proteins (GDF8 and activin A) makes muscles bigger and fat smaller, which is exactly what the claim says Bimagrumab does — even though the study used different drugs, they target the same proteins.

Study: Effect of Bimagrumab on body composition: a systematic review and meta-analysis

Bimagrumab helped people gain muscle and lose fat, which is exactly what happens when you block the proteins activin A and GDF8 — so even though the study didn’t measure those proteins directly, the results match what you’d expect.

Study: Antibody blockade of activin type II receptors preserves skeletal muscle mass and enhances fat loss during GLP-1 receptor agonism

Bimagrumab blocks a cellular switch (ActRII) that normally tells muscles to stay small and fat to grow. When blocked, muscles got bigger and fat got smaller, proving it works on the same system that activin A and GDF8 control.

Study: GDF8 and activin A blockade protects against GLP-1–induced muscle loss while enhancing fat loss in obese male mice and non-human primates

The study shows that blocking two specific proteins (GDF8 and activin A) helps keep muscle while burning more fat — and Bimagrumab is known to block those same two proteins. So yes, the claim is correct.