Follistatin 344 (FST-344 splice variant)

Follistatin is a naturally occurring protein that binds and inhibits myostatin (GDF-8) — the negative regulator of muscle growth. The 344 splice variant is one of two main circulating isoforms. Gene therapy approaches using follistatin have been tested in muscular dystrophy.^[1]

By binding myostatin, follistatin releases the natural brake on muscle growth, enabling hypertrophy beyond normal limits. It also binds activin and other TGF-β family members.

The peer-reviewed literature on Follistatin 344 is summarized below across two tiers: human research (the highest standard), and preclinical / emerging research (animal models and early-stage human work).

This table summarizes commonly discussed claims and how the published evidence weighs in. The aim is clarity — supported claims, claims that look promising but need more data, and claims that outrun the science.

Grey-market injectable; no validated clinical protocol.

Follistatin biology is genuinely interesting and the 2025 Phase 2 sarcopenia data represents real clinical progress for the class — particularly relevant given the broader interest in muscle preservation during GLP-1-driven weight loss. But the gap between the rigorous Phase 2 protein-therapy formulations and the injectable "follistatin-344" sold in grey-market research-peptide channels is substantial. Identity verification, purity, and bioactivity of grey-market product are unreliable; the effects users report often correlate more with concurrent anabolic-androgenic steroid or growth-hormone use than with the follistatin itself.

WADA-prohibited status (S4 Hormone and Metabolic Modulator) applies at all times — categorically inappropriate for any athlete subject to drug testing across professional sports, Olympic competition, NCAA athletics, and many recreational competition contexts. For the broader myostatin-inhibitor class story, see our myostatin inhibitor class history Evidence vs Myth article. The most clinically advanced myostatin-pathway approach for muscle preservation specifically during GLP-1 therapy is currently bimagrumab (Phase 3, anti-ActRIIB antibody) — see our Emerging Peptides coverage. For non-athletes with specific sarcopenia or muscle-loss contexts, the appropriate framework is clinician-supervised consideration of the emerging research-stage interventions rather than grey-market follistatin self-administration.