Follistatin 344

What it is

A full-length precursor isoform of the endogenous activin-binding glycoprotein follistatin. After signal peptide cleavage, the mature circulating isoform is FST315 (315 amino acids, ~38 kDa). Follistatin binds and neutralizes TGF-β superfamily ligands, primarily activin A/B and myostatin (GDF-8), preventing them from engaging the ActRIIB receptor and blocking downstream Smad2/3 signaling that suppresses muscle growth. In transgenic mice, follistatin overexpression increased muscle mass 194-327% versus controls. However, the injectable recombinant protein has a very short circulating half-life (~1-2 hours in animals), meaning daily subcutaneous injections produce only transient biological effects. The dramatic muscle-building results cited in community discussions come from AAV gene therapy trials (sustained local expression for months), not from injectable protein. Phase 1/2a gene therapy trials for Becker muscular dystrophy and inclusion body myositis showed promising functional outcomes, though these remain small studies with methodological limitations.

Community-reported ranges

Ranges sourced from community forums and published preclinical literature. Not dosing guidance. Dose units are mcg/day. Gene therapy trial data cited separately — not applicable to injectable recombinant protein.

Frequently discussed alongside

Based on community forum discussions. Not a recommendation to combine compounds.

Published research

Follistatin's role in muscle biology is well-established through extensive animal research. Gilson et al. (2009) demonstrated follistatin induces muscle hypertrophy through both myostatin and activin inhibition plus satellite cell proliferation. Human data exists only for AAV gene therapy delivery: Mendell et al. (2015) showed improved 6-minute walk test in Becker muscular dystrophy patients, and a follow-up study (2017) reported improved ambulation over 3-5 years. These gene therapy results cannot be extrapolated to injectable recombinant protein due to fundamental pharmacokinetic differences. Authoritative reviews note that injectable FS-344 would likely produce only transient myostatin inhibition with minimal impact on muscle mass. Community dosing protocols are not derived from clinical evidence.

Reported side effects

From community self-reports. Not from controlled studies.

Gene therapy trials showed a favorable safety profile (transient rash/urticaria in <10% of subjects). For injectable use, community users have reported injection site reactions (sometimes severe), transient fatigue, joint stiffness, and flu-like symptoms. Theoretical concerns include FSH suppression (follistatin inhibits activin-mediated FSH release), tendon weakness (myostatin deficiency weakens tendons in animal models), and off-target TGF-β modulation. One published case report documented central serous chorioretinopathy in a bodybuilder using high-dose follistatin. Significant community skepticism exists regarding injectable product efficacy given the ~1-2 hour half-life.

Regulatory status