What Is Follistatin?
Follistatin (FST — the FST-344 / FS344 isoform is the one most often referenced in muscle research) is a naturally occurring secreted glycoprotein that binds and inhibits myostatin and related TGF-β family ligands, releasing a brake on skeletal-muscle growth. Because myostatin limits muscle mass, blocking it is a target for muscle-wasting diseases and, in the fitness/peptide world, for muscle gain. Human evidence is very limited: the most solid data come from small gene-therapy trials in muscular dystrophy, not from healthy people or from injectable “follistatin peptide” products. It is an experimental biologic, not an approved drug or dietary supplement, and this entry is informational only.
Evidence
The clearest human data come from a Phase 1/2a follistatin gene-therapy trial in Becker muscular dystrophy, in which an AAV1 vector encoding the FS344 isoform was injected into the quadriceps of six patients; the study reported it was generally well tolerated and described improvements in six-minute-walk distance in some participants 1Reference 1Clinical trialA Phase 1/2a Follistatin Gene Therapy Trial for Becker Muscular Dystrophy — clinical trialView study →. This was a small, uncontrolled, disease-specific safety/feasibility study — not evidence that follistatin builds muscle in healthy adults, and not a test of the injectable “follistatin peptide” products sold in the grey market. Broader reviews note that myostatin-pathway inhibition reliably increases muscle mass in animal models, but that randomised trials of myostatin inhibitors in humans have so far often failed to translate into clear functional benefit; that translational caveat is commonly acknowledged and I did not attach a single citable trial to it in this quick pass. Claims that follistatin injections increase lean mass, cut fat, or “reverse epigenetic age” in healthy people are marketing claims I could not tie to a verified controlled human trial.
Dosage & Safety
There is no established or recommended dose of follistatin for muscle building in healthy people — the human data are from intramuscular gene-therapy delivery of an FST-encoding vector, not from a defined peptide dose, so no meaningful dosage range applies. Follistatin is an experimental biologic; it is not an approved drug and is not a legal dietary supplement, and injectable “follistatin peptide” products are unapproved research chemicals of unverified identity and purity. Long-term safety, off-target effects (follistatin also acts on activin/inhibin signalling in the reproductive and other axes), interactions, and use in pregnancy are not established and were not assessed in this pass. Informational only — this is not a recommendation to obtain or use it.
References
- Mendell JR, et al. (2015). A Phase 1/2a Follistatin Gene Therapy Trial for Becker Muscular Dystrophy — clinical trial. Molecular Therapy. https://pubmed.ncbi.nlm.nih.gov/25322757/