Follistatin-315 (FS-315)
Follistatin-315 is the shorter, circulating isoform of the endogenous glycoprotein follistatin, produced by alternative splicing. It binds and neutralises members of the TGF-beta superfamily — most notably myostatin (GDF-8) and activin A — and is marketed in the grey research-peptide market on the promise of muscle growth. Human data specific to exogenously administered FS-315 is essentially absent; nearly all supporting evidence is animal or in-vitro.
01 Overview
Follistatin exists as two main isoforms, FS-288 and FS-315, generated by alternative splicing of a single gene. FS-315 lacks the C-terminal heparin-binding tail that anchors FS-288 to the cell surface, so FS-315 is the predominant form found free in serum. Both isoforms sequester myostatin and activin, releasing the brake these ligands place on muscle growth; myostatin-null and follistatin-overexpressing animals develop dramatic hypertrophy.
The compound sold to bodybuilders as "follistatin" is typically a recombinant FS-315 or FS-344 preparation of uncertain purity. There are no controlled human trials of injected recombinant follistatin for muscle building; the strongest human-relevant work uses AAV-delivered follistatin gene therapy in muscular dystrophy, which is a different modality. Claims of rapid muscle gain rest on animal models and anecdote.
02 Mechanism
Acts as a high-affinity antagonist of myostatin and activin A, preventing these TGF-beta ligands from binding activin type II receptors and thereby de-repressing satellite-cell activation and muscle protein accretion.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Anecdotal research use | 100–300 mcg/day | SubQ | No validated human dosing exists; figures reflect underground practice only. |
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Muscle hypertrophyFollistatin overexpression roughly doubles muscle mass in mice; no equivalent effect is documented for injected FS-315 in humans. | Large in rodents | Preclinical | |
| Reduced fibrosisActivin/myostatin blockade limits fibrotic tissue in dystrophic animal muscle. | Model-dependent | Preclinical |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Uncharacterised systemic activin blockadeActivin signalling regulates the pituitary-gonadal axis, inflammation and epithelial tissue; broad neutralisation could have unpredictable endocrine effects. | Moderate | Unknown | Unconfirmed | |
| Immunogenicity / injection reactionsInjected recombinant protein of unverified purity may trigger local reactions or anti-drug antibodies. | Moderate | Unknown | Unconfirmed |