Follistatin
Myostatin Inhibitor / Growth Factor AntagonistresearchAlso known as: Follistatin-344, FS-344, FST, Activin-Binding Protein
A glycoprotein that inhibits myostatin and activin, removing the body's natural brake on muscle growth and enabling significant increases in lean muscle mass.
Overview
Follistatin is a naturally occurring glycoprotein produced in nearly all tissues that acts as a potent antagonist of myostatin (GDF-8) and activin — two TGF-beta superfamily members that limit muscle growth. By binding and neutralizing myostatin, follistatin effectively removes the body's natural cap on muscle development, allowing for enhanced hypertrophy. The most commonly discussed form in the peptide community is Follistatin-344, a precursor form that is processed into the active Follistatin-315 isoform. Gene therapy studies in animals have produced dramatic muscle growth, and the famous 'mighty mice' experiments that produced double-muscled rodents involved myostatin inhibition. Follistatin also has roles in reproductive biology, inflammation, and liver protection. It is one of the most sought-after peptides in bodybuilding, though practical results in humans from injectable forms are debated.
Mechanism of Action
Follistatin acts as a decoy receptor and antagonist: (1) Binds myostatin (GDF-8) with high affinity, preventing it from activating ActRIIB receptors on muscle cells; (2) Also neutralizes activin A and B, further removing growth inhibition signals; (3) Myostatin inhibition unleashes the Akt/mTOR pathway for enhanced protein synthesis; (4) Promotes satellite cell proliferation and fusion; (5) Reduces SMAD2/3 signaling that normally suppresses muscle growth; (6) May promote fat loss through enhanced muscle metabolism and brown fat activation; (7) Has anti-inflammatory properties through activin neutralization.
Molecular Formula
Large glycoprotein (varies by isoform)
Molecular Weight
~36,000 g/mol (FS-315); ~38,000 g/mol (FS-344 precursor)
Sequence
344 amino acids (FS-344 precursor); 315 amino acids (active form)
Dosage Protocols
Dose Range
100mcg – 200mcg
Frequency
Once daily
Route
subcutaneous
Cycle Length
10-30 days
Injectable follistatin has questionable bioavailability. Some protocols use very short cycles. Effectiveness of injectable forms is debated vs. gene therapy approaches.
Source: Community protocols (limited clinical data)
🧮 Personalized Dosage Calculator
💰 Estimated Pricing
Typical Supply
1mg vial
Last Updated
2026-02
Myostatin inhibitor protein. Research-only. Expensive due to complex production. Used in muscle/fertility research.
⚠️ Prices are estimates based on publicly available data and may vary significantly by vendor, location, and prescription status. This is not medical or financial advice.
Side Effects
| Effect | Severity |
|---|---|
| Injection site reaction | mild |
| Joint discomfort | mild |
| Reproductive effects | moderate |
| Potential tumor risk | severe |
Pros & Cons
Targets the fundamental biological brake on muscle growth — myostatin
Gene therapy animal studies show dramatic and consistent muscle growth results
May simultaneously promote fat loss through metabolic enhancement
Natural protein with established biological roles
Injectable forms have questionable bioavailability and practical effectiveness
Complex glycoprotein is difficult to manufacture and verify for quality
Long-term safety of chronic myostatin inhibition is unknown
Very expensive compared to other muscle-building peptides
Most dramatic results seen only in gene therapy — not injectable peptide — studies
Research Studies
🩸 Blood Work
No specific bloodwork requirements reported for this peptide. General health panels are always recommended before starting any peptide protocol.
Legal Status
Not FDA-approved. Available as a research chemical. Some gene therapy applications in clinical trials. Banned by WADA. Complex protein makes sourcing and quality verification challenging.
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