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The Peptide Effect
preclinicalMuscle Growth

Follistatin

Also known as: FST, FS-344, FS-315, Follistatin-344, Activin-Binding Protein

Follistatin is a naturally occurring glycoprotein that acts as a potent inhibitor of myostatin and activin — two key negative regulators of muscle growth. By neutralizing these growth-limiting signals, follistatin effectively removes the biological "brake" on muscle hypertrophy. It has attracted significant research interest for both performance applications and as a potential treatment for muscular dystrophy via gene therapy approaches.

Key Facts

Mechanism
Follistatin binds and neutralizes activin, myostatin (GDF-8), and other TGF-beta superfamily members with high affinity. Myostatin is a powerful negative regulator of skeletal muscle mass — it signals through ActRIIB receptors to suppress muscle protein synthesis and satellite cell proliferation. By sequestering myostatin before it can bind its receptor, follistatin removes this growth inhibition, allowing muscle hypertrophy to proceed unchecked. FST-344 is the common full-length isoform used in research, which is cleaved into the shorter FST-315 and FST-288 isoforms. FST-315 circulates systemically, while FST-288 binds to cell-surface heparan sulfate proteoglycans for localized action.
Research Status
preclinical
Half-Life
Not well characterized for exogenous peptide; gene therapy produces sustained expression
Molecular Formula
C₁₃₈₉H₂₁₅₅N₃₈₅O₄₂₅S₃₆
Primary Use
Muscle Growth
Table of Contents

Benefits

  • Myostatin inhibition — directly neutralizes the primary negative regulator of muscle mass, enabling greater hypertrophy potentialmoderate
  • Potential muscular dystrophy treatment — gene therapy trials have shown meaningful increases in muscle mass and function in Becker muscular dystrophy patientsmoderate
  • Improved body composition — reduced fat mass alongside increased lean mass in animal modelspreliminary
  • May enhance reproductive function — follistatin plays a role in follicle-stimulating hormone regulation and gonadal functionpreliminary
  • Potential synergy with exercise — blocking myostatin during resistance training may amplify the hypertrophic response beyond what training alone achievespreliminary

Dosage Protocols

RouteDosage RangeFrequencyNotes
Subcutaneous injection100–200 mcgOnce dailyAnecdotal dosing protocol. Typical cycles run 10–30 days. Limited clinical data exists to support optimal dosing in humans. Purity and source quality vary dramatically between suppliers.
Gene therapy (AAV vector)Single administrationOne-time intramuscular injectionUsed in clinical trials for Becker muscular dystrophy (e.g., Nationwide Children's Hospital trials). AAV1-FS344 delivered via direct intramuscular injection produces sustained local follistatin expression. Not available outside clinical trials.
Subcutaneous injection (loading protocol)200–300 mcgOnce daily for first 5 days, then 100 mcg dailyAnecdotal "front-loading" approach. No clinical validation. Users report better initial response with higher starting doses tapered to maintenance.

Medical disclaimer

Dosage information is provided for educational reference only. Always follow your prescriber's instructions and consult a qualified healthcare provider before starting any peptide protocol.

Side Effects

  • Injection site reactions — redness, swelling, or pain at the injection sitecommon
  • Limited human safety data — the long-term effects of exogenous follistatin administration are not well characterizedserious
  • Potential for excessive or uncontrolled muscle growth with sustained myostatin suppression (theoretical based on myostatin-knockout animal models)rare
  • Possible effects on reproductive hormones — follistatin interacts with activin and FSH signaling, which could alter fertility parametersrare
  • Theoretical cardiac effects — the heart is a muscle, and unregulated growth signaling could potentially affect cardiac tissueserious

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Frequently Asked Questions

How does myostatin inhibition work?
Myostatin (GDF-8) is a protein produced by muscle cells that acts as a growth limiter — it signals surrounding muscle tissue to stop growing. This evolved as a metabolic safeguard to prevent excessive muscle mass. Follistatin binds directly to myostatin and prevents it from reaching its receptor (ActRIIB), effectively removing this growth brake. Animals with naturally low myostatin (like Belgian Blue cattle or myostatin-knockout mice) develop dramatically more muscle mass than normal.
What is the difference between follistatin gene therapy and injectable follistatin?
Gene therapy uses an adeno-associated virus (AAV) to deliver the follistatin gene directly into muscle cells, which then produce follistatin continuously for months or years from a single injection. Injectable follistatin peptide requires daily dosing and has an uncertain half-life. Gene therapy trials (e.g., AAV1-FS344 for Becker muscular dystrophy) have shown more consistent results, but this approach is only available within clinical trials and carries its own risks, including immune responses to the viral vector.
Why do Belgian Blue cattle look so muscular?
Belgian Blue cattle have a natural mutation in the myostatin gene that severely reduces myostatin production. With less myostatin signaling, their muscles grow far beyond normal limits — a phenomenon called "double muscling." These cattle can have 40% more muscle mass than standard breeds. This natural example is often cited to illustrate the dramatic effect that myostatin inhibition can have and is part of what drives interest in follistatin as a myostatin-blocking agent.
What are realistic expectations for injectable follistatin?
Expectations should be heavily tempered. While the science of myostatin inhibition is compelling, injectable follistatin peptides face major challenges: uncertain bioavailability, rapid degradation, questionable purity from peptide suppliers, and no controlled human dosing studies. Animal studies and gene therapy trials show clear muscle-building effects, but translating this to a self-administered injectable peptide is a significant leap. Users should not expect Belgian Blue-like transformations from short peptide cycles.
How can I verify the quality of follistatin I purchase?
Follistatin is a complex glycoprotein that is difficult and expensive to manufacture correctly. Many products sold online are degraded, mislabeled, or contain little to no active follistatin. Look for suppliers that provide third-party Certificate of Analysis (CoA) with mass spectrometry data confirming molecular weight and purity. HPLC purity should be >95%. Be wary of prices that seem too low — legitimate follistatin is significantly more expensive than simpler peptides like BPC-157 due to manufacturing complexity.

References

  1. 1
    Follistatin directly blocks activin and myostatin signaling and is required for normal muscle development(2001)PubMed ↗
  2. 2
    Follistatin gene delivery enhances muscle growth and strength in nonhuman primates(2009)PubMed ↗
  3. 3
    Gene therapy for muscular dystrophy using follistatin: safety and efficacy in Becker muscular dystrophy patients(2015)PubMed ↗
  4. 4
    Myostatin as a therapeutic target for muscle wasting: current knowledge and future perspectives(2018)PubMed ↗

Last updated: 2026-02-14