Follistatin
Research OnlyAn endogenous glycoprotein that inhibits myostatin and activin signaling, potentially allowing muscle growth beyond genetic limits. Gene therapy trials for muscular dystrophy show promise, but injectable peptide forms remain unapproved and understudied in humans.
FST · FS-344 · FS-315 · Activin-Binding Protein
8 human studies
- Preclinical
- 79%
- Clinical
- 21%
Based on 38 cited sources
clinically demonstrated · very-low confidence
Demonstrated effect magnitude — not a recommendation or safety claim.
Research Depth 35: best human evidence is two small open-label AAV1-FS344 gene-therapy trials (Mendell 2015 BMD n=6 25322757; Mendell 2017 sIBM n=6 vs 8 matched untreated 28279643) — single-uncontrolled-study tier (1A 15), high risk of bias (1B 5), total human N<50 (1C 2), indirect to the marketed injectable-peptide route (1D 6), plus a substantial multi-model preclinical program in mice and macaques (1E 7). Mechanism 91: myostatin/activin target with measured binding (activin Kd ~45 pM), crystal structure showing receptor-site occlusion (Thompson 2005 16198295) and genetic confirmation — follistatin effect in myostatin-null background (Lee 2007 17726519) and IGF-IR dominant-negative abolishing ~63% of hypertrophy (Kalista 2011 22087027) (2A 40); near-full target-to-effect pathway incl. IGF-1R/Akt/mTOR (2B 26); in-vivo dose-response across AAV-dose cohorts and primates (2C 16, 2D 9). Plausibility 82: mechanism->surrogate(muscle mass)->clinical(6MWT) chain supported for gene therapy, coherent with cross-species myostatin-null biology, class support from myostatin/activin inhibitors (sotatercept), with the peptide-delivery link unproven keeping it below full marks. Global Coverage 54: human trials concentrated in one Nationwide Children's group, preclinical independently replicated (JHU, Belgium/Finland, OSU) across 4-6 countries; AAV-FS344 carries FDA clinical-trial registration (4D 5). Community Experience 48: niche but persistent bodybuilding interest in black-market FS-344/FS-315, mixed/inconsistent reports and serious product authenticity issues (WADA found only 9/17 products contained follistatin). Effectiveness basis clinical (very-low): gene-therapy 6MWT gains (+58-125 m BMD; +56 vs -25.8 m/year sIBM) exceed the ~30 m MCID on a hard functional outcome, but from tiny uncontrolled trials in a delivery modality distinct from the marketed injectable peptide.
Follistatin’s mechanisms are well-characterized through structural biology and gene therapy trials, though injectable peptide effects in humans remain unvalidated.
How It Works (Simplified)
Follistatin acts as a “molecular sponge” that neutralizes muscle-limiting proteins:
Binds and sequesters myostatin (GDF-8), the body’s primary muscle growth limiter, preventing it from signaling muscles to stop growing.
Binds activin A with extremely high affinity (Kd ~45 pM), reducing catabolic signaling that promotes muscle wasting.
Two follistatin molecules encircle each ligand, physically blocking access to ActRIIA/ActRIIB receptors on muscle cells.
Hypertrophic effects require functional IGF-1 receptor signaling; follistatin removes inhibition rather than directly stimulating growth.
Key Research: Crystal structures PMID:16198295 and gene therapy trials PMID:25322757 confirm mechanism.
Important Limitations
- All human efficacy data comes from gene therapy (AAV delivery), not injectable peptides
- Native follistatin has very short half-life (~4 min initial, ~130 min terminal) limiting systemic effects
- Gene therapy produces sustained local expression; peptide injection kinetics are entirely different
- Black market FS-344/FS-315 products have significant quality issues (WADA found only 9/17 contained follistatin)
- Translation from gene therapy success to injectable peptide effects is not scientifically supported
Net effect.IGF-1R inhibition reduces the effect by ~63%.
Reported positives
- Myostatin inhibition concept appeals to muscle building community
- Improved recovery and muscle growth reported by some
- Theoretical muscle preservation benefits during cutting
- Growing research interest in muscle wasting applications
Reported negatives
- Very expensive with minimal evidence of effectiveness
- Extremely limited human data for injectable use
- Product quality and authenticity difficult to verify
- Potential concerns about unregulated cell proliferation
“Niche interest in bodybuilding. Concept of myostatin inhibition generates discussion but evidence is thin.”
Self-reported, unverified accounts — not clinical evidence and no substitute for the cited research. Anecdotes are prone to selection bias and placebo effects.
Native follistatin has very short half-life (~4 min initial, ~130 min terminal). Injectable peptide effects would be transient. Gene therapy produces sustained local expression over months to years.
PMID:25322757In AAV gene therapy studies, muscle improvements developed over months and were durable in primate studies. This timeline is specific to gene therapy, not peptide injection.
PMID:20368179Long-term safety of sustained myostatin inhibition is uncertain. Concerns include effects on cardiac muscle, tendons, and other tissues. Gene therapy trials continue to monitor safety.
Research-based observations
This timeline reflects observations from published clinical and preclinical studies. Individual responses may vary significantly. This is not a guarantee of effects or a dosing schedule. Consult qualified healthcare providers for personalized guidance.
Good Signs (4 indicators)
Warning Signs (4 indicators)
Bad Signs (5 indicators)
For Research Evaluation Only
These quality indicators are general guidelines based on typical peptide characteristics. Professional laboratory testing (HPLC, mass spectrometry) provides definitive quality verification. This checklist is for initial visual evaluation only.
Igf-1-Lr3
SynergisticFollistatin's hypertrophic effects require intact IGF-1R/Akt/mTOR pathway. IGF-1 LR3 activates this pathway directly. Combined use may amplify muscle growth but also compounds unknown risks.
Hgh
CompatibleHGH increases IGF-1, which is required for follistatin's full effect. Theoretical synergy, but combined safety not established.
BPC-157
CompatibleDifferent regenerative targets - follistatin for muscle growth via myostatin inhibition, BPC-157 for tissue healing. No known interactions.
TB-500
CompatibleDifferent mechanisms - TB-500 for tissue repair via actin regulation, follistatin for muscle growth via myostatin/activin sequestration.
Follistatin already inhibits myostatin. Adding other myostatin inhibitors is redundant and may cause unpredictable effects on the activin/myostatin/follistatin balance.
Research Note: Interaction data is based on published literature, mechanistic understanding, and theoretical considerations. Most peptide combinations lack direct clinical study. This information is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare providers.
Key Studies Cited
Full reference list available on request. All citations link to PubMed for verification.
This dossier synthesizes available evidence from peer-reviewed literature, regulatory documents, and clinical trial registries. Evidence strength ratings follow a modified GRADE approach.
For complete methodology details, see our Methodology page.
Important Disclaimer
This dossier is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider before making health decisions.
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