Follistatin
| Category | Other Peptides |
| Goals | Muscle & Performance |
| Evidence level | Preclinical (animal/cell; early gene-therapy trials in disease) |
| Legal status | Research-only — not approved for general human use |
| FDA status | Not approved; investigational gene therapy in clinical trials only |
| Half-life | Not well established (rapidly cleared protein) |
| Routes | Subcutaneous · Intramuscular (non-clinical) · AAV gene therapy (research) |
| CAS / MW / Sequence | Not well established · ~37.8 kDa · 344-aa glycoprotein (FST-344) |
| Last reviewed | 2026-06-05 |
In one line
A naturally occurring glycoprotein that blocks myostatin and activin — the body’s “brakes” on muscle growth — so inhibiting it can drive substantial increases in muscle mass, most convincingly shown in animals and gene-therapy research, not in everyday injectable use.
Evidence at a glance
Follistatin’s muscle-building effect is dramatic in animal and gene-therapy models, but the popular injectable recombinant “Follistatin-344” has no controlled human efficacy or safety evidence. As a large glycoprotein it is fragile, hard to verify, and unlikely to survive/act like the gene-therapy versions. The real human data comes from AAV gene therapy in muscular dystrophy, a very different context. See Evidence Grading Explained and the Disclaimer.
Key Takeaways
- A 344-amino-acid glycoprotein (the “FST-344” isoform; ~37.8 kDa) — not a small peptide but a full secreted protein.
- Works by binding and neutralizing myostatin and activin A, two TGF-β-family proteins that limit muscle growth.
- Because it blocks two brakes (myostatin and activin), follistatin overexpression produces larger muscle gains than myostatin knockout alone in animals (reported ~194–327% muscle-mass increase in transgenic mice vs ~100% for myostatin knockout).
- Strongest real human data is from AAV gene therapy (e.g. Becker muscular dystrophy, inclusion-body myositis) — not from injecting recombinant protein.
- The injectable “Follistatin-344” sold to consumers is research-only, unproven in humans, and difficult to authenticate.
What Is It
Follistatin is a naturally occurring secreted glycoprotein found throughout the body. Its best-known role for muscle is as a high-affinity antagonist of myostatin (GDF-8) and activin A — signaling proteins in the TGF-β superfamily that normally restrain skeletal-muscle growth. By soaking up these inhibitors, follistatin releases the brakes on muscle, allowing hypertrophy.
Several isoforms exist; the commonly referenced FST-344 is a 344-residue form that, after processing, circulates systemically (in contrast to the cell-surface-bound FST-315). It is being explored two ways: as AAV-delivered gene therapy (the form actually tested in patients) and as a recombinant protein sold non-clinically for muscle goals.
Mechanism of Action
- Myostatin (GDF-8) inhibition (established) — binds myostatin and prevents it from signaling through activin receptors, removing a primary limiter of muscle size.
- Activin A inhibition (established) — also neutralizes activin A; blocking this second brake is why follistatin out-performs pure myostatin blockade in animals.
- Downstream SMAD signaling (established) — by blocking these ligands it reduces SMAD2/3 activation, shifting the balance toward muscle growth.
- Systemic distribution (FST-344) (established) — the 344 isoform enters circulation, enabling body-wide rather than purely local effects.
Limitations
Because activin signaling is broad (reproductive, inflammatory, fibrotic roles), strong systemic follistatin activity has off-target potential that is poorly characterized outside gene-therapy disease trials. As a large, fragile glycoprotein, an injected recombinant version may behave very differently from gene-expressed follistatin.
Evidence by Outcome
| Outcome | Evidence | Notes |
|---|---|---|
| Muscle mass (transgenic / gene-overexpression) | Preclinical | Large increases in mice (~194–327%) |
| Ambulation / function in muscular dystrophy (AAV gene therapy) | Clinical (early) | Phase 1/2a Becker MD: ~11.5% 6-min-walk improvement at 6 mo |
| Inclusion-body myositis (gene therapy) | Clinical (early) | Reported functional improvement in small trials |
| Muscle growth from injected recombinant FST-344 | Anecdotal | No controlled human data; large fragile protein |
| Fat loss / body recomposition | Anecdotal / Preclinical | Extrapolated; not established in humans |
Reported Dosing
Not medical advice
There is no established human dose for recombinant follistatin, and the only human data uses gene therapy, not injections. Figures below are as reported non-clinically and documented for education only — not a recommendation. See Reconstitution & Dosing Math and Injection Technique.
| Route | Dose (reported) | Frequency | Notes |
|---|---|---|---|
| Subcutaneous / IM | ~100 µg/day | Daily, short blocks | Anecdotal; unproven and unverifiable |
| AAV gene therapy | Single dose (vector genomes) | One-time (research) | Clinical-trial setting only — not consumer use |
Pharmacokinetics
Follistatin PK in the injectable form is not well established. As a large (~37.8 kDa) glycoprotein, circulating follistatin is generally cleared relatively quickly and is susceptible to proteolysis, which is a key reason the meaningful human research uses gene therapy (continuous endogenous expression) rather than repeated protein injections. Reliable human half-life data for recombinant FST-344 is lacking. See Half-Life & Pharmacokinetics.
Side Effects & Risks
- Off-target activin blockade — activin has roles in reproduction, inflammation, and fibrosis; broad systemic inhibition has poorly characterized consequences.
- Theoretical proliferation/tumor concern — myostatin/activin pathways are tumor-suppressive in some tissues; long-term systemic blockade is not adequately studied.
- Tendon/connective-tissue mismatch — very rapid muscle growth could outpace tendon adaptation (a concern seen with strong anabolic interventions).
- Gene-therapy-specific risks (in trials): immune responses to the viral vector — a different risk profile from injections.
- Sourcing risk is severe — a large glycoprotein is hard to make, easy to mislabel, and frequently not what the label claims. See Side Effects & Risk Management, Bloodwork & Monitoring, Red Flags & Scams.
Cycling
No evidence-based cycling standard exists for follistatin. Anecdotal injectable protocols use short blocks, but given the absence of human efficacy/safety data and authentication problems, no meaningful regimen can be stated. (Gene therapy is, by design, a one-time intervention.) See Cycling.
Stacks It Appears In
- Discussed alongside other muscle-targeted compounds such as IGF-1 LR3 (growth-factor pathway) and GH-axis peptides like MK-677 or CJC-1295 + Ipamorelin — though follistatin’s myostatin-inhibition mechanism is distinct.
Comparisons
- vs IGF-1 LR3 — both target muscle but oppositely: IGF-1 LR3 adds growth signaling via the IGF-1 receptor; follistatin removes growth inhibition by blocking myostatin/activin.
- vs GH secretagogues (Sermorelin, Hexarelin) — those raise GH/IGF-1; follistatin does not touch the GH axis.
- Recombinant protein vs AAV gene therapy — only the gene-therapy form has produced human functional data; the injectable protein remains unproven.
Sourcing & Quality
Follistatin is one of the hardest peptides/proteins to source authentically: it is a large glycoprotein, expensive to produce correctly, and the consumer “Follistatin-344” market is rife with mislabeled or inactive product. Treat any claim with heavy skepticism and verify rigorously: How to Read a CoA, Third-Party Testing, Red Flags & Scams. Reconstitution and storage (proteins are fragile): Reconstitution & Dosing Math, Storage & Handling. No vendors are endorsed here.
Legal & Regulatory Status
(As of 2026-06-05.) Follistatin is not FDA-approved for any general use. Its legitimate human research exists only as investigational AAV gene therapy in clinical trials (e.g. Becker muscular dystrophy, inclusion-body myositis), which is tightly regulated and not commercially available. Recombinant “Follistatin-344” sold for muscle gain is a research chemical, not for human use, and myostatin inhibitors are prohibited in sport by WADA. Status varies by country. See Regulatory & Legal Status.
FAQ
Is follistatin a peptide? Loosely grouped with peptides in this context, but it is actually a large 344-amino-acid glycoprotein, not a short peptide.
Does the injectable “Follistatin-344” build muscle in humans? There is no controlled human evidence that it does. The dramatic results come from animals and gene therapy — different forms and contexts.
Why is gene therapy different from injecting it? Gene therapy makes the body produce follistatin continuously, sidestepping the rapid clearance and fragility of an injected protein — which is why that is where the human data exists.
How does it differ from myostatin inhibitors generally? Follistatin blocks both myostatin and activin A, so it can exceed the muscle gains of blocking myostatin alone.
References
- Mendell J.R. et al. (2015). “A Phase 1/2a Follistatin Gene Therapy Trial for Becker Muscular Dystrophy.” Molecular Therapy (PMC4426808).
- Kota J. et al. (2009) / Haidet A.M. et al. (2008). “Long-term enhancement of skeletal muscle mass and strength by single gene administration of myostatin inhibitors.” PNAS.
- Mendell J.R. et al. (2017). “Follistatin Gene Therapy for Sporadic Inclusion Body Myositis Improves Functional Outcomes.” Molecular Therapy.
- Lee S.J., McPherron A.C. — foundational work on myostatin/follistatin and muscle mass.
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