Follistatin-344
Also known as: FS-344, Myostatin inhibitor
Sold on the strength of a photograph of an extraordinarily muscular mouse, from gene-therapy research that has nothing to do with what is in the vial.
What it is
Follistatin is a protein that binds and inhibits myostatin, the body's brake on muscle growth. Animals — and the very rare humans — with defective myostatin signalling develop dramatically increased muscle mass, and that observation is the entire source of interest in this compound.
Composition
A recombinant form of the 344-amino-acid follistatin isoform. Sold online as a research chemical.
Proposed mechanism
It binds myostatin and prevents it signalling, removing the brake on muscle growth. It also binds activins and other members of the TGF-beta family, which is where the selectivity problem begins.
Claimed benefits
Rapid, large muscle gain, and permanent changes in the body's growth ceiling.
What the evidence shows
Unproven· animal data only. The famous images came from gene therapy — delivering a follistatin gene by viral vector so the animal produces it continuously within its own muscle tissue. That is a fundamentally different intervention from injecting a protein with a short half-life and no selective delivery to muscle. No human trial has shown the injected version produces muscle growth.
The more damning evidence is what happened when this was pursued properly. Pharmaceutical companies have developed myostatin inhibitors for muscular dystrophy and sarcopenia, and the results have been consistently disappointing: increases in muscle mass without matching improvements in strength or function, and several programmes discontinued. That is a substantial finding, and it is almost never mentioned alongside the mouse photograph.
Legal & regulatory status
Not a licensed medicine. Myostatin inhibitors are prohibited at all times under the WADA list.
Known risks & unknowns
Follistatin binds more than myostatin — it also interacts with activins and other TGF-beta family members involved in reproduction, wound healing and tumour suppression. What happens when that network is interfered with systemically in humans is not known, and given what those pathways do, that is not a reassuring unknown.
Sources
Frequently asked questions
- What is Follistatin-344?
- A protein that binds and inhibits myostatin, the body's brake on muscle growth. Animals and the very rare humans with defective myostatin signalling develop dramatically increased muscle mass, which is where the entire interest in this comes from.
- Does injecting follistatin build muscle?
- The famous results came from gene therapy — delivering a follistatin gene by viral vector so the animal produces it continuously in its own muscle tissue. That is a fundamentally different intervention from injecting a protein that has a short half-life and is not selectively delivered to muscle. No human trial has shown that the injected version produces muscle growth.
- What happened to myostatin inhibitors as medicines?
- Pharmaceutical companies have pursued them seriously for muscular dystrophy and sarcopenia, and the results have been consistently disappointing: trials produced increases in muscle mass without matching improvements in strength or function, and several programmes were discontinued. That is a substantial and often-omitted piece of evidence.
- What are the risks?
- Follistatin binds more than myostatin — it also interacts with activins and other TGF-beta family members involved in reproduction, wound healing and tumour suppression. The consequences of systemically interfering with that network in humans are not known, and that is not a reassuring unknown.
Related compounds
- IGF-1 LR3 (Unproven)
An evidence-tiered explainer on IGF-1 LR3: a modified growth factor sold as a muscle builder that was created as a cell-culture reagent and never tested in people.
- MK-677 (Ibutamoren) (Silver)
An evidence-tiered explainer on MK-677 (ibutamoren): an oral ghrelin-receptor agonist, technically not a peptide, that raises GH and IGF-1, with real but modest human trial data and no approval.
- MOTS-c (Unproven)
An evidence-tiered explainer on MOTS-c: a mitochondrial-derived peptide with striking mouse data, sold as exercise in a vial on the strength of it.
Written and reviewed by Mathew Beale, MSc Biotechnology, University of Reading.
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