In plain language
Follistatin 315 is the longer, freely circulating isoform of follistatin, the protein that inhibits myostatin and activins to allow muscle growth. It is the main form found in blood, with lower affinity for cell surfaces than the shorter isoform. Evidence is largely preclinical, and injectable forms lack human trials.
What it is explored for
Follistatin 315 is the well-defined circulating form of follistatin, which makes it a tidy starting point for understanding how this protein works in the bloodstream. That said, the evidence is largely preclinical, and there are no human trials of the administered peptide, so any muscle benefits remain a research question. Here is where interest currently sits.
- Muscle growth and lean mass
- Myostatin and activin research
- Circulating follistatin biology
- Strength and performance interest
- Body-composition research
These are areas of active interest and reported use, not proven outcomes. This peptide carries a preliminary evidence rating, see the evidence summary below for how strong the science actually is.
How it works
Follistatin 315 is the longer splice isoform that circulates freely in serum, binding myostatin and activins with less tendency to stick to cell-surface heparan sulfate than the shorter FST-288 form.
- Myostatin and activin binding. Neutralizes myostatin and activins, TGF-beta family factors that restrain muscle growth.
- Circulating isoform. Its lower cell-surface affinity makes it the predominant freely circulating form in the bloodstream.
- Systemic versus local action. Compared with the cell-bound FST-288 isoform it is thought to act more in the circulation than at tissue surfaces.
Isoform behavior is characterized mainly in biochemical and animal studies. There are no human trials of administered follistatin 315 peptide.
Evidence summary
Evidence for follistatin 315 is preliminary. Its biochemistry as the circulating isoform is reasonably defined, and the broader follistatin and myostatin field shows strong animal effects, but there are no controlled human trials of administered follistatin 315 for muscle or other outcomes. Benefit and safety in people are not established.
Reported safety & side effects
Human safety data for administered follistatin 315 is lacking. As a broad inhibitor of TGF-beta family signaling, theoretical concerns extend beyond muscle to other tissues and reproduction. Long-term effects in people are unknown.
Frequently asked
Is follistatin 315 FDA-approved?
No. Follistatin 315 is not approved by the FDA for any use. It is sold as a research chemical and is not an approved medicine.
How is follistatin 315 different from follistatin 344?
They are different splice isoforms. Follistatin 315 is the shorter, freely circulating serum form with lower cell-surface affinity, while follistatin 344 (often processed to the 288 form) binds cell surfaces more tightly and acts more locally.
Is there human evidence for muscle growth?
No. The data is biochemical and animal. There are no controlled human trials of administered follistatin 315, so muscle benefit and safety in people are not established.