# Follistatin 315

> Follistatin 315 is the more abundant circulating isoform of follistatin, lacking the heparin-binding domain present in the 344 variant, which makes it behave more as a freely circulating systemic inhibitor of activin and myostatin. It is studied in research settings for the same underlying rationale as follistatin 344: reducing myostatin-driven suppression of muscle growth. Because it does not bind cell surfaces as readily, its distribution and activity profile in tissue is understood to differ somewhat from the 344 isoform, though comparative human data are minimal. It remains an emerging research compound rather than an established therapeutic.

- URL: https://www.lifeconverted.com/peptides/follistatin-315
- Also known as: FST-315, FS315
- Primary category: Muscle & Performance
- Effectiveness rank: #14 in Healing & Recovery; #16 in Muscle & Performance
- Evidence level: Emerging research
- Route: Subcutaneous
- Dose range: 100-300 mcg
- Dose tiers: low 100 mcg; moderate 200 mcg; high 300 mcg
- Half-life: Estimated at several hours; not well characterized in humans
- Timing: Morning
- Cycle: 4-8 weeks on, 8-12 weeks off (As with other follistatin isoforms, conservative cycling is generally discussed given the absence of long-term safety data.)
- Storage: Store lyophilized peptide frozen or refrigerated away from light; reconstitute with bacteriostatic water and refrigerate, using within 2-3 weeks.

## Mechanism of action

Follistatin 315 binds myostatin and activin A with high affinity in the bloodstream, preventing them from reaching the activin type IIB receptor on muscle cell membranes and blocking the resulting Smad2/3-mediated suppression of muscle protein synthesis. Lacking a strong heparin-binding domain, it is thought to circulate more freely rather than concentrating at cell surfaces, potentially favoring broader systemic distribution over localized muscle-tissue action.

## Reported benefits

- Proposed systemic reduction in myostatin/activin signaling
- Suggested support for muscle mass preservation and growth in research models
- May offer a different distribution profile than follistatin 344 for those studying systemic effects

## Reported side effects

- Unestablished long-term human safety profile
- Theoretical disruption of activin-dependent reproductive and tissue-repair processes
- Injection site irritation
- Unpredictable systemic effects from broad TGF-beta superfamily inhibition

## Common protocols

- Daily subcutaneous injection over multi-week research cycles

## Stacking notes

Sometimes discussed as an alternative to follistatin 344 in research contexts; no direct human comparative data exist to guide combined or sequential use.

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