Editorial note: this article summarises published research on follistatin and myostatin inhibition. Baclabs does not sell follistatin-344 or any peptide; we supply bacteriostatic water and reconstitution consumables. Nothing here is medical advice.
Follistatin-344 is sold as a myostatin inhibitor on the strength of one of the most striking observations in muscle biology: humans and animals that lack functional myostatin grow visibly, dramatically more muscle. The logic is straightforward, if myostatin is a brake, remove it and the muscle grows.
There is a real, published human trial involving FS-344, but it delivered the follistatin gene inside a viral vector into muscle tissue, not follistatin protein into a syringe. Meanwhile the wider myostatin field has spent fifteen years discovering that added muscle mass and added muscle function are not the same thing.
Myostatin: what it does and why blocking it is attractive
Myostatin (GDF-8) is a TGF-beta superfamily member secreted by skeletal muscle itself. It signals through the activin type IIB receptor and the Smad2/3 pathway to restrain fibre growth, a negative feedback loop keeping muscle mass within a set range.
The proof of concept in humans is a single case report: Schuelke and colleagues described in the New England Journal of Medicine in 2004 a child with gross muscle hypertrophy carrying a myostatin splice-site mutation [1]. Similar phenotypes exist in Belgian Blue cattle and whippets. That case is the origin of the entire myostatin inhibitor industry.
Follistatin is a natural antagonist of this family. It is best characterised as an activin-binding protein [5], and it also binds and neutralises myostatin, blocking receptor engagement upstream.
FS-344, FS-315 and FS-288: the isoforms matter
“Follistatin-344” refers to the 344-amino-acid precursor transcript. After signal peptide cleavage it yields FS-315, the longer circulating isoform with low affinity for cell-surface heparan sulfate. Alternative splicing produces FS-288, a shorter form that binds heparan sulfate proteoglycans avidly and therefore stays associated with cell surfaces rather than circulating freely.
This distinction is not academic. The cell-surface-associated form is the one that acts locally, and preclinical work using a follistatin-288-Fc fusion protein has shown localised growth of skeletal muscle [6]. Whether a vial sold as “follistatin-344” contains the precursor, the processed FS-315, correctly folded and glycosylated protein, or something else is not verifiable without independent analysis, and glycosylation depends on the expression system, which a bacterial one will not reproduce.
The Becker muscular dystrophy gene therapy trial
Mendell, Al-Zaidy and colleagues ran a phase 1/2a dose-ascending trial of AAV1-FS344 in six ambulatory patients with Becker muscular dystrophy and confirmed dystrophin gene mutations [2][3]. The vector was injected directly into muscle: four injections across three of the four quadriceps muscles (vastus lateralis, vastus medialis and rectus femoris) in each leg. Three patients received 3 × 10¹¹ vector genomes per kg per leg (6 × 10¹¹ vg/kg total) and three received 6 × 10¹¹ vg/kg per leg (1.2 × 10¹² vg/kg total) [3].
Two of three patients in each cohort improved their six-minute walk distance, a statistically significant average improvement of 11.5% (p = 0.02) at six months. Patients with extensive muscle fibrosis showed no functional improvement. The procedure was well tolerated with no adverse effects directly related to gene transfer. Follow-up was up to 12 months for the low-dose cohort and 6 months for the high-dose cohort [3].
Why injectable follistatin protein is not the same intervention
The distinction is the single most important point in this article.
- Delivery. AAV1-FS344 transduces muscle cells so that they transcribe and translate follistatin themselves, producing correctly processed, correctly glycosylated protein inside the tissue. An injection of exogenous protein deposits a bolus into the interstitial space.
- Duration and localisation. AAV-mediated expression in post-mitotic muscle is effectively continuous for months to years, and the vector was placed in specific quadriceps muscles. Systemic injection of a short-lived protein has neither property; the pharmacokinetics of injected follistatin in humans do not appear to have been published.
- Population, endpoint and scale. Six patients with a dystrophinopathy, assessed on walking distance over six to twelve months. That is a phase 1/2a safety and signal study, not evidence of hypertrophy in healthy trained adults.
None of this makes the trial unimportant. It makes it a poor basis for claims about a vial of follistatin-344 protein bought online.
The wider myostatin field has been disappointing
If myostatin inhibition were a reliable route to functional muscle, the pharmaceutical programmes would have shown it. A 2022 review of myostatin pathway inhibition in motor neuron disease summarised the pattern across human trials: muscle volume increases of 3–9%, against 10–30% in mice, and increased muscle mass that “did not result in clinically meaningful improvement of muscle strength, functional motor scales or self-reported physical function” across trials in muscular dystrophy, inclusion body myositis, cachexia and COPD [4].
Two exceptions involved elderly populations: bimagrumab improved grip strength and gait metrics in sarcopenic adults aged 65 and over, and LY2495655 improved stair-climbing and chair-rising time in adults aged 75 and over [4].
Safety has also bitten. ACE-031, a soluble decoy activin type IIB receptor, was discontinued after epistaxis and telangiectasias emerged in a Duchenne muscular dystrophy trial, attributed to off-target inhibition of BMP-9 and BMP-10 [4]. The TGF-beta superfamily shares receptors, and broad blockade has vascular consequences. Follistatin binds activins as well as myostatin [5], so it is not narrow-spectrum either.
| Intervention | Type | Human outcome |
|---|---|---|
| AAV1-FS344 | Gene therapy, intramuscular | +11.5% mean 6MWT at 6 months, n=6 BMD patients [2][3] |
| ACE-031 | Decoy ActRIIB receptor | Discontinued, epistaxis, telangiectasias [4] |
| Bimagrumab | Anti-ActRII antibody | Mass gains; function improved only in sarcopenic 65+ [4] |
| LY2495655 | Anti-myostatin antibody | Improved stair climb and chair rise in adults 75+ [4] |
| Follistatin-344 protein | Injected recombinant protein | No published human trial located |
Follistatin 344 muscle growth claims and dosing
Vendor and forum material describes short daily injection courses of follistatin-344 lasting a few weeks, sometimes with claims of several kilograms of lean mass. The figures quoted have no traceable source, and we do not repeat them. No dose-finding study, controlled trial, pharmacokinetic study or registry entry for injected follistatin-344 in humans could be located. There is no published safety data at any exposure, no monitoring framework, and no information on immunogenicity, a relevant concern for a glycoprotein produced in a non-mammalian expression system.
The theoretical risks deserve naming: follistatin and activins regulate reproductive function, inflammation, wound healing and fibrosis, not only muscle, and systemic activin blockade has produced vascular adverse effects in a related programme [4].
Anti-doping and UK legal status
Myostatin inhibitors are prohibited in sport at all times. WADA section S4 names “myostatin inhibitors such as: agents reducing or ablating myostatin expression, myostatin-binding proteins (e.g. follistatin, myostatin propeptide)” and “activin receptor IIB competitors such as decoy activin receptors (e.g. ACE-031)”, all as non-specified substances [7][8]. Gene doping provisions apply separately to vector-delivered approaches.
Follistatin-344 holds no UK marketing authorisation. Supplying it for human use engages the Human Medicines Regulations 2012 and MHRA enforcement.
Reconstitution and diluent choice
Follistatin is supplied lyophilised and is a large, fragile glycoprotein, shaking risks denaturation, so add diluent slowly down the vial wall and swirl gently. Multi-dose vials need a preserved diluent. Baclabs supplies UK-stocked bacteriostatic water in 30 mL vials; the reconstitution guide covers the concentration arithmetic, bacteriostatic vs sterile water covers diluent choice, and storage and shelf life explains the 28-day in-use rule.
For the wider picture, see the muscle growth peptides hub. For the growth factor side of the same class, see IGF-1 LR3 vs IGF-1 DES.
Frequently asked questions
Does follistatin-344 build muscle in humans?
No published human trial of injected follistatin-344 protein exists. The human evidence involves AAV1-FS344 gene therapy delivered into the quadriceps of six Becker muscular dystrophy patients, which produced a mean 11.5% improvement in six-minute walk distance at six months [2][3]. That is a different intervention in a disease population, not a hypertrophy result in healthy adults.
What is the difference between follistatin-344, FS-315 and FS-288?
FS-344 is the precursor transcript. Processing yields FS-315, the longer, freely circulating isoform, while alternative splicing yields FS-288, which binds cell-surface heparan sulfate and stays local. A follistatin-288-Fc fusion has been shown to promote localised skeletal muscle growth in preclinical work [6].
What dose of follistatin-344 is used in research?
There is no established research dose for injected follistatin-344 in humans. The daily microgram figures and multi-week courses circulating online are unattributed community claims with no trial, pharmacokinetic study or dose-finding work behind them. The one human study used AAV vector doses of 3 × 10¹¹ to 6 × 10¹¹ vector genomes per kg per leg, which is not convertible to a protein dose [3].
Is myostatin inhibition safe?
Not established. ACE-031 development was stopped after epistaxis and telangiectasias attributed to off-target BMP-9 and BMP-10 inhibition [4]. Follistatin binds activins as well as myostatin [5], and activins regulate reproduction, inflammation and fibrosis, so broad blockade has consequences beyond muscle that have not been characterised for this compound.
Is follistatin banned in sport?
Yes. WADA section S4 explicitly lists myostatin-binding proteins including follistatin among prohibited myostatin inhibitors, non-specified and banned at all times [7][8].
References
- Schuelke M et al. Myostatin mutation associated with gross muscle hypertrophy in a child. New England Journal of Medicine, 2004
- Mendell JR et al. A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy. Molecular Therapy, 2015
- Al-Zaidy SA et al. Follistatin gene therapy improves ambulation in Becker muscular dystrophy. Journal of Neuromuscular Diseases, 2015
- Inhibition of myostatin and related signaling pathways for the treatment of muscle atrophy in motor neuron diseases. PMC, 2022
- Follistatin and its role as an activin-binding protein. PubMed, 1997
- Follistatin-288-Fc fusion protein promotes localized growth of skeletal muscle. PubMed, 2019
- WADA Prohibited List S4: Hormone and metabolic modulators. Drugs.com summary of the WADA list
- The Prohibited List. World Anti-Doping Agency
Disclaimer: This content is for educational purposes only and does not constitute medical advice. Bacteriostatic water is supplied for reconstitution of substances intended for research and for use as directed by a healthcare professional. Peptides and medicines discussed here may be unlicensed in the UK or prescription-only; consult a qualified clinician before using any medicine. Baclabs does not sell peptides.