Peptide education
Follistatin-315
Your blood already carries follistatin-315. It is the major soluble circulating isoform of follistatin, an endogenous glycoprotein that binds myostatin and activin-family ligands, and the muscle interest is logical: myostatin restrains muscle growth, and blocking it works in animals. What the reputation skips is that the human studies people cite used FS344 gene transfer in six-patient disease trials, not FS315 protein. Direct FS315 has no peer-reviewed human trial at all, and drug-development researchers concluded the native protein is poorly suited to systemic dosing, which is why serious programs engineer variants or deliver genes instead.
FS315 is the body's own circulating follistatin, but the muscle evidence attached to its name belongs to a different isoform delivered by gene therapy. No human trial has ever tested the product sold under this name.
Overview
Quick answer
Follistatin-315 is different from Follistatin-344 gene transfer, Follistatin-288, ACE-031, myostatin propeptide, and short synthetic fragments sold under a follistatin name. The identity matters because the human disease studies used intramuscular AAV1-FS344 gene transfer, while many online listings use the Follistatin-315 name for poorly characterized vials or fragment products.
What is Follistatin-315?
The major circulating isoform of human follistatin, a liver-derived glycoprotein that binds and neutralizes activin-family ligands including myostatin. It is real human biology, and it is a complex biologic, not a short peptide with a simple synthesis.
What do people use or talk about it for?
Muscle gain, myostatin blockade, recomposition, recovery, fat loss, longevity, and performance, spread across vendor pages, clinic marketing, and offshore gene-therapy promotions. The FS344 disease studies get quoted as if they tested retail FS315 in healthy people. They tested neither.
What study and market details are available?
Every concrete human regimen on this page is an FS344 gene-transfer protocol: intramuscular AAV1.CMV.FS344 in Becker muscular dystrophy and inclusion body myositis, plus a Duchenne protocol with heavy monitoring. For injected FS315 protein there is no peer-reviewed human schedule, amount, or outcome to cite.
What is the product-quality problem?
The name covers too much ground. Testing of 17 follistatin-labeled products found only 9 contained any follistatin, and some held entirely different growth-promoting peptides. One current vendor markets "Follistatin 315" as a 50-amino-acid synthetic fragment, which is not the full-length circulating isoform, and a purity percentage does not fix a wrong-molecule problem.
What human evidence is missing?
Everything direct: no trial has injected FS315 into healthy adults for muscle, fat loss, recovery, longevity, or performance. The closest human data are small disease gene-transfer studies using a different isoform, a different delivery system, and sick patients.
Reported practice
Commonly reported protocol
Community myostatin-inhibitor protocols. Community-reported patterns, not verified by controlled human trials and not a use recommendation. Full use-pattern detail
Evidence
Evidence snapshot
FS315 is described in the literature as the major soluble circulating follistatin isoform, with different tissue-binding behavior than shorter isoforms such as FS288.
Published human muscle-function findings came from intramuscular AAV1-FS344 studies in Becker muscular dystrophy and sporadic inclusion body myositis, not from direct FST315 product administration.
Public human literature has no peer-reviewed healthy-human trial showing that retail Follistatin-315 vials improve muscle, fat loss, recovery, or performance.
Black-market testing and vendor descriptions show that a follistatin label can hide wrong identity, non-follistatin contents, fragment products, or unverified biologic comparability.
Claims
Common claims vs evidence
| Claim | Human evidence | Mechanistic evidence | Anecdotal evidence | Verdict |
|---|---|---|---|---|
| Follistatin-315 builds muscle in healthy adults. | No peer-reviewed healthy-adult FST315 efficacy trial was found. The human muscle studies used FS344 gene transfer in muscular disorders. | Follistatin can bind myostatin and activin-family ligands, which is why the muscle-growth claim has a real biological hook. The issue is whether a marketed FST315 vial contains the right material and produces a predictable muscle outcome in people. | Vendor, clinic-style, and gray-market discussion commonly presents the name as a myostatin-blocking muscle or performance product. | Easy to understand as a search theme, but retail Follistatin-315 still lacks healthy-adult outcome data. |
| The human studies show follistatin works for muscle disease. | Small studies in Becker muscular dystrophy and sporadic inclusion body myositis reported functional or histologic findings after intramuscular FS344 gene transfer. The sIBM interpretation was later challenged in published correspondence. | Local muscle gene transfer fits the idea of altering myostatin/activin signaling in diseased muscle, and preclinical AAV1-FS344 work helped set up that direction. | Online marketing often borrows these disease-study findings for broader muscle-building claims. | The disease-study evidence is relevant background, but it is small, investigational, and not direct support for commercial FST315 use. |
| Retail Follistatin-315 vials match endogenous FS315. | No clinical source shows that current retail vials or fragment products match endogenous full-length circulating FS315 or the material used in gene-transfer studies. | Follistatin is a biologic glycoprotein. Full-length identity, glycosylation, folding, potency, impurities, sterility, and endotoxin status matter more than a name on a vial. | One current vendor description presents "Follistatin 315" as a 50-amino-acid synthetic fragment, while another offshore market promotes gene-therapy-style offerings with body-composition claims. | A product name alone cannot show that the material matches endogenous FS315. |
| Follistatin is only a myostatin blocker. | Human muscle studies are easiest to explain through myostatin biology, but the broader physiology sources show follistatin biology outside skeletal muscle. | Follistatin also interacts with activin-family signaling. Circulating follistatin is liver-derived and metabolically regulated, and higher circulating levels have been associated with mortality, heart failure, and cardiometabolic outcomes in observational work. | Marketing usually compresses the biology into "block myostatin, gain muscle," leaving out reproductive, metabolic, immune, and long-term unknowns. | The myostatin-only explanation is incomplete. Systemic follistatin biology reaches beyond muscle. |
| A purity percentage or COA settles the quality question. | No human study ties generic gray-market FST315 COAs to clinical equivalence, safety, or potency. | Glycosylation-sensitive clearance and biologic structure make this a comparability problem, not just a purity-number problem. | Vendor listings often emphasize purity, but black-market analysis found frequent mislabeling and some products containing other growth-promoting peptides. | Not enough. Identity, full-length structure, glycosylation, potency, sterility, endotoxin, and lot-specific testing matter. |
Bottom line
Main takeaway
Follistatin-315 is a real human protein whose muscle claims come from different material, FS344 gene therapy, tested in muscle-disease patients rather than in anything sold under its name.
Sort every claim by which "follistatin" it means: endogenous FS315, FS344 gene transfer, full-length recombinant protein, a 50-amino-acid fragment, or an offshore gene-therapy offer. The evidence changes completely with each answer.
Schneyer 2004 for isoform identity, the Mendell FS344 studies for the borrowed human data, Datta-Mannan for why native FS315 makes a poor systemic drug, and the black-market analysis for what a label is worth.
Identity
What it is
FS315 is a soluble follistatin isoform encoded by the FST gene, described as the major circulating form in human serum. Compared with isoforms like FS288, it binds cell-surface proteoglycans weakly and stays in circulation.
The attention comes from what it binds. Follistatin neutralizes myostatin and activin-family ligands, and myostatin is the best-known brake on muscle growth, so the name pulls in bodybuilding, performance, muscle-wasting, and recovery interest.
The borrowing problem defines the whole page. The human muscle studies used FS344 gene transfer in disease populations, not commercial FS315 protein, and the drug-development literature has described native FS315 as a poor candidate for broad systemic dosing, which pushed real programs toward engineered variants and gene delivery. The molecule your body makes is not the molecule the market sells.
Market usage makes the ambiguity worse: "Follistatin-315" on a listing can mean a full-length protein idea, a short synthetic fragment, a research-use vial, or an offshore gene-therapy offer. Each has a different evidentiary and safety meaning, and the listings rarely say which one is in the vial.
How people talk about it online
The themes are myostatin inhibition, muscle gain, recomposition, fat loss, recovery, and longevity, and community sources treat FS315 and FS344 as near-interchangeable despite the isoform distinction. None of that conversation is controlled human outcome evidence.
Vendor and clinic pages can make Follistatin-315 sound like a clean, well-defined muscle peptide. The documented market reality is mixed identity: fragment products, mislabeled contents, and unverified biologic comparability.
Offshore gene-therapy marketing runs as a separate thread, using body-composition language that borrows credibility from the small disease studies. Those studies do not make offshore claims established clinical results.
Use context
Routes, doses, and cycle patterns
Concrete human dosing details belong to investigational FS344 gene-transfer studies, not direct FST315 protein. Outside those studies, FST315 appears mostly in muscle, myostatin, research-use-vial, fragment, and offshore gene-therapy claims; those sources show market interest but not a direct FST315 regimen.
Human studies and product labels
Becker muscular dystrophy FS344 gene-transfer study
- Purpose
- Muscle function and histology in Becker muscular dystrophy
- Context
- Small open-label human disease study
- Route
- Direct bilateral intramuscular quadriceps injection
- Amount
- 3 x 10^11 vg/kg/leg in cohort 1; 6 x 10^11 vg/kg/leg in cohort 2
- Frequency
- One gene-transfer administration pattern described by cohort
- Duration
- Follow-up assessed functional and histologic outcomes after treatment
The abstract reported no adverse effects and some 6-minute-walk and histologic findings, but this was a six-patient disease study using AAV1.CMV.FS344, not direct Follistatin-315 administration in healthy adults.
Sporadic inclusion body myositis FS344 gene-transfer study
- Purpose
- Muscle function in sporadic inclusion body myositis
- Context
- Small human disease study with matched untreated comparison and exercise context
- Route
- Intramuscular delivery to quadriceps muscles of both legs
- Amount
- 6 x 10^11 vg/kg rAAV1.CMV.huFS344
- Frequency
- Single gene-transfer administration as reported in the study
- Duration
- Annualized 6-minute-walk change was reported; study context included follow-up over time
The paper reported favorable annualized 6-minute-walk findings in some treated subjects, but later correspondence challenged parts of the efficacy interpretation because of design and concurrent-intervention concerns.
Duchenne muscular dystrophy FS344 study record
- Purpose
- Safety and functional testing in Duchenne muscular dystrophy
- Context
- ClinicalTrials.gov gene-transfer study listing
- Route
- Intramuscular injections divided across gluteals, quadriceps, and tibialis anterior
- Amount
- Proposed total dose 2.4E12 vg/kg
- Frequency
- Study-plan gene-transfer dose, not repeated retail peptide use
- Duration
- Safety and immune monitoring described through 24 months
The listing matters because it shows monitoring intensity: laboratory, immune, pulmonary, antibody, T-cell, circulating follistatin, and adverse-event follow-up were built into the plan.
Real-world discussion
Community myostatin-inhibitor protocols
- Purpose
- Muscle growth discussion
- Context
- Forums, vendors, and protocol blogs
- Route
- Subcutaneous injection
- Amount
- Typically around 100 to 200 mcg per day for runs of 10 to 20 days
- Frequency
- Once daily during a run
- Duration
- Commonly described as 10 to 20 day cycles
Follistatin-315 is the other commonly-sold isoform, and community sources treat the two as near-interchangeable despite the isoform distinction. Human outcome data are absent. Informational context, not advice.
What varies
- Identity: endogenous FS315, FS344 gene transfer, FS288, ACE-031, myostatin propeptide, fragment products, and unlabeled mixtures are different things.
- Route: the human study regimens are intramuscular gene-transfer regimens; retail injection assumptions are a separate product-quality issue.
- Amount: vector-genome doses in studies do not convert into milligram vial amounts.
- Frequency: one-time or protocol-level gene transfer does not translate into repeated personal-use schedules.
- Product quality: full-length identity, glycosylation, folding, potency, impurities, sterility, endotoxin, concentration, storage, and chain of custody all matter.
Human data
Human evidence
For FS315 as a product, the human evidence is indirect. The muscle findings attached to the follistatin name came from intramuscular FS344 gene transfer: six Becker muscular dystrophy patients, six inclusion body myositis patients, and a heavily monitored Duchenne protocol, none of which administered FS315 protein. No peer-reviewed trial has given commercial Follistatin-315 to healthy adults for any outcome. The observational literature on endogenous circulating follistatin adds caution rather than support, since higher levels track with mortality and cardiometabolic outcomes.
Evidence maturity
Follistatin-315 is a real circulating human isoform, but no peer-reviewed human trial has tested direct FST315 products; the muscle evidence belongs to FS344 gene transfer.
FS315 is characterized as the major soluble circulating follistatin isoform, with structural work supporting activin and myostatin binding.
The human muscle findings attached to the follistatin name came from intramuscular AAV1-FS344 gene transfer in small disease studies, not FST315 administration.
None found, and drug-development literature describes native FST315 as poorly suited for broad systemic use, pushing work toward engineered variants or gene delivery.
No approved product exists, and retail listings include short synthetic fragments and material with documented mislabeling.
| Study / evidence area | Population | Design | Product context | Main outcome | Limitations | Weight |
|---|---|---|---|---|---|---|
| Becker muscular dystrophy AAV1.CMV.FS344 study | Six patients with Becker muscular dystrophy | Small open-label disease study | Investigational intramuscular FS344 gene-transfer product | Two cohort-1 patients improved 58 m and 125 m on 6-minute walk distance, one showed no change; in cohort 2, two improved 108 m and 29 m while one showed no improvement. The abstract also described histologic changes consistent with reduced fibrosis and hypertrophy. | Six patients, disease-specific population, open-label design, direct muscle gene transfer, and no direct FST315 product administration. | weak |
| Sporadic inclusion body myositis AAV1.CMV.huFS344 study | Six treated subjects with sporadic inclusion body myositis | Small human disease study with matched untreated comparison | Investigational intramuscular FS344 gene-transfer product plus exercise context | The report described an annualized median 6-minute-walk change of +56.0 m/year versus -25.8 m/year in matched untreated subjects, with 4 of 6 treated subjects improving substantially. | Small size, disease-specific population, concurrent exercise context, and published correspondence questioning parts of the efficacy interpretation. | weak |
| Duchenne muscular dystrophy AAV1.CMV.huFollistatin344 protocol | Six planned Duchenne muscular dystrophy participants | Phase I/IIa registry / protocol-level safety study | Investigational FS344 gene-transfer protocol | The protocol specified a proposed 2.4E12 vg/kg total dose divided across multiple muscles and included extensive laboratory, antibody, T-cell, pulmonary, circulating-follistatin, and adverse-event monitoring. | Protocol and registry information exist, but no peer-reviewed efficacy readout was found. | weak |
| Black-market follistatin product analysis | Seventeen black-market follistatin-labeled preparations | Analytical product-testing study | Black-market products, not regulated clinical material | Only 9 of 17 tested preparations actually contained follistatin, and some products contained other growth-promoting peptides such as MGF or GHRP-2. | Product testing does not measure clinical efficacy, but it strongly affects whether any product claim can be trusted. | moderate |
Cautions
Safety and unknowns
- Direct human safety data for commercial Follistatin-315 products were not found.
- The gene-transfer protocols treated follistatin intervention as biologically complex, with monitoring for liver enzymes, bilirubin, glucose, coagulation, CBC, creatine kinase, renal indices, urinalysis, immune antibodies, T-cell responses, circulating follistatin, adverse events, and pulmonary function.
- Systemic follistatin biology extends beyond muscle. Sources describe liver-derived circulating follistatin, metabolic regulation, observational cardiometabolic and mortality associations, and reproductive-axis biology involving FSH suppression.
- Long-term questions remain around endocrine feedback, reproductive effects, immune response, off-target activin-family signaling, cardiac and metabolic outcomes, oncologic caution, and long-lasting gene-expression effects.
- A marketed vial or fragment product adds risks that the human gene-transfer studies do not answer: wrong identity, wrong amount, degraded material, impurities, sterility failure, endotoxin exposure, and unverified potency.
Product quality
A vial label is only a starting point
Product quality is central for Follistatin-315 because the name can point to a full-length endogenous glycoprotein, a gene-transfer construct, a short synthetic fragment, or a mislabeled vial.
A black-market analytical study found frequent mislabeling in follistatin-labeled products, including preparations that did not contain follistatin and products containing other growth-promoting peptides.
Follistatin-based biologics can be sensitive to glycosylation and clearance differences, so a purity percentage leaves comparability to endogenous FS315 or investigational gene-transfer work unresolved.
Full-length identity
A short fragment marketed as Follistatin-315 is not necessarily equivalent to the full-length circulating isoform described in the biology literature.
Glycosylation and folding
Follistatin is a biologic glycoprotein, and related work shows that glycosylation can affect clearance and comparability.
Potency
The product has to bind the relevant ligands as intended; identity and purity do not by themselves establish biological activity.
Sterility and endotoxin
Any injectable-use context raises sterile-product issues that a generic research-use label or headline purity claim leaves untouched.
Mislabeled mixtures
Black-market testing found products that contained different growth-promoting peptides, which changes both risk and interpretation.
Mechanism
How it is proposed to work
Follistatin binds activin-family signals, including myostatin. Since myostatin helps restrain muscle growth, blocking that pathway is the reason follistatin attracts muscle and performance interest. The limit is that follistatin also touches broader activin and endocrine biology, so the mechanism is not limited to "more muscle."
FS315 is the major soluble circulating follistatin isoform. Compared with more proteoglycan-binding isoforms, it is less tied to cell-surface proteoglycans and is more prominent in circulation.
Structural work supports follistatin interaction with activin and myostatin complexes, which explains the rationale for muscle-disease gene-transfer studies.
Drug-development literature has described native FST315 as poorly suited for broad systemic parenteral biotherapeutic use, which helps explain why research directions moved toward engineered variants or gene delivery rather than simple native-FS315 injections.
Broader human physiology sources tie circulating follistatin to liver and metabolic regulation, cardiometabolic associations, and reproductive-axis signaling, so chronic systemic exposure remains a large unanswered safety question.
FS315 is the mature circulating form; FS344 is the precursor used as cargo in gene-transfer work. Both neutralize myostatin-family ligands; the community treats them as interchangeable despite unresolved functional differences between isoforms.
FAQ
Common questions
How is follistatin-315 different from follistatin-344?
They are different isoforms of the same protein; community sources treat them as near-interchangeable despite the distinction. Neither has human evidence for muscle claims.
What schedules get reported online?
Commonly 100 to 200 mcg per day for 10 to 20 day cycles.
Is there human evidence?
No human trials for physique use. The myostatin-pathway data are animal work, and the serious human programs in this space use gene therapy, not peptide injections.
Details
Technical details
Sources
References
- 1.
NCBI FST gene. NCBI Gene, FST gene record
Gene-level identity and isoform nomenclature
- 2.
Becker FS344 trial. Mendell et al., 2015, Becker muscular dystrophy trial abstract
Main human disease-study efficacy and regimen data
- 3.
sIBM FS344 trial. Mendell et al., 2017, sIBM trial abstract
Main human disease-study efficacy and regimen data
- 4.
Black-market follistatin analysis. Reichel C et al. Detection of black market follistatin 344.
Product-identity testing behind the 9-of-17 finding and the mislabeled-contents claims
- 5.
FDA cell and gene therapy list. FDA approved cellular and gene therapy product list
No approved follistatin product on current list
- 6.
FDA orphan designation. FDA orphan-drug designation entry for adeno-associated virus transgene of follistatin
Designated, not approved
- 7.
FDA compounding pages. FDA approval and compounding pages; current 503A nominated bulks document
Compounded drugs are not FDA-approved; follistatin absent from current nominated bulks document searched here
- 8.
FDA peptide warning letters. FDA warning letters to Xcel, Gram Peptides, and USApeptide
Research-use labeling does not neutralize human-drug intended use; injectables pose serious risk
- 9.
Engineered follistatin variant. Datta-Mannan et al., engineered human follistatin variant
Native FST315 poor PK and PD for broad parenteral systemic use
- 10.
Follistatin-Fc glycosylation. Datta-Mannan et al., 2015, follistatin-Fc glycosylation PK paper
Glycosylation affects clearance and product comparability
- 11.
Hansen liver follistatin. Hansen et al., 2016
Circulating follistatin is liver-derived and metabolically regulated
- 12.
Pan 2024 outcomes. Pan et al., 2024
Observational association of elevated circulating follistatin with mortality and cardiometabolic outcomes
- 13.
Endocrine-axis studies. Recombinant follistatin endocrine-axis studies
FSH suppression biology and endocrine caution
- 14.
Follistatin structure studies. Structural studies of activin-FS315 and myostatin-follistatin complexes
Mechanism and ligand-binding basis
- 15.
Primate FS344 study. Kota et al., 2009, nonhuman primate AAV1-FS344 study
Preclinical safety and translational rationale
- 16.
Vendor fragment listing. Current vendor page marketing “Follistatin 315” as a 50-aa synthetic fragment
Gray-market identity mismatch signal
- 17.
Vendor and offshore claims. Current vendor and offshore clinic pages claiming purity, body-composition effects, or gene-therapy outcomes
Vendor and offshore-clinic claims; no efficacy data.
- 18.
FTC claims guidance. FTC Health Products Compliance Guidance and substantiation policy
Health-claim substantiation and limitations language.
- 19.
sIBM critique. Greenberg critique and related correspondence on sIBM paper
Interpretation caution for claimed efficacy in sIBM
- 20.
UniProt mechanism summary. UniProt / structural summaries on follistatin ligand antagonism
Broader TGF-β-family mechanism context