What Is Follistatin-344?

1. What does the 344 in follistatin-344 mean?

Follistatin was isolated in 1987 from pig follicular fluid as a single-chain protein of about 35,000 daltons. It suppressed the release of follicle-stimulating hormone, or FSH, from cultured rat pituitary cells at 2.5–6.0 ng/mL, and it was given a name that set it apart from inhibin, the other known suppressor of FSH release (Ueno and colleagues’ isolation paper).

When the human gene was cloned a year later, the same messenger RNA turned out to yield two precursor proteins through alternative splicing: one 344 amino acids long and one 317 (Shimasaki and colleagues’ 1988 report). Each sheds a 29-amino-acid signal sequence on secretion, leaving two mature proteins built from repeated follistatin and Kazal domains (the UniProt entry for human follistatin).

Researchers chose FS344 for muscle work specifically to limit interference with the pituitary activin and inhibin axis that governs FSH, which had been the main reservation about using follistatin as a muscle drug (Al-Zaidy and colleagues’ account of the program).

So the number is a transcript length, not a dose, a potency or a purity grade. A vial labeled with it carries no verified relationship to the construct used in any trial, and because no follistatin product is approved there is no pharmacopeial monograph or regulator-reviewed potency standard to check it against.

2. How does follistatin act on muscle?

Myostatin signals through activin type II receptors. Purified myostatin binds ActRIIB, and follistatin blocks that binding (Lee and McPherron’s receptor work). Follistatin also neutralizes activin A and GDF-11, which makes it a wider net than an antibody aimed at myostatin alone (Yaden and colleagues’ regeneration studies).

That breadth shows up in mouse genetics. A muscle-specific follistatin transgene produced muscle growth comparable to deleting the myostatin gene, and crossing the transgene into myostatin-null mice pushed muscle mass to roughly four times normal, which the author took as evidence that follistatin blocks more than myostatin alone (Lee’s 2007 quadrupling experiment). These are genetically engineered animals, not people given a drug.

Follistatin is also not muscle-specific in the body. In healthy volunteers, plasma follistatin rose about 7-fold after three hours of cycling and peaked three hours into recovery, and arterial-venous sampling showed the liver, not the working leg, released it (Hansen and colleagues’ exercise study). Circulating follistatin is part of a normal metabolic response rather than a muscle-only signal.

3. Why is all the human work gene therapy rather than injections?

The obstacle is pharmacokinetics. A drug-development program that characterized native follistatin-315 judged the protein unsuitable for systemic dosing, and fusing it to an antibody Fc fragment while removing its heparan-sulfate binding improved terminal half-life roughly 100-fold and total exposure roughly 1,600-fold in mice. Only the engineered version produced a dose-dependent effect in mouse atrophy models (Datta-Mannan and colleagues’ pharmacokinetic study).

Gene transfer sidesteps that problem by turning muscle fibers into a local source that secretes the protein for months or years after one injection. In cynomolgus macaques, human follistatin stayed above 20 ng per milligram of quadriceps tissue 5 and 15 months after a single AAV1-FS344 injection (Kota and colleagues’ primate study).

That same primate study is where the striking muscle numbers come from, and they are animal numbers.

The 2009 primate paper also carries a 2026 erratum, which is worth noting when the figures are quoted secondhand.

A half-life of around 24 hours circulates in compound databases for “follistatin-344.” No published human measurement supports it. Manufacturing matters here too: follistatin is a glycoprotein, and differences in glycosylation changed clearance substantially in animals (the follow-up glycosylation analysis), so the clearance of an unapproved preparation is not merely unpublished but unverifiable. A half-life model can draw a curve from an assumed input, but it cannot supply the input that does not exist.

4. What did the human gene-transfer studies find?

Three published reports cover 15 patients in total, all open-label phase 1 or 1/2 studies with no placebo group.

Becker muscular dystrophy (6 patients)

Six ambulatory men received AAV1.CMV.FS344 by direct bilateral quadriceps injection, three at 3 × 10¹¹ vector genomes per kilogram per leg and three at 6 × 10¹¹ per leg. All took prednisone from a month before injection, tapered off by day 60, to blunt an immune reaction to the viral capsid (Mendell and colleagues’ 2015 trial report).

Quadriceps strength did not rise consistently in either cohort. Biopsies at 6 months showed reduced fibrosis between fibers, fewer nuclei sitting centrally within fibers, and fiber sizes distributed more normally. No adverse event was attributed to the gene transfer, and FSH, LH, estradiol and testosterone stayed within normal limits.

Sporadic inclusion body myositis (6 patients)

The same vector was given at 6 × 10¹¹ per leg, again with prednisone, this time alongside a prescribed exercise program of stationary cycling and weighted leg lifts three times a week. Annualized to one year, treated patients gained 56.0 m on the 6-minute walk test while eight untreated patients from the same clinic, matched for age, sex and baseline walking distance, lost 25.8 m (p = 0.01). Four of six improved by 58–153 m and two by 5–23 m, with no treatment-related adverse events reported (Mendell and colleagues’ 2017 report).

Duchenne muscular dystrophy (3 patients)

A separate study injected a total of 2.4 × 10¹² vector genomes per kilogram across gluteal, quadriceps and tibialis anterior muscles in three boys. The registry record reports no dose-limiting toxicities, one serious adverse event from a fall, none of 3 participants with an increase in 6-minute walk distance, and 1 of 3 with an improved North Star Ambulatory Assessment at two years (the posted NCT02354781 results).

Those vector doses are reported here to describe what the studies did. They measure quantities of virus delivered by physicians into specific muscles under a trial protocol, and they cannot be converted into any amount of a protein product.

5. Why are the positive results disputed?

The inclusion body myositis result drew a detailed rebuttal in the same journal. The critique argued that the design cannot support an efficacy claim: it was open-label with no randomized comparator, patients received four potentially active interventions simultaneously (gene transfer, high-dose prednisone, exercise and trial participation), the registered primary endpoint was safety rather than walking distance, the annualized walking measure was constructed after the fact, and adverse-event rates were calculated per event rather than per patient, so a fall rate printed as 6% represented 2 of 6 patients (Greenberg’s published critique).

That is not a dispute about whether follistatin does anything in biology. It is a dispute about whether these particular studies can show a functional benefit, and the Duchenne registry numbers point the same direction, with no walking improvement in any of the three participants.

Two plasmid gene-therapy studies in volunteers are listed as completed with no posted results or publications, one with 43 participants (NCT06411366) and one with 14 (NCT07285629), and an AAV-follistatin study in age-related muscle loss is recruiting 12 participants with a plan to combine it with a VEGF plasmid (the recruiting NCT07443826 record). Nothing in the registry describes giving follistatin protein by injection (the Becker and myositis registry entry).

6. What happened when a follistatin drug met a placebo group?

The most rigorously tested molecule in this family is not follistatin-344 but ACE-083, an engineered follistatin-based protein designed to stay in the muscle it is injected into. Its record matters because both studies were randomized and placebo-controlled.

Put plainly: when follistatin biology was tested against placebo in people, it reliably enlarged the injected muscle and did not reliably make anyone stronger or more capable. Injection-site reactions, mostly mild to moderate, were the most common adverse events.

That dissociation is the single most useful result for anyone weighing follistatin claims. Size and strength are different outcomes, and a bigger measured cross-section is not a performance result.

7. Is follistatin-344 approved, and is it banned in sport?

• No approval anywhere. There is no follistatin product in Drugs@FDA, no approved indication, and no reviewed label whose claims can be checked.
• Not a controlled substance in the US. Follistatin is a protein rather than an anabolic steroid, and it does not appear in the Schedule III list (21 CFR 1308.13). Not scheduled is not the same as legal to sell or use as a medicine.
• Prohibited in sport at all times. The WADA 2026 Prohibited List names follistatin as an example of a myostatin-binding protein under S4.3, agents preventing activin receptor IIB activation, and those substances are non-Specified. Gene transfer of follistatin falls separately under M3.1, gene and cell doping.

8. What are the risks, and what cannot be undone?

For anything sold as follistatin-344, the honest answer about risk is that it is unknown. No registered trial has injected the protein into a person, so there is no adverse-event table, no exposure data and no basis for predicting what a given amount does.

The gene-transfer trials observed no treatment-attributed adverse events across 15 patients, with normal liver, kidney and bone-marrow panels and no change in FSH, LH, estradiol or testosterone (the Becker report; the myositis report). Those are short follow-ups in very small groups of patients with muscle disease, and the way that safety information was presented was itself criticized (Greenberg’s critique). No signal in 15 people is not evidence of safety.

Three concerns shaped how the drug was designed, and they remain the ones worth understanding:

• Pituitary and reproductive signaling. Follistatin was discovered as an inhibitor of FSH release (the original isolation paper), and the FS344 form was picked to reduce interference with the activin and inhibin axis (Al-Zaidy and colleagues). The trials monitored FSH, LH, estradiol and testosterone for exactly that reason, which is also the monitoring anyone discussing fertility or hormone effects should ask about.
• Breadth of blockade. Because follistatin also binds activin A and GDF-11, which act well beyond muscle, broad systemic exposure is not pharmacologically the same as blocking myostatin alone (Yaden and colleagues).
• Immune response. T-cell responses to the viral capsid and to follistatin were measurable in some gene-therapy patients, although anti-follistatin antibody titers stayed low (the Becker report).

Gene transfer adds a problem no other category has: it cannot be stopped. Macaque muscle was still producing human follistatin 15 months after one injection (the primate study). An unwanted effect of a transferred gene does not resolve by skipping the next dose, because there is no next dose.

9. Does any of this apply to muscle loss on a GLP-1?

This is the question that brings most readers here, and the sourced answer is that nobody has tested it. No trial has combined follistatin in any form with semaglutide, tirzepatide or any other GLP-1 medicine, so there is no evidence on added muscle retention and none on interactions. Muscle loss during GLP-1 treatment is a real and measurable concern, which is a reason to want an answer here, not a substitute for having one.

The placebo-controlled ACE-083 results are the closest relevant data, and they cut against optimism: larger injected muscles, no consistent strength or function benefit. Growth confined to an injected muscle also has little to do with preserving whole-body lean mass during a calorie deficit, which is what matters in weight loss.

What can actually be measured

Follistatin itself is measurable in blood, but a single value is hard to interpret because the liver releases it in response to exercise, with a roughly 7-fold rise after prolonged cycling (the exercise study). Any single value therefore has to be read against when the person last exercised.

What the trials tracked is a more useful template than a follistatin level: walking distance over a fixed six minutes, measured strength at defined joints, imaging-based muscle volume, and the hormone panel of FSH, LH, estradiol and testosterone. Those are the outcomes a clinician can interpret, and the hormone panel is the one the trials watched for a specific mechanistic reason.

A personal log cannot verify that any product contained what its label claimed, let alone that it reached a muscle. It can keep dates, products, training load, food intake, weight, measured strength and lab results side by side, which is what makes a timeline legible to a clinician later. The tracking guide covers keeping those records in one place; if several things change at once, no log can tell you which one mattered.

10. Follistatin-344 FAQ

• What is follistatin-344 used for?

  It has no approved use. In research, the FS344 gene was delivered by viral vector to small groups of patients with Becker muscular dystrophy, sporadic inclusion body myositis and Duchenne muscular dystrophy. The muscle-growth interest outside those centers rests on animal work, not on human results.

• Is follistatin-344 legal?

  No regulator has approved a follistatin product. In the United States follistatin is not on the Schedule III anabolic steroid list, which is a separate question from being an approved medicine. WADA prohibits follistatin at all times under class S4.3, and follistatin gene transfer is additionally covered as gene doping under M3.1.

• Is follistatin-344 a peptide or a protein?

  It is a full-size secreted glycoprotein, measured at roughly 35,000 daltons when it was first isolated, not a short peptide. The number 344 is the length of the precursor coded by the gene, before a signal sequence is removed during secretion.

• What are the side effects of follistatin-344?

  For injected follistatin protein there is no answer, because no registered trial has administered it to a person. The gene-transfer reports described no adverse events attributed to treatment across 15 patients, but those groups were tiny and uncontrolled, and that safety reporting was itself criticized in print. Pituitary FSH signaling and immune responses to the vector were the concerns investigators watched.

• How long does follistatin stay in your system?

  No human half-life for follistatin protein has been published. In mice, native follistatin-315 was cleared too fast to work as a systemic injected drug. Gene transfer is a different situation: monkey muscle was still producing human follistatin 15 months after one injection.

• Can follistatin be used with semaglutide or tirzepatide?

  No study has combined follistatin with a GLP-1 medicine, so there is no evidence that it protects muscle during GLP-1 weight loss and none on how the two might interact. The placebo-controlled results from a follistatin-based drug showed bigger muscles without reliably better strength, which is a reason to be skeptical of that use.

• What dose of follistatin-344 has been studied?

  No human dose of follistatin protein has been established. The only amounts given to people were gene-transfer doses expressed in vector genomes per kilogram, which describe how much virus was injected and cannot be translated into an amount of protein.

• Is follistatin-344 the same as ACE-083?

  No. ACE-083 is an engineered follistatin-based fusion protein built to stay in the muscle it is injected into. Its trial findings describe that specific molecule and cannot be carried over to anything labeled follistatin-344.

11. Sources