A naturally occurring protein that inhibits myostatin — the body's 'muscle growth brake' — studied for muscle wasting and aging.
Follistatin 344 is a naturally occurring glycoprotein that binds and neutralizes activin A, myostatin (GDF-8), and other TGF-β superfamily members. Myostatin is the body's primary negative regulator of muscle growth — it acts as a 'brake' on muscle development. By binding myostatin, follistatin effectively releases this brake, allowing enhanced muscle growth and regeneration.
The 344 designation refers to the amino acid count of this specific follistatin isoform (FST344), which is the most commonly studied variant. Other isoforms include FST303 and FST315. Follistatin is produced naturally in many tissues including the liver, skin, and ovaries.
Natural examples of myostatin inhibition are dramatic: myostatin-knockout animals (Belgian Blue cattle, Bully Whippet dogs, the 'mighty mouse' model) show extreme muscular hypertrophy. In humans, rare loss-of-function myostatin mutations produce exceptional muscularity and low body fat from birth.
Animal studies demonstrate follistatin gene therapy produces significant muscle mass increases and strength improvements without exercise. In Duchenne muscular dystrophy models, follistatin treatment improved muscle pathology and function. Human gene therapy trials using follistatin have shown preliminary safety and efficacy in muscular dystrophy.
Jerry Mendell's group at Nationwide Children's Hospital has conducted follistatin gene therapy trials for Becker muscular dystrophy showing functional improvements. However, injectable follistatin peptide is different from gene therapy — the protein is rapidly cleared and may not achieve sustained myostatin inhibition.
📚 Key Reference: PMID: 25544560 (follistatin gene therapy)
Injectable follistatin has limited human safety data. Gene therapy approaches have shown preliminary safety. Myostatin inhibition could theoretically affect cardiac muscle, tendons, and other tissues. Reproductive effects are possible (activin inhibition affects fertility). Research use only. Consult your provider.
NOT FDA-approved. Research use only. Gene therapy versions are in clinical trials for muscular dystrophy.