USN BCAA Bodybuilding Sports Nutrition Supplement BCAA is a high quality bodybuilding sports nutrition supplement from USN. This is a complete Branched Chain Amino Acid product with isoleucine leucine amp valine. n Glutamine is the most abundant amino acid found in the human body & is arguably the most important in the process of building muscle & gaining strength. Citrulline malate again may enhance Nitric Oxide production also increases natural arginine levels promote aerobic energy