USN BCAA Syntho Amino Stack is a combination of the Branched Chain Amino Acids (BCAAs), uptake enhancers and testosterone maintaining zinc. The BCAAs, which comprise of the essential amino acids Leucine, Isoleucine and Valine, make up approximately one third of skeletal muscle in the human body. Usn bcaa syntho amino stack is a combination of the branched chain amino acids (bcaas), uptake enchancers and testoerone maintaining zinc. Zinv contributes to normal protein synthesis, and the maintenance of normal testosterone levels in the blood. Can aid muscle recover during you workout. Bcaas are vital to any fitness enthusiast, as they are the building blocks of protein and an essential for muscle recovery and growth. 20 servings per pack.