BCAAs And The Body : Amino acids are the building blocks of protein and therefore the building blocks of lean muscle. Branched Chain Amino Acids (BCAAs) refer to three specific amino acids - Leucine Isoleucine and Valine - that the body must obtain from either food or BCAA supplementation for recovery and Lean Muscle Growth. BCAAs And Exercise : BCAAs can be used by muscle cells as an energy source which may help offset muscle breakdown during endurance exercise and help support recovery and repair after weight training. As a result BCAAs are a popular and relevant supplement among those who exercise regularly.