BCAAs are a group of three essential amino acids – isoleucine, leucine, and valine – that the body cannot produce and needs to obtain from dietary sources. They take part in several metabolic processes in the body, are precursors in the synthesis of alanine and glutamine, and make up about one-third of skeletal muscle. During aerobic exercise of long duration (such as long-distance running, cycling, or swimming), the body may break down muscle in order to release BCAAs to be used as energy. Supplementing with BCAAs during or after a workout is believed to prevent muscle breakdown, however research on the benefits of BCAAs for muscle enhancement is mixed.