What Is L-Leucine L-leucine, sometimes simply referred to as Leucine, is a branched chain amino acid (BCAA) that plays an important role in energy production and muscle maintenance. L-leucine can be found in meats, dairy, wheat, and soy and is most commonly taken by bodybuilders and athletes to help boost athletic performance. L-Leucine Benefits L-leucine is the most important of the BCAAs and is commonly taken as part of a preworkout regimen to aid muscle recovery and promote lean muscle growth.*