Leucine Leucine is an essential amino acid that, along with other amino acids (valine and isoleucine), is part of the BCAA. In BCAA, leucine acts as the most potent protein activator in the anabolic process, i.e. in the muscle protein synthesis. At the same time, leucine is the most well-known amino acid that prevents the catabolism and muscle breakdown. In addition, it lowers blood glucose, which is rather helpful in fat burning and diet.