Leucine is an essential amino acid. It is one of the three amino acids, which create the group of the branched-chain amino acid (BCAA). Leucine has a direct influence on the growth of muscle cells because when its intake is in appropriate amount it can make the nitrogen balance in the organism (the balance between synthesis and decomposition of protein molecules) positive. The increased protein synthesis in the muscle cells provides complete recovery of the musculature after a physical activity. This stimulates muscle growth in muscle building programs and protects the muscles from decomposition in weight loss programs. Leucine is contained in animal products (meat, dairy products, fish) as well in some foods of plant origin. The only way to supply the body with enough leucine in the proper time, in order to achieve the mentioned results, is through supplementation. The amino acid leucine is of crucial importance in muscle building programs because it directly stimulates the protein synthesis, respectively – muscle growth. It has also a key role in the weight loss programs because in calorie deficit it protects the muscles from disintegration.