Lysine, or L-lysine, is an essential amino acid, which means that it is necessary for human health but cannot be made by the body. For this reason, lysine must be obtained from food. Amino acids are the building blocks of protein. Lysine is important for proper growth, and it plays an essential role in the production of carnitine, a nutrient responsible for converting fatty acids into energy and helping to lower cholesterol. Lysine appears to help the body absorb and conserve calcium, and it plays an important role in the formation of collagen, a substance important for bones and connective tissues including skin, tendon, and cartilage.