L-Lysine is an essential amino acid that must be obtained through the diet or through supplementation. Lysine is necessary for the production of all proteins in the body, and is required for the maintenance of the structural proteins collagen and elastin, which form all connective tissue such as skin, tendon, and bone. L-Lysine is also a precursor to carnitine, which is needed for fat metabolism and energy production. In addition, L-Lysine may help to support a healthy immune system and proper cardiovascular function.