Lysine is an amino acid that is essential for protein synthesis. Because it s not synthesized in the body, it must be obtained in the diet. The primary dietary sources of lysine are meat, poultry, and milk; therefore vegans and vegetarians can benefit from supplementation. Lysine has many biological functions, including the synthesis of connective tissues such as bone, skin, collagen, and elastin; the synthesis of carnitine, which converts fatty acids to energy; support for muscle growth and recovery; and maintenance of healthy immune function.