L-Lysine is an essential amino acid which acts as a precursor for several other amino acids, including l-citrulline (needed in the body for normal protein metabolism) and l-carnitine (needed for fat metabolism). l-Lysine is crucial for the formation of collagen, a major part of the body’s connective tissues. l-Lysine also contributes to energy production when converted to acetyl coenzyme A, one of the principal fuels for the Krebs cycle.