Source Naturals L-Lysine is an essential free-form amino acid which acts as a precursor for several other amino acids, including L-Carnitine (needed for fat metabolism). L-Lysine is crucial to 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 principle fuels for the Krebs cycle.