L-Lysine-d3 is intended for use as an internal standard for the quantification of L-lysine by GC- or LC-MS. L-Lysine is an essential amino acid. It is catabolized via the saccharopine pathway to produce acetyl-coenzyme A in liver mitochondria. L-Lysine is also a precursor in the biosynthesis of L-carnitine , a conditionally essential nutrient that has roles in energy production and fatty acid metabolism. L-Lysine (400 mg/kg per day) reduces viral shedding in cats latently infected with feline herpesvirus type 1 (FHV-1).Administration of an L-lysine-deficient diet increases fecal pellet excretion induced by restraint stress, indicating increases in anxiety, in rats. Formulations containing L-lysine have been used as dietary supplements.