vitamin (role) Vitamin K CHEBI:28384 The term "vitamin K" refers to a group of chemically similar fat-soluble compounds called naphthoquinones: vitamin K1 (phytonadione) is found in plants and is the primary source of vitamin K for humans through dietary consumption, vitamin K2 compounds (menaquinones) are made by bacteria in the human gut, and vitamin K3 (menadione) is a water-soluble preparation available for adults only. Vitamin K is necessary for the liver to produce the coagulation factors II, VII, IX, and X, as well as the clotting factors protein C, protein S, and protein Z; vitamin K deficiency can result in deficiencies of these coagulation factors and excess bleeding. An injection of vitamin K is routinely given to newborn infants to prevent vitamin K deficiency bleeding, also known as hemorrhagic disease of the newborn. Vitamin K deficiency is rare in adults but may result from chronic malnutrition or an inability to absorb dietary vitamins. NCIT:C943