vitamin K, a group of fat-soluble vitamins known as quinones that are essential for the synthesis of prothrombin in the liver and of several related proteins involved in the clotting of blood. The vitamin is widely distributed in foods, especially leafy green vegetables, pork liver, yogurt, egg yolk, kelp, alfalfa, fish-liver oils, and blackstrap molasses, and is synthesized by the bacterial flora of the GI tract. It is also produced synthetically. Deficiency results in hypoprothrombinemia, characterized by poor coagulation of the blood and hemorrhage, and usually occurs from inadequate absorption of the vitamin from the GI tract or the inability to use it in the liver. It is used to reduce the clotting time in patients with obstructive jaundice and in hemorrhagic states associated with intestinal diseases and diseases of the liver; it is given prophylactically to infants to prevent hemorrhagic disease of the newborn. Natural vitamin K is stored in the body and produces no toxicity. Excessive doses of synthetic vitamin K may cause anemia in newborns and hemolysis in people with glucose-6-phosphate deficiency. See also vitamin K1, vitamin K2, menadione.
