Vitamin K: One of two naturally occurring fat-soluble vitamins (vitamin K1 and vitamin K2) needed for the clotting of blood because of an essential role in the production of prothrombin (a clotting factor). The term vitamin A may also refer to a synthetic compound that is closely related chemically to the natural vitamins K1 and K2 and has similar biological activity.
Vitamin K is required (as a cofactor) for the body to make four of the blood's coagulation (clotting) factors: particularly prothrombin and also factors VII, IX, and X.
Vitamin K1 is made by plants, whereas vitamin K2 is of bacterial origin and is the important form for people. All other forms of vitamin K are converted to vitamin K2 in the body. There are a number of closely related compounds of the vitamin K2 series.
Vitamin K deficiency only rarely occurs because an adequate supply of the vitamin is usually present in the diet and the vitamin is synthesized by bacteria in the intestine. Deficiency of vitamin K may occur following the administration of certain drugs that inhibit the growth of the vitamin-synthesizing bacteria or as a result of disorders affecting the production or flow of bile which is necessary for the intestinal absorption of vitamin K. In newborn babies, the absence of large intestinal bacteria coupled with the absence of body stores of vitamin K may result in hemorrhagic disease of the newborn. This is a dangerous condition because there can be bleeding into critical organs such as the brain. This disorder can be prevented by the administration of vitamin K to the baby shortly after birth or to the mother during labor.
A fat-soluble substance present in green leafy vegetables was found in 1929 to be needed for coagulation of the blood to take place. The substance came to be named vitamin K. The K was for Koagulation (Danish for coagulation). A pure form of the vitamin was isolated and analyzed in 1939. Several related compounds with vitamin K activity have also been synthesized.
The 1943 Nobel Prize in physiology or medicine was shared by the Danish researcher Henrik Carl Peter Dam (1895-1976) for his (original) discovery of vitamin K and the American worker Edward Adelbert Doisy (1893-1986) for his discovery of the chemical nature of vitamin K.