vitamin: Vitamin K
Vitamin K consists of substances that are essential for the clotting of blood. It was identified in 1934 by Danish biochemist Henrik Dam. Two types of K vitamins have been isolated: K1, an oil purified from alfalfa concentrates, and K2, synthesized by the normal intestinal bacteria. Both can be derived from the synthetic compound menadione (sometimes called vitamin K3), a yellow crystalline solid that is as potent in its ability to promote blood clotting as the natural vitamins. The best sources are leafy green vegetables, such as cabbage and spinach, and intestinal bacteria (which produce most of the body's supply of vitamin K). Vitamin K is required for the synthesis in the liver of several blood clotting factors, including prothrombin. Coumarin derivatives, used in medicine to prevent blood coagulation in certain cases, act by antagonizing the action of vitamin K. In the deficiency state an abnormal length of time is needed for the blood to clot, and there may be hemorrhaging in various tissues. Deficiency occurs in hemorrhagic disease of the newborn infant, in liver damage, and in cases where the vitamin is not absorbed properly by the intestine. It can also occur in coumarin therapy or when normal intestinal bacteria are destroyed by extended antibiotic therapy. Vitamin K does not treat hemophilia. Deficiency is rarely of dietary origin. The estimated safe and adequate intake for adults is 70 to 140 micrograms.
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