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This is a fat-soluble vitamin, and rarely mentioned as it's seldom deficient in the human body. It occurs in three forms: K1 (phylloquinone) - the natural vitamin K from plants; K2 (menaquinone) - derived from intestinal bacteria; and K3 (menadione) - a synthetic version available for those who can't absorb it from food.
The best natural sources are obtained from cultured milk products such as yogurt and buttermilk, as well as vegetable oils; and green vegetables - such as broccoli, cabbage and lettuce. Antibiotics and over-consumption of sugar and sweets inhibit vitamin K absorption.
Deficiency symptoms include delayed blood-clotting of wounds, haemorrhages such as nose bleeds, and a low level of blood platelets. Deficiencies are usually caused by a defect in metabolism - such as a malfunction of the liver.
There is no official Recommended Daily Allowance (RDA) for this vitamin, but 300 mcg is generally considered adequate for an adult. See 'Vitamin', 'Vitamin B Complex', 'Vitamin B2', 'VitaminB5', 'Vitamin B6', 'Vitamin B12','Vitamin B15'; 'Vitamin C', 'Vitamin P'. For more information on nutrition, see 'Nutrients A to Z' by Dr. Michael Sharon; Prion Books.