Vitamin K is known as the antihemorrhagic vitamin.
Three compounds which have the biological activity of vitamin K are
1. Phylloquinone, which is the normal dietary source, being found in green leafy vegetables.
2. Menaquinones, which are a family of closely related compounds synthesized by the intestinal bacteria, with differing lengths of the side chain.
3. Menadione, a synthetic compound which can be metabolised to yield phylloquinone (Fig 8.4).
Vitamin K is needed for the formation of prothrombin, a substance necessary for blood clotting.
Intestinal bacteria normally synthesize substantial amount of vitamin K. Because vitamin K is fat soluble, its absorption is facilitated in the presence of bile. Small amount of vitamin k are stored in the liver, heart, skin, muscle and kidneys.
The best source of vitamin K are the green leafy vegetables eg. spinach, cabbage, kale etc. Good sources are cauliflower, wheat germ, etc. Carrots and potatos are fair sources. Milk, meat and fish are poor sources.
Vitamin K requirement depends on the amount of vitamin K formed by the intestinal bacteria. The more the endogenous vitamin K formation less will be the dietary requirement.
Being fat-soluble, its absorption is enhanced by sufficient amount of bile salts mainly in the jejunum by the way of lymphatics. Liver stores appreciable amounts. It is present in blood stream in significant amount. All tissues contain small amounts of vitamin K.
The deficiency of vitamin K leads to a lowering of prothrombin level and increased clotting time of blood. This may lead to hemorrhagic conditions. Vitamin K deficiency causes hemorrhagic disease of the newborn.