Vitamin K is a group of fat-soluble vitamins required for the synthesis of proteins needed for blood coagulation and for calcium binding in bones and other tissues.

These proteins undergo modification by vitamin K, which allows them to bind calcium ions. This is essential to control bleeding and without vitamin K, bleeding disorders occur. The bones may also become weak and the arteries and other tissues are susceptible to calcification.
 

Vitamin K is so named because its discovery was first reported in a German journal, where it was referred to as “Koagulations vitamin.”

Chemically, the vitamin K family is made up of 2-methyl-1 derivatives. Two natural forms of vitamin K exist, which are vitamin K1 and vitamin K2. Vitamin K1 is also called phylloquinone, phytomenadione or phytonadione and is synthesized in plants, particularly green, leafy vegetables because it is involved in photosynthesis. Vitamin K2, which is the main form stored in animals, has a number of subtypes referred to as menaquinones, homologues of the vitamin that are characterized by the different lengths of their isoprenoid side chains.

Bacteria in the large intestine can convert vitamin K1 into vitamin K2 , as well as being able to lengthen the isoprenoid side chains of vitamin K2, to give a range of vitamin K2 homologues.

People are generally able to get their daily requirement of vitamin K through a healthy and balanced diet and deficiency is rare. Furthermore, all fat-soluble vitamins such as vitamins A, D, E and K, get stored in the body’s fat tissues and are eliminated at a much slower rate than water-soluble vitamins. There are also three synthetic forms of vitamin K (vitamins K3, K4, and K5), although vitamin K3 has demonstrated toxic effects in some studies.

According to the U.S. Dietary Reference Intake, the intake of vitamin K a 25 year old man requires each day is 120 micrograms. For a woman, the amount is 90 micrograms/day; for infants 10–20 micrograms/day and for children and teenagers, the daily requirement is 15–100 micrograms/day.

Although it is possible to have an allergic reaction to vitamin K supplementation, there is no known risk of vitamin K1 or vitamin K2 having toxic effects and no tolerable upper intake level has therefore been set.