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

Chemically, the vitamin K family is made up of 2-methyl-1 derivatives. Two natural forms of Vitamin K exist, namely 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 K1 is also abundant in various fruits such as avocado and kiwi fruit and some herbs contain very high amounts of vitamin K. For example, two tablespoons of parsley is known to contain more than 150% of the recommended daily amount of vitamin K.

Vitamin K2, which is the main form stored in animals, has is 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.

There are also three synthetic forms of vitamin K, which are vitamins K3, K4, and K5, although vitamin K3 has shown toxic effects in some studies.

 

As a fat-soluble vitamin, absorption of vitamin K1 is often greater in the presence of butter or oil. The tight binding of vitamin K1 to thylakoid membranes in chloroplasts means its bioavailability is poor unless fats are added. The bioavailability of vitamin K is 5% in cooked spinach, for example, whereas adding fat to the spinach increases the bioavailability to 13%.

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 would be 90 micrograms/day; for infants 10–20 micrograms/day and for children and teenagers, the daily requirement would be 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.

Newborn babies are at risk of vitamin K deficiency because they do not possess the gut bacteria required to help synthesise this vitamin. Furthermore, vitamin K does not cross the placenta to reach the fetus during pregnancy and human milk is low in this vitamin, meaning breast feeding does not supply adequate amounts to the newborn.

Breast-fed babies can be low in vitamin K for several weeks until their guts start to develop the bacteria required to synthesize vitamin K2. These babies are at risk of a condition called hemorrhagic disease of the newborn, which can lead to severe bleeding that sometimes causes brain damage. As a preventative measure, babies are routinely given vitamin K injections after birth. Infant formula is made to contain vitamin K, but even formula-fed babies may be low in vitamin K for a few days.