The various forms of vitamin K are all fat-soluble molecules, each with the same methylated naphthoquinone ring structure. The two main forms of vitamin K 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 K1 contains the functional naphthoquinone ring, an aliphatic side chain and a phytl side chain.
Vitamin K2, which is the main form stored in animals, has a number of subtypes referred to as menaquinones, homologues that are characterized by the different lengths of their isoprenoid side chains. The most common number of isoprenoid residues is four because enzymes present in animals can convert vitamin K1 into menaquinone-4. This occurs in tissues such as the pancreas, arterial walls and testes and is achieved through replacement of the phytl side chain with an unsaturated geranylgeranyl tail that contains the four isoprene residues.
Menaquinones are represented by the abbreviation MK-n, where the M represents menaquinone, K stands or vitamin K and n represent the number of residues in the isoprenoid chain.
As a fat-soluble vitamin, the 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 it has a poor bioavailability unless fats are added. The bioavailability of this vitamin is 5% in cooked spinach, for example, whereas adding fat to the spinach increases the bioavailability to 13%.
People are generally able to get their daily requirement of vitamin K through a healthy and balanced diet. 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.