Vitamin K is a family of structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements.

[2] The complete synthesis involves final modification of these so-called "Gla proteins" by the enzyme gamma-glutamyl carboxylase that uses vitamin K as a cofactor.

Research suggests that deficiency of vitamin K may also weaken bones, potentially contributing to osteoporosis, and may promote calcification of arteries and other soft tissues.

[4] Vitamin K2, in turn, consists of a number of related chemical subtypes, with differing lengths of carbon side chains made of isoprenoid groups of atoms.

Vitamin K1 is made by plants, and is found in highest amounts in green leafy vegetables, being directly involved in photosynthesis.

These are similar in structure in that they share a quinone ring, but differ in the length and degree of saturation of the carbon tail and the number of repeating isoprene units in the side chain (see figures in Chemistry section).

[1][6][7] Vitamin K has several roles: an essential nutrient absorbed from food, a product synthesized and marketed as part of a multi-vitamin or as a single-vitamin dietary supplement, and a prescription medication for specific purposes.

[9] The European Union and Japan also reviewed safety and concluded – as had the United States – that there was insufficient evidence to set an upper limit for vitamin K.[9][10] For US food and dietary supplement labeling purposes, the amount in a serving is expressed as a percentage of daily value.

According to the Global Fortification Data Exchange, vitamin K deficiency is so rare that no countries require that foods be fortified.

[19] Bleeding in infants due to vitamin K deficiency can be severe, leading to hospitalization, brain damage, and death.

Intramuscular injection, typically given shortly after birth, is more effective in preventing vitamin K deficiency bleeding than oral administration, which calls for weekly dosing up to three months of age.

They block the regeneration and recycling of vitamin K. Some of the 4-hydroxycoumarin anticoagulant class of chemicals are designed to have high potency and long residence times in the body, and these are used specifically as second generation rodenticides ("rat poison").

[34] An increase in prothrombin time, a coagulation assay, has been used as an indicator of vitamin K status, but it lacks sufficient sensitivity and specificity for this application.

[40] The US Food and Drug Administration has banned this form from sale as a human dietary supplement because overdoses have been shown to cause allergic reactions, hemolytic anemia, and cytotoxicity in liver cells.

[42] Vitamin K1 appears as a yellow viscous liquid at room temperature due to its absorption of violet light in the UV-visible spectra obtained by ultraviolet–visible spectroscopy.

[43] The structures of menaquinones, vitamin K2, are marked by the polyisoprenyl side chain present in the molecule that can contain four to 13 isoprenyl units.

[5] The large size of Vitamin K1 gives many different peaks in mass spectroscopy, most of which involve derivatives of the naphthoquinone ring base and the alkyl side chain.

[49] 17 human proteins with Gla domains have been discovered; they play key roles in the regulation of three physiological processes: Vitamin K is absorbed through the jejunum and ileum in the small intestine.

Estimates for absorption are on the order of 80% for vitamin K1 in its free form (as a dietary supplement) but much lower when present in foods.

[65] These snails produce a venom containing hundreds of neuroactive peptides, or conotoxins, which is sufficiently toxic to kill an adult human.

In plants but not cyanobacteria, knockout of this gene show growth restriction similar to mutants lacking the ability to produce K1.

In the vitamin K2 synthesizing bacteria, menaquinone transfers two electrons between two different small molecules, during oxygen-independent metabolic energy production processes (anaerobic respiration).

[70] Some of these reactions generate a cellular energy source, ATP, in a manner similar to eukaryotic cell aerobic respiration, except the final electron acceptor is not molecular oxygen, but fumarate or nitrate.

[70] In 1929, Danish scientist Henrik Dam investigated the role of cholesterol by feeding chickens a cholesterol-depleted diet.

Three groups of physicians independently found this: Biochemical Institute, University of Copenhagen (Dam and Johannes Glavind), University of Iowa Department of Pathology (Emory Warner, Kenneth Brinkhous, and Harry Pratt Smith), and the Mayo Clinic (Hugh Butt, Albert Snell, and Arnold Osterberg).

[76] The first published report of successful treatment with vitamin K of life-threatening hemorrhage in a jaundiced patient with prothrombin deficiency was made in 1938 by Smith, Warner, and Brinkhous.

[79] The risk of osteoporosis, assessed via bone mineral density and fractures, was not affected for people on warfarin therapy – a vitamin K antagonist.

[80] Studies investigating whether vitamin K supplementation reduces risk of bone fractures have shown mixed results.

[83] These observations led to a theory that in humans, inadequately carboxylated MGP, due to low dietary intake of the vitamin, could result in increased risk of arterial calcification and coronary heart disease.

[4] From a review of observational trials, long-term use of vitamin K antagonists as anticoagulation therapy is associated with lower cancer incidence in general.