Definition:

• Vitamin K: Naturally occurring forms of vitamin K include phylloquinone (vitamin K₁) and a family of molecules called menaquinones (MKs or vitamin K₂).
• Vitamin K2 or menaquinones are classified according to the number of isoprenyl units on the side chain. Thus, vitamin K2 with 4 isoprenyl units is called menaquinone-4 (MK-4; also known as menatetrenone), and vitamin K2 with 7 units is called menaquinone-7 (MK-7)
• Vitamin K is the essential cofactor for the carboxylation of glutamic acid residues in many vitamin K-dependent proteins (VKDPs) that are involved in blood coagulation, bone metabolism, prevention of vessel mineralization, and regulation of various cellular functions.
• Vitamin K deficiency increases the risk of excessive bleeding (hemorrhage). An injection of vitamin K is recommended to protect all newborns from life-threatening bleeding within the skull
• Vitamin K deficiency may impair the activity of VKDPs and increase the risk of osteoporosis and fractures. Some, but not all, observational studies have found lower vitamin K status to be associated with lower bone mineral density and higher fracture incidence. Overall, intervention trials have been inconclusive regarding the role of supplemental vitamin K (vitamin K₁ or vitamin K₂) in further reducing bone loss in otherwise calcium– and vitamin D–replete
• Abnormal mineralization of blood vessels increases with age and is a major risk factor for cardiovascular disease. Vitamin K inadequacy may inactivate several VKDPs that inhibit the formation of calcium precipitates in vessels. There is only limited evidence of the benefit of supplemental vitamin K in the prevention of vessel calcification and cardiovascular
• Phylloquinone (K1) is found at high concentrations in green leafy vegetables and certain plant oils, while most menaquinones are usually found in cheese, animal products, and certain fermented

Drug interactions

• Treatment with antibiotics can decrease the number of bacteria in the intestinal tract that synthesizes vitamin K.30 When antibiotic therapy is combined with decreased oral intake, a hemorrhagic tendency may develop.
• Anticonvulsants may decrease vitamin K levels by decreasing the half-life of the vitamin.
• The stress of highdose chemotherapy may lead to temporary vitamin K deficiency by decreasing appetite and interfering with intestinal absorption and hepatic metabolism of vitamin K.
• Warfarin works by inhibiting the vitamin K-dependent activation of coagulation factors II, VII, IX, and X. Increasing vitamin K intake inhibits the action of warfarin, whereas decreasing vitamin K intake has the opposite effect. For this reason, patients taking warfarin should keep their dietary intake of vitamin K consistent. In patients taking warfarin, supplementation with 100–150 μg/day of vitamin K1 resulted in fewer fluctuations of the INR outside the normal range, thereby reducing the risk of thrombotic events resulting from undertreatment and the risk of hemorrhagic events resulting from overtreatment.

Dosage and administration

• Vitamin K1 supplements are usually administered in dosages of 100–1,000 μg/day.
• MK-4 has frequently been used at a dosage of 45 mg/day. Some commercially available supplements provide 45–90 μg of MK-7. Vitamin K should be taken with a meal that contains fat.