Vitamin K2

Vitamin K₂ or menaquinone (/ˌmɛnəˈkwɪnoʊn/) has nine related compounds, generally subdivided into the short-chain menaquinones (with MK-4 as the most important member) and the long-chain menaquinones, of which MK-7, MK-8 and MK-9 are nutritionally the most recognized.

Vitamin K₂, the main storage form in animals, has several subtypes, which differ in isoprenoid chain length. These vitamin K₂ homologues are called menaquinones, and are characterized by the number of isoprenoid residues in their side chains. Menaquinones are abbreviated MK-n, where M stands for menaquinone, the K stands for vitamin K, and the n represents the number of isoprenoid side chain residues. For example, menaquinone-4 (abbreviated MK-4) has four isoprene residues in its side chain. Menaquinone-4 (also known as menatetrenone from its four isoprene residues) is the most common type of vitamin K₂ in animal products since MK-4 is normally synthesized from vitamin K₁ in certain animal tissues (arterial walls, pancreas, and testes) by replacement of the phytyl tail with an unsaturated geranylgeranyl tail containing four isoprene units, thus yielding menaquinone-4. This homolog of vitamin K₂ may have enzyme functions distinct from those of vitamin K₁.

Menaquinone-7 is different from MK-4 in that it is not produced by human tissue. MK-7 may be converted from phylloquinone (K₁) in the colon by E. coli bacteria. However, bacterially derived menaquinones (MK-7) appear to contribute minimally to overall vitamin K status. MK-4 and MK-7 are both found in the United States in dietary supplements for bone health.

The U.S. Food and Drug Administration (FDA) has not approved any form of vitamin K for the prevention or treatment of osteoporosis; however, MK-4 has been shown to decrease the incidence of fractures up to 87%. MK-4 (45 mg daily) has been approved by the Ministry of Health in Japan since 1995 for the prevention and treatment of osteoporosis.

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