menaquinone

Menaquinone, commonly called vitamin K2, is a group of fat-soluble compounds in the vitamin K family. Like vitamin K1 (phylloquinone), menaquinones function as cofactors for the gamma-carboxylation of glutamate residues in vitamin K–dependent proteins, enabling calcium binding in bone and blood vessels.

Chemically, menaquinones share a naphthoquinone ring linked to an isoprenoid side chain. The side chain length varies and the compounds are designated MK-n, where n is the number of isoprene units. The best known forms are MK-4 and MK-7, with other forms ranging up to MK-13.

Sources and synthesis: Menaquinones are produced by bacteria. In foods, MK-7 is abundant in natto (a fermented soybean product); MK-4 is found in small amounts in meat, eggs, and some cheeses. The human gut microbiota also contributes small amounts.

Absorption and distribution: As fat-soluble vitamins, menaquinones are absorbed with dietary fats and carried in chylomicrons and lipoproteins. MK-7 generally has a longer half-life and higher bioavailability than MK-4, leading to sustained gamma-carboxylation of extrahepatic proteins.

Physiological role and health considerations: Vitamin K–dependent carboxylation activates clotting factors and proteins involved in bone mineralization and vascular health. Evidence on bone health is favorable but variable; MK-7 supplementation is popular for this purpose. People on anticoagulants should consult a clinician before taking menaquinone supplements.