Vitamin K and Bone
The human diet contains different forms of vitamin K. Green vegetables, notably spinach, kale, broccoli, and sprouts, are the main sources of phylloquinone (vitamin K1); menaquinones (vitamin K2) form a family of closely related compounds produced by a number of microorganisms including lactic acid and propionic acid bacteria, as well as by Bacillus subtilis natto. In Europe and Northern America, cheese and curd cheese are the main dietary sources of vitamin K2, whereas in Japan, natto (fermented soy beans), a popular food, is extremely rich in menaquinone-7, one of the long-chain menaquinones (Booth et al., 1993; Shearer and Bolton-Smith, 2000; Schurgers and Vermeer, 2000). Also, bacteria in the gut produce large amounts of vitamin K2, but hardly any of it is absorbed because of the lack of bile salts at the site of production. Dietary vitamin K2 accounts for approximately 10% of the total vitamin K intake, but its better absorption and much longer half-life than K1 make it an important contributor to total human vitamin K status.