Vitamin K includes a group of compounds that contain a central ring structure with varying side chains. These differing side chains account for variations in absorption, bioavailability, transport and tissue distribution. Vitamin K1, phylloquinone, is the most common form and is found in plants and green vegetables. The vitamin K2 family, menaquinones, are less common and are found in meats, eggs, some cheeses, and fermented food, such as Japanese natto. The most common menaquinones are menaquinone–4 (MK–4) and menaquinone–7 (MK–7).
Studies indicate that vitamin K1 and MK–4 are primarily concentrated in the liver, while MK–7 concentrates in extra–hepatic tissues, including vascular tissues, and has a longer half–life. Vitamin K enhances bone formation by carboxylating osteocalcin. This enables osteocalcin to bind to calcium and promote healthy bone mineralization. Adequate intake of vitamin K1 and K2 may be especially important for optimal carboxylation in aging individuals, as indicated by a recent study involving Japanese women. By activating matrix Gla–protein (MGP), vitamin K also helps maintain healthy calcium metabolism in vascular tissues, supporting healthy vascular elasticity. Recent studies indicate that vitamin D also potentiates the activity of MGP and that the combination of vitamin K and D provides synergistic support for blood vessel function and bone health. Vitamin K supplementation is contraindicated for those taking Coumadin/warfarin anticoagulant medication.
