Transport of vitamin K to bone in humans

Molecules with vitamin It activity are important for optimal bone health. The major compound of this group in bone is vitamin K-1 (phylloquinone), which is derived exclusively from plant foods in the diet. Vitamin K-1 is absorbed along with dietary fat from the small intestine and transported by chylomicrons in blood. In serum obtained after an overnight fast from healthy men more than half of the vitamin K-1 was recovered from the density fraction that contains chylomicrons and chylomicron remnants (CR), and only a quarter was associated with the major lipoprotein in serum, low density lipoprotein. The concentration of vitamin K-1 in serum is closely related to the triglyceride concentration. Another determinant of vitamin K-1 concentration in serum is the presence of specific variants of apolipoprotein E (apoE). ApoE is a small protein through which the vitamin It-rich CR bind to lipoprotein receptors. The three most common variants of apoE promote CR clearance from circulation with very different efficiency, in the order E2>E3>E4. The variant that promotes CR clearance best is associated with low vitamin K-1 concentration in serum and increased response to vitamin K antagonists. Vitamin K-1 concentration in serum is linked to vitamin It status of bone. The bone protein osteocalcin tends to be less completely carboxylated in people with low vitamin K concentrations in serum. Many hemodialysis patients with a history of bone fractures have indications of poor vitamin It status. The same patients also appear to have a greatly increased prospective bone fracture risk.