Effects of vitamin K on calcium and bone metabolism

The K vitamins, a group of napthoquinones, are required for the carboxylation of a limited number of proteins including the bone matrix protein osteocalcin. Vitamin K, (phylloquinone) and vitamin K-2 (menaquinones), differ regarding food source (green vegetables and fermented products, respectively), bioavailability and intermediate metabolism. Epidemiological studies provide evidence for an association between a low vitamin K intake and an enhanced osteoporotic fracture risk. Doses of vitamin K-1 up to 15 times the current recommended dietary allowance have successfully been used to reduce the percentage of undercarboxylated osteocalcin in the circulation. Studies demonstrating clear beneficial effects on bone health, however, are still lacking. In contrast, therapy with very high pharmacological doses of the vitamin K-2 menatetrenone has impressively been used to prevent further bone mineral loss and fracture risk in osteoporotic patients. Curr Opin Clin Nutr Metab Care 4:483-487. (C) 2001 Lippincott Williams Wilkins.