Vitamin K is known primarily for its effect on blood clotting. However, this vitamin is also required to synthesize osteocalcin, a protein found uniquely and in large amounts in bone. Osteocalcin is the protein matrix upon which calcium crystallizes. The component of osteocalcin that attracts calcium ions is a modified amino acid, gamma-carboxyglutamic acid, formed by the vitamin K-dependent carboxylation of glutamic acid. Because of its role in osteocalcin production, vitamin K is essential for bone formation, remodeling, and repair.
It is generally assumed that vitamin K deficiency is rare. However, assessment of vitamin K status is based on relatively insensitive tests, such as prothrombin time. Recent advances have made it possible to measure vitamin K levels in blood. In a series of 16 patients with osteoporosis, mean serum vitamin K concentration was only 35% that of age-matched controls. If osteocalcin synthesis is sensitive to changes in serum vitamin K levels, then the low levels in osteoporotic patients may have clinical significance. That possibility was supported by a recent study in which vitamin K supplementation of a typical Western diet increased urinary excretion of gamma-carboxyglutamic acid by 23%.
Vitamin K deficiency is probably more common than previously believed. Deficiency may occur in individuals whose vegetable consumption is low. Another factor that could promote deficiency is frequent use of antibiotics, which can destroy naturally occurring vitamin K-producing bacteria in the intestines.
Rats fed a vitamin K deficient diet had significantly increased urinary calcium excretion. Furthermore, vitamin K supplementation accelerated the healing of experimental fractures in rabbits, even though they were already receiving "adequate" levels in their diet. In a preliminary study of osteoporotic patients, treatment with vitamin K reduced urinary calcium loss by 18-50%. The evidence suggests that, when accelerated bone formation is desirable, as in osteoporosis or after a fracture, a greater amount of vitamin K is required.