Vitamin K functions as a coenzyme in the synthesis of the active form of proteins involved in blood coagulation and bone metabolism.30 It is required to catalyze the post-translational g-carboxylation of glutamic acid residues for the normal coagulation of the blood. Carboxylated proteins include plasma prothrombin (coagulation factor II) and the plasma procoagulants, factors VII, IX, and X. Another vitamin K-dependent protein, osteocalcin, which is localized in the bone matrix, may have a role in the calcification of bone. A symptom of vitamin K deficiency is an increase in prothrombin time, which increases risk of spontaneous hemorrhage. Although vitamin K deficiencies are rare, individuals with fat malabsorption, liver diseases, or chronic antibiotic therapy are at risk of vitamin K deficiency. Newborn infants may be at risk of abnormal vitamin K status due to poor placental transfer of vitamin K and a lack of intestinal microflora to produce vitamin K2. Because of the lack of data to estimate an average requirement for vitamin K, AIs have been established based on dietary intake data from healthy individuals.
The AIs of vitamin K for adult men and women are 120 and 90 μg/d, respectively. Vitamin K1 is the major dietary form of vitamin K. Green leafy vegetables and plant oils are major sources of dietary vitamin K. Also, vitamin K1 is the form of the vitamin generally utilized in dietary supplements. No evidence of toxicity has been associated with large intakes of either vitamin K1 or vitamin K2.
