Influence of vitamin k on anticoagulant therapy depends on vitamin k status and the source and chemical forms of vitamin k.

Warfarin therapy requires close monitoring to avoid excessive bleeding and to maintain the effective therapeutic concentration assessed with the internationalized ratio (INR). High vitamin K intake can decrease the therapeutic effectiveness of warfarin, while poor vitamin K status appears to increase the sensitivity to small changes in vitamin K intake, especially from supplements. Very large amounts of vitamin K from a single meal with vegetables (400 g of vegetables with 700 to 1500 microg of vitamin K1) can measurably change INR, but occasional typical servings (<100 g) would probably have little lasting impact on INR. Warfarin requirements may change in those altering their intake of dark-green vegetables. The 2005 Dietary Guidelines for Americans recommends 3 cups/week of dark-green vegetables, which contain about 100 to 570 microg/serving of vitamin K1. Less well-known sources and chemical forms of vitamin K, such as MK-7 in natto (a fermented Japanese product), also measurably influence INR. Additional research is needed in warfarin-treated patients to fully quantify the interactions among various sources and chemical forms of vitamin K, age, genotype, and other factors.