The effect of dietary vitamin K on warfarin‐induced anticoagulation

We examined the effect of vitamin-K-rich vegetables, vitamin-K-poor vegetables and phytomenadione on the stability of warfarin-induced anticoagulation. Patients on stable anticoagulant treatment were randomized to either 1 (n = 5), 2 (n = 7) or 7 (n = 13) d with high intake of vitamin-K-rich vegetables (median daily vitamin K intake 1100 micrograms) or high intake of vitamin-K-poor vegetables (daily vitamin K intake 135 micrograms) for 6 d (n = 7), or habitual diet supplemented with 1000 micrograms of phytomenadione daily (n = 5). Nine patients (69%; 95% CI, 39-91%) who consumed vitamin-K-rich vegetables for 7 d reached activities above the therapeutic level. Two (40%; 95% CI, 5-85%) and three patients (43%; 95% CI, 10-86%) who consumed vitamin-K-rich vegetables for 1 and 2 d, respectively, exceeded the upper therapeutic limit. No changes were observed in the vitamin-K-poor group. All patients who received phytomenadione exceeded the upper therapeutic limit. Dietary vitamin K should be regarded as an important environmental factor contributing to unwanted disturbances in warfarin-induced anticoagulation.