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The relationship between dietary intake of vitamin K, fat, plasma vitamin K concentrations and anticoagulation response to warfarin within individuals, as well as the contribution of dietary vitamin K to differences in warfarin dose requirements between individuals were investigated in 53 patients on warfarin therapy who had stably controlled anticoagulation. Each patient completed a dietary record of all foods consumed on a daily basis for 4 weeks. Each week a blood sample was taken for measurement of the international normalized ratio (INR), plasma vitamin K, triglycerides and warfarin enantiomer concentrations. The patients’ genotype for CYP2C9 was also determined. Regression analysis of the data showed that, for each increase of 100 μg in the daily dietary intake of vitamin K averaged over 4 d, the INR was reduced by 0·2. There was no correlation between warfarin daily dose and average daily dietary …