Summary
The effect of dietary vitamin K intake on warfarin sensitivity is known only from case reports and few small clinical studies. We followed 50 patients commencing warfarin and consuming their regular diets (for 8 weeks) to study this relationship. A one-week recall dietary questionnaire was completed at weeks 2 and 8. Daily intake of nutrients and vitamin K was calculated from standard tables. Warfarin sensitivity index (WSI) was defined as final INR/final warfarin dose (mg/day/m2 of body surface area) (week 8).
Vitamin K intake was 17-974 (median: 179) μg/day. Median WSI was 0.82 (0.31-4.47). A WSI value of 1.1 significantly separated excess (≥250 μg/day) from normal (<250 μg/day) vitamin K consumers (16/18 vs. 15/32, respectively, p <0.01). The former had lower day 5 INR (median: 1.9 vs. 3.0, p <0.001), needed more warfarin to achieve INR ≥2.0 (32.0 ± 9.2 mg vs. 25.4 ± 6.4 mg, p = 0.009) and required a higher maintenance steady state warfarin dose (5.7 ± 1.7 mg/day vs. 3.5 ± 1.0 mg/day, p <0.001).
We conclude that in 32% (16/50) of anticoagulated patients under usual dietary conditions sensitivity to warfarin is decreased by vitamin K intake ≥250 μg/day.
