Avoidance of Vitamin K-Rich Foods Is Common among Warfarin Users and Translates into Lower Usual Vitamin K Intakes.

Background: Warfarin users should aim for stable daily vitamin K intakes. However, some studies report that patients are often advised to avoid eating green vegetables. Whether this advice impacts vitamin K intakes is unknown.

Objective: Our aim was to describe the nature and sources of vitamin K-related dietary recommendations that patients received at the initiation of warfarin therapy, assess their adherence to these recommendations, and examine whether usual vitamin K intakes vary according to these recommendations.

Design: We conducted a retrospective cohort study with patients enrolled in the Québec Warfarin Cohort Study. Patients were asked to report dietary recommendations they had received at warfarin initiation and their adherence to these recommendations. Usual vitamin K intakes were assessed using a validated semi-quantitative food frequency questionnaire.

Participants/setting: Three hundred seventeen patients aged 36 to 97 years who initiated warfarin between 2011 and 2012 and were treated for 12 months or longer with a target international normalized ratio range of 2.0 to 3.0 or 2.5 to 3.5.

Statistical Analyses Performed: Patients were classified according to vitamin K-related recommendations reported: limit or avoid vitamin K-rich foods; aim for stable consumption of vitamin K-rich foods; or no vitamin K-related advice. A one-way analysis of covariance was used to compare mean usual vitamin K intakes between patients after adjustment for covariates.

Results: Most patients (68%) reported being advised to limit or avoid vitamin K-rich foods, particularly green vegetables, 10% reported being advised to aim for stable consumption of vitamin K-rich foods, and 22% did not recall receiving any vitamin K-related recommendation. Mean usual vitamin K intakes of patients adhering to the recommendation to limit or avoid vitamin K-rich foods was 35% to 46% lower than those of other patients (P<0.001), a difference resulting almost entirely (82%) from a lower consumption of green vegetables.

Conclusions: In contrast with current dietary recommendation, most warfarin users reported avoiding vitamin K-rich foods, which translated into lower usual vitamin K intakes.