Interaction Between Dietary Vitamin K Intake and Anticoagulation by Vitamin K Antagonists: Is It Really True?

Educational advice is often given to patients starting treatment with vitamin K Antagonists (VKAs). A great emphasis is made on nutritional information. Common belief is that dietary vitamin K intake could counteract the anticoagulant effect by VKAs and for many years, patients have been discouraged to consume vitamin-K-rich foods, such as green leafy vegetables.The objective of this study is to summarize the current evidence supporting the putative interaction between dietary vitamin K intake and changes in INR with the VKAs.Data sources are MEDLINE via PubMed and Cochrane database.All clinical studies investigating the relationship between dietary vitamin K and measures of anticoagulation were included. We excluded all studies of supplementation of vitamin K alone.We performed a systematic review of the literature up to October 2015, searching for a combination of "food," "diet," "vitamin K," "phylloquinone," "warfarin," "INR," "coagulation," and "anticoagulant."Two dietary interventional trials and 9 observational studies were included. We found conflicting evidence on the effect of dietary intake of vitamin K on coagulation response. Some studies found a negative correlation between vitamin K intake and INR changes, while others suggested that a minimum amount of vitamin K is required to maintain an adequate anticoagulation. Median dietary intake of vitamin K1 ranged from 76 to 217 μg/day among studies, and an effect on coagulation may be detected only for high amount of vitamin intake (>150 μg/day).Most studies included patients with various indications for VKAs therapy, such as atrial fibrillation, prosthetic heart valves, and venous thromboembolism. Thus, INR target was dishomogeneous and no subanalyses for specific populations or different anticoagulants were conducted. Measures used to evaluate anticoagulation stability were variable.The available evidence does not support current advice to modify dietary habits when starting therapy with VKAs. Restriction of dietary vitamin K intake does not seem to be a valid strategy to improve anticoagulation quality with VKAs. It would be, perhaps, more relevant to maintain stable dietary habit, avoiding wide changes in the intake of vitamin K.