This research dates from a period when warfarin was the primary oral anticoagulant in clinical use. At that time, patients were routinely warned to avoid foods rich in vitamin K, such as spinach and other green leafy vegetables. Newer anticoagulant medications now exist that do not act through vitamin K and do not require the same dietary restrictions.
Anticoagulant medications such as warfarin work by interfering with the body’s utilization of vitamin K. Because of this mechanism, patients taking warfarin have traditionally been advised to avoid foods and supplements that contain vitamin K.
Research published in Blood (November 1, 2004; Vol. 104, No. 9, pp. 2682–2689) examined the effects of vitamin K supplementation in healthy subjects who were stably taking warfarin. Participants were given vitamin K1 supplements, starting at 50 micrograms per day for one week, with the dose gradually increased each week.
The researchers found that doses below 150 micrograms per day did not interfere with anticoagulant therapy. Meals containing foods naturally high in vitamin K also had no effect on anticoagulation control. At doses above 150 micrograms per day, the INR (International Normalized Ratio—a measure of blood clotting tendency) decreased, meaning the blood clotted more readily. The authors concluded that consuming vitamin K–rich foods, such as spinach and broccoli, or taking a multivitamin containing vitamin K did not interfere with warfarin therapy when intake remained within moderate ranges.
Additional research appearing in Blood (March 15, 2007; Vol. 109, No. 6, pp. 2419–2423) found that patients with unstable warfarin control—defined by wide fluctuations in INR—tended to be deficient in vitamin K. In this study, patients were randomly assigned to receive either a placebo or 150 micrograms of vitamin K daily for six months.
Patients receiving vitamin K experienced less variation in anticoagulation control compared to those receiving placebo. Among 70 patients with unstable warfarin control, 34 of those receiving vitamin K supplementation showed improved stability, with 19 meeting the study’s criteria for stable anticoagulation. In contrast, only seven patients in the placebo group achieved stable control.
The authors concluded that vitamin K supplementation may improve anticoagulation stability in patients with poorly controlled warfarin therapy. Patients taking warfarin should discuss dietary and supplement use with their physician and should not self-medicate with vitamin K.
