There are several indications for using chronic oral anticoagulation to prevent adverse outcomes from thromboembolic phenomenon. Because of its tendency to be erratic and the risk of bleeding, physicians are reluctant to use oral anticoagulation. Despite efforts to establish protocols and monitor International Normalized Ratios (INRs), many patients still have levels that are outside therapeutic ranges. Part of this variation may be related to fluctuations that occur within the patient or to new medications such as analgesics and antipyretics. Consumption of alcohol or vitamin K also can alter INRs. Despite its potential influence on INRs, the impact of dietary vitamin K has been evaluated primarily with uncontrolled case series, case reports, or small retrospective studies. Franco and colleagues assessed the effect of dietary intake of vitamin K on anticoagulation stability in patients receiving chronic oral anticoagulation therapy.

The trial was a prospective, randomized, crossover study of 12 patients in an outpatient anticoagulation clinic. Before the study, vitamin K-rich food intake was assessed using a recall method. Intake of broccoli, cauliflower, green tea, liver, cabbage, green peas, lettuce, spinach, watercress, spring greens, and other greens was assessed. The patients were then assigned to greater than usual intake, usual intake, or less than usual intake groups. If patients had a stable INR within the therapeutic range and were on a stable oral anticoagulation dosage, they were randomly assigned to a diet rich (500 percent increase) or poor (80 percent decrease) in vitamin K for four consecutive days. After a one- to two-week washout period, the patients were then switched to the other diet. All meals were prepared by a dietician, and between-meal snacking was limited. INRs were obtained at the start of the study, day 4, and day 7 of each intervention.

There was a statistically significant inverse association between vitaminK intake score and different anticoagulation levels. Vitamin K intake also was found to be independently associated with over- and undercoagulation. In the patients who had the vitamin K-depleted diet, the INR increased from a mean of 2.6 to 3.3 by day 7, which was a significant increase. Patients who received the vitamin K-enriched diet had an INR that dropped from a mean of 3.1 to 2.8 by day 4.

The authors conclude that vitamin K interacts with coumadin, interfering with anticoagulation stability. They add that stable intake of vitamin K is an essential component of anticoagulation therapy and should be addressed by health care professionals who are educating patients about coumadin therapy.