Vitamin K and Warfarin Interaction

Mechanism of Action

Warfarin works by inhibiting vitamin K-dependent clotting factors (II, VII, IX, X) through interference with the vitamin K epoxide reductase cycle, and green leafy vegetables contain high amounts of vitamin K (phylloquinones) that can bypass warfarin's mechanism of action 1
One cup of raw spinach contains 144.9 µg of vitamin K, while one cup of cooked frozen spinach contains 1027.3 µg, highlighting the significant vitamin K content in green leafy vegetables 2

An increase in dietary vitamin K intake may result in a subtherapeutic INR, increasing the risk for thromboembolic events, particularly in patients consuming high daily amounts of green leafy vegetables 2
Dark green vegetables like broccoli, spinach, lettuce, and salad dressing are the most common dietary sources of vitamin K, and patients should be aware of their vitamin K content to maintain consistent intake 2

Patients on warfarin should maintain consistent vitamin K intake rather than avoiding green leafy vegetables altogether, as advised by the American College of Cardiology 1
Patients should be educated about foods high in vitamin K and develop a plan for consistent weekly intake with minimal fluctuations, to minimize the risk of thromboembolic events 2
If INR becomes subtherapeutic due to increased vitamin K intake, warfarin dose can be adjusted upward rather than restricting dietary intake, as recommended by the American College of Cardiology 2, 4

More frequent INR monitoring is required during changes in diet, particularly when patients alter their consumption of green leafy vegetables, to ensure timely detection of INR fluctuations 3