Vitamin K is a fat-soluble vitamin that is found in kale, Swiss chard, broccoli, parsley and most other green plants. Animal sources of vitamin K include liver, egg yolks, milk, yogurt and fish liver oils. Polyunsaturated oils, such as canola oil, also contain small amounts of vitamin K. According to the Linus Pauling Institute at Oregon State University, the "K" is derived from the German word "koagulation," referring to vitamin K's role in blood coagulation. If you take Coumadin, or warfarin, to prevent clotting, you may have heard that you should not consume vitamin K.
The Coagulation Cascade
Your blood's ability to clot following an injury depends on the presence of healthy platelets, which are cellular fragments released from your bone marrow, and on an array of blood-borne proteins, called clotting factors, produced by your liver. Injury to a blood vessel simultaneously triggers the aggregation of platelets and the activation of clotting factors at the damaged site. Activation of clotting factors involves the sequential conversion of at least a dozen different proteins in a "cascade" that quickly leads to the formation of a thrombus, or clot.
Vitamin K Is Required for the Synthesis of Clotting Factors
Several clotting factors are not fully functional until they are converted to their mature forms in your liver, which requires vitamin K and an enzyme to alter the clotting factors' structure. During this conversion, vitamin K itself is chemically modified, and it must be transformed back to its "native" state before it can participate in any further maturation reactions. Yet another enzyme, called vitamin K epoxide reductase complex, or VKORC, returns vitamin K to its native form. According to a 2008 "Vitamins and Hormones" review, the recycling of vitamin K during clotting factor conversion is called the "vitamin K cycle."
Coumadin Inhibits VKORC
Coumadin, or warfarin, is a drug that reduces your blood's clotting ability. Coumadin exerts its effects by inhibiting VKORC, thereby preventing the recycling of vitamin K. With less native vitamin K available, fewer fully functional clotting factors are produced. However, if you consume extra vitamin K, Coumadin's effectiveness is reduced, your liver's ability to convert clotting factors is restored, and your blood clots more quickly.
Considerations and Precautions
Some people suffer from medical conditions -- artificial heart valves, heart rhythm abnormalities, or an increased tendency to produce clots -- which require "thinning" of their blood to prevent the formation of life-threatening blood clots. Coumadin accomplishes this purpose. If you eat vitamin K-containing foods or take vitamin K supplements while you are also taking Coumadin, your blood will clot more readily. If you take Coumadin, ask your doctor before you use supplements or make any significant changes in your diet.
