Anticoagulants (blood thinners) have some of the narrowest therapeutic windows of all the drugs available on the market. With Coumadin (warfarin) available in 0.5-1mg increments, it's easy to see that the tiniest alteration in dose can cause problems with too much clotting or too much blood thinning.
Vitamin K, a fat soluble vitamin, is required for a normal procession of the clotting cascade. There is also some evidence that it plays a role in bone growth and density maintenance. Warfarin works by disabling Vitamin K-dependent synthesis of clotting factors in the liver. An increase in Vitamin K in the body causes a decreased effectiveness of warfarin - ideal in the event of accidental overdosage or ingestion of bromadiolone, a 4-hydroxycoumarin which is the active ingredient in some rodenticides.
Because of Vitamin K's role in body functions aside from coagulation (such as bone health), it is not actually recommended for patients taking warfarin to avoid Vitamin K all together, but rather to ensure a moderate, regular amount of Vitamin K in their diet. In order for patients to do this, they need to know what foods contain Vitamin K.
Foods considered very high in Vitamin K contain 500mcg or more of Vitamin K and should generally be avoided in patients on anticoagulant therapy. They cause a sudden spike in Vitamin K levels in the blood and can decrease the effectiveness of therapy. These include most of the "leafy greens"- spinach, kale, turnip greens and the like, as well as a few surprises, like green tea.
Patients on anticoagulants can eat items high in Vitamin K in moderation; high levels are considered 100-500mcg. These include vegetables like broccoli, cauliflower, and collard greens as well as protein sources like lentils, egg yolks, soybeans and beef liver.
Medium level (25-100mcg) sources of Vitamin K can be eaten daily and include asparagus, lettuce, green apples, chicken and pork livers, and coffee.
Additionally, some multivitamin supplements, especially those geared toward women, contain Vitamin K. It may appear on a label under the names phylloquinone, phytomenadione, phytonadione. And patients taking anticoagulants should take care to avoid supplements with Vitamin K in them. There are many available without Vitamin K. Patients should be advised to ask their doctor or pharmacist for help choosing a multivitamin if they are on anticoagulants.
A synthetic form of Vitamin K, menadione, has shown some evidence of toxicity through hemolytic anemia (rupture of blood cells) and liver damage. Synthetic Vitamin K has been banned in over the counter supplements in the United States because of these damaging effects, but supplements imported from overseas may not observe these regulations.
They key is for patients to avoid sudden fluctuations in Vitamin K intake, otherwise their doctor may have to change their warfarin dose frequently. This can result in increased bleeding risk or increased risk of thrombosis and clotting.
Best,
C. Samuels
Resources
"High Vitamin K Foods." Vaughn's Summaries. Vaughn's Summaries, 2005. Web. 21 May 2011. <http://www.vaughns-1-pagers.com/food/vitamin-k-foods.htm>.
Parks, Robin. "Anticoagulants: Vitamin K and your diet." Health.com. Heath.com, 26 Feb 2008. Web. 21 May 2011. <http://www.health.com/health/library/topic/0,,tb1790_tb1791,00.html>.
Theberge, Christopher. "Anticoagulants and Vitamin K." Scribd.com. The Nutrition and Food Web Archive, 2007. Web. 21 May 2011. <http://www.scribd.com/doc/123024/Anticoagulants-and-Vitamin-K>.
"Vitamin K." Wikipedia. 2011. Web. <http://en.wikipedia.org/wiki/Vitamin_K>.
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