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I get the question all the time about how much Vitamin K is right for folks taking Coumadin® (warfarin). Unfortunately, there's no perfect study to guide just how much Vitamin K is too much for those taking Coumadin. Most physicians recommend limiting foods that contain very high or even moderate amounts of Vitamin K. At the same time, there's never been a recommendation to severely limit Vitamin K intake.
The Recommended Daily Allowance (RDA) for Vitamin K is 80 micrograms (mcg) for males and 70 mcg for females. The majority of ingredients contain small amounts - in the under 15 mcg range - so keeping an eye on foods that contain more than 20 - 25 mcg per serving is a good rule of thumb.
Avoiding all Vitamin K might be just as much of a problem as getting too much, however. Studies have clearly shown that eating foods that are higher in Vitamin K will have an effect on the effectiveness of Coumadin and the INR. Most people who either take warfarin themselves or those who help patients manage their anti-coagulation know this, but there is some research now that shows eating too little Vitamin K can have the same effect.
So what to do? How much is too much? How little is too little? It appears, from what research we have, that in those folks who were taking Coumadin 29 mcg was too little and 76 mcg just right to keep their INR stable.
While it's not a perfect way to look at the issue of how much is too much Vitamin K in the diet for warfarin users, another study showed an effect on the INR in those taking 150 - 200 mcg per day in Vitamin K supplements. These are the levels found in Vitamin K rich foods such as spinach, collard greens and broccoli.
For the number of folks who use this medication and the fact that there's no great replacement on the horizon, it is a shame that a large study has not been done to help answer this question more clearly. For the time being, we have to be content with the small studies that point toward an optimum near the RDA guidelines for Vitamin K.