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Vitamin K and Warfarin: What You Should Know
============================================

Marie B. Walker
Henry I. Bussey, Pharm.D., FCCP, FAHA
May, 2004
Updated January, 2008

Many people recognize that there is a relationship between warfarin
and vitamin K, but often, they do not fully understand this
relationship. So, what do you need to know about vitamin K and
warfarin?

First, you should know that your liver uses vitamin K to make blood
clotting proteins. In doing so, vitamin K plays a role in your body's
natural clotting process. Warfarin works against vitamin K.
Specifically, warfarin reduces your liver's ability to use vitamin K
to produce normally functioning forms of the blood clotting proteins.
By reducing the liver's ability to use vitamin K to produce normally
functioning forms of the blood clotting proteins, warfarin reduces
your risk of forming a blood clot.

So how does my intake of vitamin K affect my warfarin therapy? A
significant change in your intake of vitamin K can result in a
significant, and potentially dangerous, change in your INR. For
example, if you reduce the amount of vitamin K in your diet, your INR
will increase. Also, reducing the amount of vitamin K in your diet may
make it more difficult to manage your warfarin therapy. Patients who
have a low intake of vitamin K have been found to have more
fluctuation in their INR, which is the test used to measure the effect
of warfarin and to adjust the dose of warfarin. For more information
on the INR, please review the Understanding the PT-INR Test section of
ClotCare.

Why would a diet low in vitamin K make my INR more difficult to
manage? Suppose you have a diet that is extremely low in vitamin K.
Now suppose you eat a spinach salad at dinner one night. Spinach salad
is high in vitamin K, so you have just increased your vitamin K intake
for the week. Because your diet is so low in vitamin K, this increase
in vitamin K intake represents a huge change in your vitamin K intake
for the week. For example, maybe eating the spinach salad increased
your vitamin K intake by 50%. The result of this significant increase
in vitamin K will be a significant drop in your INR. On the other
hand, say you have a diet that has a moderate intake of vitamin K. In
eating the spinach salad, you may have increased your vitamin K intake
for the week, but rather than increasing your intake by 50%, you
increased it by only 5%. The bottom line is that if your diet is
extremely low in vitamin K, then any changes to your vitamin K intake
represent large fluctuations in your vitamin K intake, which may
translate to large fluctuations in your INR.

So, if I am on warfarin, should I avoid eating foods that contain
vitamin K? By now you should know that the answer to this question is
"no." It is a common misconception that people on warfarin should
avoid vitamin K. As is indicated above, reducing your vitamin K intake
can cause your INR to increase and may make it more difficult to
control. Rather than avoiding vitamin K, you should maintain a
consistent intake of vitamin K by maintaining a consistent diet. In
other words, from week to week, you should eat the same types of
foods.

In order to maintain a consistent intake of vitamin K, you need to
know something about the vitamin K content of the foods you eat. As a
"rule of thumb", green vegetables, especially leafy green vegetables,
and certain oils have a high content of vitamin K. Most fruits, meats,
dairy products, and grains are low in vitamin K. As a resource to use
in making consistent dietary decisions, ClotCare provides a list of
certain foods and their relative content of vitamin K (i.e. low,
moderate, or high). Click here to download this list. You can also
click here for information on the vitamin K content of foods available
on the NutritionData website. QAS also has a vitamin K registry on its
PTINR.com site.

Finally, probably the best thing you can do to help your
anticoagulation clinician manage your warfarin therapy effectively is
to communicate to him any changes in your life that might affect your
warfarin. You should inform your clinician of any dietary changes you
make, preferably before making them.

For answers to other frequently asked questions about warfarin
therapy, go to our FAQs About Warfarin (Coumadin) section.

As always, you are welcome to contact us with questions, comments, or
suggestions at webmaster@clotcare.com.

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ClotCare Online Resource covers topics such as Coumadin, warfarin,
blood clot, stroke, heart attack, INR, PT, low molecular weight
haparin, heparin, dvt, anticoagulation, antithrombotic therapy, A.
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