Vitamin K and Anticoagulants (warfarin, Coumadin®, Jantoven®, Marfarin®)
Vitamin K is a fat-soluble vitamin and plays a key role in the clotting cascade. The clotting cascade is a series of chemical reactions that occur in the body and result in coagulation to stop bleeding after an injury. Warfarin lengthens the amount time it takes for the clotting cascade to form a clot by decreasing the activity of vitamin K.
Found primarily in green leafy vegetables, vitamin K inhibits the effects of warfarin. While warfarin works to increase International Normalization Ratio (INR), or lengthen the amount of time it takes to form a clot, vitamin K works against it to decrease INR, or shorten the amount of time it takes to form a clot. It is therefore vital that all patients on warfarin eat a consistent amount of vitamin K daily. This way, the amount of warfarin needed to achieve the INR goals can be adjusted to account for the amount of vitamin K ingested. Sporadic consumption of vitamin K will result in either an increase or decrease of INR and the associated risks, which range from clot formation if INR is too low to excessive bleeding if INR is too high. Both of these adverse events can be fatal.
Due to the risks associated with irregular vitamin K intake, it is essential that patients taking warfarin consume a consistent amount of vitamin K daily so their warfarin dose can be adjusted to that level of vitamin K intake (1). This is the best possible way to avoid constant adjustments of the warfarin dose, decrease the number of times INR must be tested each month, and, most importantly, decrease the likelihood of a potentially fatal adverse event.
In addition to eating a consistent amount of vitamin K daily, it is important for patients taking warfarin to only consume up to the recommended daily intake (RDI) of vitamin K. The current RDI is 90 micrograms for women and 120 micrograms for men (2). It is recommended not to exceed that amount daily; however, always consult your physician before making changes to your diet.
Most complete multivitamins will contain approximately 15 to 25 micrograms of vitamin K. This means that a patient taking warfarin and a multivitamin could be getting over 25% of their RDI of vitamin K solely from the multivitamin. To make matters worse, many multivitamins and supplements contain other ingredients besides vitamin K that can interfere with warfarin including soy, niacin, and large amounts of vitamin E, vitamin C and magnesium (3).
K Free Daily is the safest complete multivitamin available for patients on warfarin. It is formulated without vitamin K, soy or niacin, and with clinically-proven safe levels of vitamin E, vitamin C and magnesium. K Free Daily also includes all the essential vitamins and minerals found in the leading multivitamin brands.
Foods High in Vitamin K
In general, dark, green leafy vegetables are the foods highest in vitamin K. The current RDI of vitamin K is 90 micrograms for women and 120 micrograms for men (2). It is important that patients taking warfarin do not exceed the recommended daily intake. Below is a selection of common vitamin K containing foods and their vitamin K content (4).
|Food||Measure||Content Per Measure (micrograms)|
|Cooked Kale||1 cup||1062.1|
|Cooked Spinach||1 cup||888.5|
|Cooked collards||1 cup||836|
|Beet greens||1 cup||697|
|Turnip Greens||1 cup||529.3|
|Mustard Greens||1 cup||419.3|
|Brussels Sprouts (cooked)||1 cup||218.9|
|Broccoli (cooked)||1 cup||220.1|
|Onions (Raw)||1 cup||207|
|Butterhead Lettuce||1 head||116.7|
|Spinach (raw)||1 cup||144.9|
|Sauerkraut (canned)||1 cup||135|
|Iceberg Lettuce||1 head||129.9|
|Broccoli (raw)||1 cup||89.4|
|Cabbage (cooked)||1 cup||73.4|
|Romaine Lettuce (raw)||1 cup||57.4|
|Celery (cooked)||1 cup||56.7|
|Broccoli (cooked)||1 spear||52.2|
|Cucumber (raw, w/ peal)||1 large||49.4|
|Spinach (raw)||1 leaf||48.3|
|Asparagus (cooked)||4 spears||48|
|Peas (cooked)||1 cup||40|
|Pumpkin (canned)||1 cup||39.2|
|Fish, canned tuna||3 oz||37.4|
|Carrot juice||1 cup||36.6|
|Kiwi Fruit (raw)||1 medium||30.6|
|Tomatoes (canned paste)||1 cup||29.9|
|Artichoke (cooked)||1 cup||24.9|
|Parsley (dried, spice)||1 tbsp||17.7|
|Canola Vegetable Oil||1 tbsp||17.1|
|Margarine-butter blend||1 tbsp||14.7|
|Carrots (raw)||1 cup||14.5|
|Tomatoes (raw)||1 cup||14.2|
Below are some of the most common nutrients (food and supplements) that may interact with warfarin (4, 5).
- Cat’s Claw
- Coenzyme Q10
- Fish Oil
- Green Tea
- Horse Chestnut
- Red Clover
- St. John’s Wart
- Wheat Grass
- Willow Bark
2. Warfarin diet: What foods should I avoid? [Internet].: Mayo Clinic; 2010. Available from: http://www.mayoclinic.com/health/warfarin/AN00455.
3. Harris JE. Interaction of dietary factors with oral anticoagulants: Review and applications. J Am Diet Assoc. 1995 May;95(5):580-4.
4. USDA national nutrient database for standard reference, release 17 [Internet]. Available from: www.nal.usda.gov/fnic/foodcomp/Data/SR17/wtrank/sr17w430.pdf.
5. Wittkowsky AK. Drug interactions update: Drugs, herbs, and oral anticoagulation. J Thromb Thrombolysis. 2001 Sep;12(1):67-71.