What is vitamin K?
Vitamin K is the generic name for a family of compounds which contain a common chemical structure. It is a fat-soluble vitamin naturally present in some foods and available as a dietary supplement. The main form of dietary vitamin K is known as phylloquinone (vitamin K1) and is present primarily in green leafy vegetables. Another form of vitamin K is menaquinone, which are present in small amounts in various animal-based and fermented foods. Almost all menaquinones can also be produced by the bacteria in the human gut. Vitamin K is present in the liver, brain, heart, pancreas and bone. Compared to other fat-soluble vitamins, considerably small amounts of vitamin K circulate in the blood because vitamin K is rapidly metabolized and excreted.
Vitamin K acts as a coenzyme for the synthesis of proteins involved in blood clotting and bone metabolism, along with other functions in the body.
Warfarin and some other anticoagulants antagonize the activity of vitamin K, which then affects blood clotting. Therefore, individuals taking these drugs need to maintain a consistent vitamin K intake.1
How much do I need?
For males age 19 and up, Health Canada recommends 120 mcg/day of vitamin K.
For females age 19 and up, Health Canada recommends 90 mcg/day of vitamin K.2
Food Sources of Vitamin K
- kale – 561 mcg per ½ cup cooked, 499 mcg per 1 cup raw
- spinach – 469 mcg per ½ cup cooked, 153 per 1 cup raw
- dandelion greens – 452 mcg per 1 cup raw
- parsley – 260 mcg per ¼ cup raw
- Brussel sprouts – 118 mcg per 4 sprouts
- broccoli – 116 mcg per ½ cup cooked
- green leaf lettuce – 103 mcg per 1 cup raw
- matcha green tea powder – 60 mcg per 2 g
- Kiwi – 28 mcg per fruit
- Bbueberries – 22 mcg per ½ cup3
Am I at risk of vitamin K deficiency?
Signs of severe vitamin K deficiency are bleeding and hemorrhage but can also be characterized by reduced bone mineralization and osteoporosis. Vitamin K deficiency in adults is very rare and is usually limited to people with malabsorption of nutrients and those taking drugs that interfere with vitamin K metabolism. Healthy individuals consuming a varied diet are at very low risk of not consuming enough vitamin K to alter the clinical tests used to measure vitamin K status (blood clotting time).1
What if I am taking warfarin?
Some diseases and conditions can cause the blood to clot excessively. This is dangerous because blood clots can form in the body and block blood flow, causing a heart attack, stroke, deep vein thrombosis and other health problems. If this occurs, your physician may prescribe warfarin. In contrast to vitamin K, which helps blood to clot quickly, warfarin is a drug that prevents and slows the formation of blood clots. Your warfarin dose is balanced with the amount of vitamin K consumed in your usual diet to ensure your blood clots at a safe rate. Therefore, consuming the same amount of vitamin K each day is important to ensure your medication works optimally.
If you are taking warfarin, it is important to follow a healthy and well-balanced diet. This doesn’t mean you have to stop eating foods rich in vitamin K, you can eat any of the foods listed above, as long as the total mcg of vitamin K consumed is relatively constant day to day.
Here are some tips for individuals taking warfarin:
- Avoid large changes in the amount of vitamin K you consume from one day to another
- Prepare foods in the same way – for example, if you normally eat cooked leafy greens, avoid suddenly eating them raw. This is because green leafy vegetables wilt when cooked, therefore the cooked version has more vitamin K than the raw version
- Always consult with your physician if you plan to make any changes to your diet4
- National Institutes of Health – Office of Dietary Supplements. (2020, February 24). Vitamin K Fact Sheet for Health Professionals. Retrieved from https://ods-od-nih-gov.subzero.lib.uoguelph.ca/factsheets/vitaminK-HealthProfessional/
- Health Canada. (2006, June 29). Dietary Reference Intakes. Retrieved from https://www.canada.ca/en/health-canada/services/food-nutrition/healthy-eating/dietary-reference-intakes/tables/reference-values-elements-dietary-reference-intakes-tables-2005.html
- Dietitians of Canada. (2018, November 19). Food Sources of Vitamin K. Retrieved from https://www-pennutrition-com.subzero.lib.uoguelph.ca/viewhandout.aspx?Portal=UbY=&id=JMbpUAc=&PreviewHandout=bA==
- Dietitians of Canada. (2019, January 9). Healthy Eating Guidelines for People Taking Warfarin Anticoagulants (Coumadin®). Retrieved from https://www-pennutrition-com.subzero.lib.uoguelph.ca/viewhandout.aspx?Portal=UbY=&id=J8LrXQw=&PreviewHandout=bA==