Vitamin K is an important nutrient that plays a large role in blood clotting. This fat-soluble vitamin is found naturally in foods such as leafy green vegetables, as well as in dietary supplements. While there is a recommended daily dose for vitamin K, excessive intake of most forms of vitamin K do not cause symptoms from toxicity.
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Natural Vitamin K
Vitamin K1, also known as phylloquinone, and vitamin K2, also called menaquinone, are both natural forms of vitamin K. They are not associated with toxicity, even if taken in large amounts. The body is limited in the amount of vitamin K it can store and this lack of accumulation contributes to the safety of natural vitamin K.
Synthetic Vitamin K
Unlike the natural forms of vitamin K, a man-made form known as vitamin K3, or menadione, can produce toxicity, especially in young children. It can cause liver damage or destruction of red blood cells, leading to a variety of symptoms including fatigue or jaundice -- yellow discoloration of the eyes and skin. Vitamin K3 is now rarely used in the United States because of these potential toxic effects. Indeed, the United States Food and Drug Administration has banned its use in all over-the-counter nutritional supplements.
Vitamin K and Blood Thinners
Given the role of vitamin K in helping the blood to clot, it is not surprising that high doses may interfere with the effectiveness of certain blood-thinners like warfarin. If you take one of these blood-thinners, ask your doctor how much vitamin K is appropriate for you to consume. He may suggest that you limit your vitamin K intake and/or consume about the same amount every day.
- Merck Manual Professional Version: Vitamin K
- Oregon State University Linus Pauling Institute: Micronutrient Information Center
- Visualizing Nutrition, Everyday Choices; Mary B. Grosvenor, MS, RD et al.
- National Institutes of Health: Menadione
- U.S. Pharmacist: The Emerging Role of Vitamin K2