Vitamin K is a nutrient that the body requires in small, regular amounts. It is essential for the formation of several substances called coagulation factors that work together to clot the blood when injuries to blood vessels occur. Insufficient vitamin K can lead to excessive bleeding and easy bruising. Vitamin K is also thought to play an important role in the prevention of bone loss. A low concentration of vitamin K within the bloodstream has been associated with low bone density, and there is some evidence that adequate concentrations of vitamin K can improve bone health while reducing the risk of fractures.
There are two natural forms of vitamin K:
- Vitamin K1 (phylloquinone or phytomenadione) is the natural from of vitamin K that comes from foods, especially green leafy vegetables but also dairy products and vegetable oils. K1 is considered as the "plant form" of vitamin K, but it is also produced commercially to treat some conditions associated with excess bleeding.
- Vitamin K2 (menaquinones) is made by bacteria, the normal flora in the intestines. Bacteria in the intestines can also convert K1 into K2. Vitamin K2 supplements K1 from the diet to meet the body's requirements.
Since the body is not able to produce a sufficient amount of vitamin K, a certain amount must be taken in through the diet. It is present in a wide variety of foods, and the normal diet in the U.K. typically supplies enough. Examples of different foods that contain high amounts, often more than the Recommended Daily Allowance (RDA), include leafy green vegetables such as kale, collards, spinach, turnip greens, mustard greens, green leaf lettuce, and other vegetables such as broccoli, green onions, parsley, asparagus, Brussels sprouts, and cabbage. Other sources include dairy products, cereals, vegetable oils, and soybeans. K1 and K2, the types provided by the diet and produced by the body, are both fat-soluble and are stored in the body's fat tissue and in the liver. An adult typically stores about a week's worth of vitamin K.
Interactions with the anticoagulant drug warfarin
People who are on the anticoagulant drug warfarin (COUMADIN®) must be careful of the amount of vitamin K present in the foods that they eat. Vitamin K is an antagonist to warfarin. This means that it counteracts the action of warfarin, reverses its drug activity, and makes it less effective in treating the condition for which it is prescribed. Warfarin is given to people with a variety of conditions such as deep vein thrombosis (DVT) and cardiovascular disease (CVD) to "thin" their blood and prevent inappropriate clotting. The drug works by inhibiting production of vitamin K-dependent clotting factors. Warfarin affects people differently and must be carefully monitored, typically with a prothrombin time (PT) test that gives results as an INR (International Normalised Ratio) value. The INR must be kept within a narrow therapeutic range. Too little warfarin can result in dangerous blood clots, but too much can cause bleeding episodes. Rather than avoid foods rich in vitamin K, it is more important for people on this drug to consume a consistent amount of vitamin K each day. Significant increases or decreases in vitamin K can affect how warfarin works in their bodies.