Vitamin K is a fat-soluble nutrient essential to the health of the human body, where it plays crucial roles in blood coagulation and bone formation. There are three major forms of vitamin K: vitamin K-1 (phylloquinone), the natural form found in plants; vitamin K-2 (menaquinone), produced by bacteria in the intestines; and vitamin K-3 (menadione), a synthetic form available only by prescription. The preferred form used in multivitamin and individual supplements is vitamin K-1. There is also a water-soluble form of vitamin K (phytonadione) which is used in some topical formulations.
Some of the richest dietary sources of vitamin K are seaweed and green leafy vegetables. Other good sources include liver, cauliflower, soybeans, cheese, egg yolk, strawberries, tomatoes, and potatoes.
The reference daily intake (RDI) for vitamin K is 80 mcg/day for adults, an amount which can easily be obtained through intake of green leafy vegetables. While excessive consumption of synthetic vitamin K (K-3) may cause liver damage, there are no known side effects or toxicity associated with consumption of natural-source vitamin K (K-1 or K-2). Because vitamin K may counteract the activity of some prescription blood-thinners (such as warfarin), anyone taking these drugs should not supplement vitamin K without consulting a physician.