Vitamin K is a fat-soluble vitamin, the most common form of which is a compound known as phylloquinone. Some dietary consumption also results from a secondary form, dihydrophylloquinone, though it appears to be significantly less active.
      The primary role of vitamin K in the human body is in the synthesis of several proteins that are required for the normal clotting of blood. Not all bleeding disorders are related to vitamin K, but in its absence, one would likely bleed to death.
      Vitamin K has more recently been reported to also be involved in the synthesis of proteins that help build and maintain bone. The amount of vitamin K that is actually needed in this regard, particularly for older individuals, has not yet been established.

      The Recommended Dietary Allowance for vitamin K was only first established in 1989. Recommendations made at that time were 80 mcg/day for adult men, and 65 mcg/day for adult women.
      The recently released report from the National Academy of Sciences Dietary Reference Intakes study (January 2001), which has reviewed all of the current scientific literature, has recommended a daily intake for this vitamin slightly higher for women and somewhat higher for men than the RDA referenced amount currently used on food labels in the U.S. The figures, listed in the following table, are based upon slightly different age ranges, which accounts for the overlapping age groups.

      The usual dietary sources of vitamin K are such foods as leafy vegetables (spinach and kale), broccoli, green beans, some dairy products, especially egg yolks, some meats (liver in particular), and to a lesser degree, cereals, fruits and other vegetables.
      In addition to the mentioned food sources, vitamin K is also synthesized in the body by intestinal bacteria.

      Vitamin K deficiency is generally uncommon, as most individuals consume sufficient amounts in a normal diet and, as previously mentioned, the vitamin is synthesized in the body. When a deficiency does occur, either from lack of dietary intake or disorders resulting in malabsorption of the vitamin, the single most important consideration is bleeding disorders. While it is not likely that a minor deficiency would lead to serious bleeding problems, individuals on anticoagulant therapy who are also vitamin K deficient are at greater risk.
      There are several herbals with anticoagulant properties, ginkgo and garlic being the most common. Aspirin is also a mild anticoagulant, for that matter, but all of these compounds have a different biochemical effect on blood clotting so interactions are difficult to predict. Individuals who are on prescribed anticoagulants are generally provided dietary guidelines by their physicians.

      Vitamin K supplements, as a standalone product, are generally not available (in the U.S.) without a prescription. There are a number of multivitamins, however, that do contain upwards to 100% of the Reference Daily Intake (RDI) of this vitamin. The primary indication of supplemental vitamin K would be to correct a blood clotting deficiency resulting from vitamin K deficiency, the management of which would properly be in the hands of your health care provider.

      Prolonged antibiotic therapy in combination with vitamin K deficiency could lead to potential bleeding problems as a result of the destruction of intestinal bacteria involved in the synthesis of vitamin K.

      Vitamin K does not enjoy the level of scientific attention given to the other vitamins, but the work that is being done has generally focused on its ability to enhance bone health, particularly in older individuals. This research is in its early stages, so any definitive information as to how much vitamin K might be beneficial, and to what degree it might be so, has yet to be established.