Vitamin K

Vitamin K is found in green leafy vegetables and also, in small amounts, in cereals, fruits, and meats. Bacteria in the intestines also produce vitamin K, so the absolute dietary requirement is not known. This vitamin is needed for the formation of prothrombin, which is required for blood to clot. It is possible for people who regularly take antibiotics that destroy the bacteria in the intestines to be at increased risk for vitamin K deficiency. A deficiency would cause an increase in bleeding and hemorrhages. The DRI for vitamin K is 90 micrograms per day for adult women and 120 micrograms per day for adult men. There is no established UL for this vitamin. Vitamin K appears to be relatively nontoxic, but high intakes of synthetic forms may cause jaundice. Supplemental doses also interfere with anticoagulant drugs. People taking warfarin (a blood thinner) must be aware that vitamin K or foods containing vitamin K may reduce the effectiveness of their medication.

Vitamin K has also been linked to bone density. People with low levels of vitamin K have lower bone density, which can be improved with vitamin K supplementation. In addition, women obtaining a minimum of 110 micrograms of vitamin K are at significantly lower hip fracture risk than women who have a lower intake. The Framingham Heart Study also found a relationship between higher vitamin K intake and reduced hip fracture risk.Vitamin K is found in so many foods, and with bacterial production in the gut, it would seem difficult to not get enough. Nevertheless, a survey has found that an important segment of Americans, particularly children and young adults, does not obtain sufficient vitamin K.

No studies have examined the relationship between vitamin K and athletic performance. Further, it is difficult to think of a theoretical framework in which such a relationship might exist. It seems clear that, especially for athletes involved in contact sports, adequate vitamin K is necessary to avoid excessive bruising and bleeding. However, there is no documented evidence that athletes are at a particularly high risk for a deficiency.