VITAMIN K SOURCES
About 10 % of the vitamin K absorbed each day comes from bacterial synthesis in the colon. The remainder comes from dietary sources. Although the vitamin K contents of individual foods varies, green leafy vegetables (e.g., kale, turnip greens, parsley, salad greens, cabbage, and spinach), broccoli, peas, and green beans are the best sources.
Vegetable oils, such as soy and canola, also are good sources. Vitamin K is relatively stable to heat processing, but it can be destroyed by exposure to light.
VITAMIN K NEEDS
For women, the Adequate Intake for vitamin K is 90 μg/day; for men, it is 120 μg/day. These Adequate Intakes are based on the apparent adequacy of usual intakes and the lack of information to determine an EAR and RDA. Although current intakes provide adequate vitamin K for blood clotting functions, it is not known whether increased intakes might be beneficial for other functions.
FUNCTIONS OF VITAMIN K
Vitamin K is needed for the synthesis of blood-clotting factors by the liver and the conversion of preprothrombin to the active blood-clotting factor called prothrombin.
In the body, vitamin K is converted to an inactive form once it has activated these clotting factors. It must be then reactivated for its biological action to persist. Drugs such as warfarin, which strongly inhibit this reactivation process, act as powerful anticoagulats. People taking warfarin to lessen blood clotting should maintain a consistent dietary vitamin K intake and avoid vitamin K supplementation.
Vitamin K might also play a role in bone metabolism. Three additional vitamin K-dependent Gla proteins are known to be synthesized in bone. The functions of there proteins (called osteocalcin, matrix Gla protein, and protein S) are not clearly understood. However, the synthesis of these proteins in reduced in vitamin K deficient animals and results in changes in bone health. Vitamin K also may help protect the body from inflammation, thereby providing protection against cardiovascular disease and osteoporosis.
VITAMIN K DEFICIENCY
A deficiency of vitamin K is rare, but it can occur with a prolonged use of antibiotics that disrupt vitamin K synthesis or with impaired fat absorption. Vitamin K deficiency also can occur in newborns. Vitamin K stores are typically low at birth, so infants are at risk of defective blood clotting and hemorrhage. To prevent this possible vitamin K deficiency. newborn infants in North America are given vitamin K injections within 6 hours of delivery.
Laboratory animal studies have shown that excessive amounts of vitamin A and vitamin E negatively affect the actions of vitamin K. Vitamin A is thought to interfere with the absorption of vitamin K from the intestine, whereas large doses of vitamin E can lead to a decrease in vitamin K-dependent clotting factors and increased bleeding tendency.
VITAMIN K TOXICITY
To date, no Upper Limit has been set for vitamin K. Although vitamin K can be stored in the liver and the bone. it is more readily excreted than other fat-soluble vitamins. When used in its natural forms of phylloquinones or menaquinones, increased amount of vitamin K have not cause harmful effects. In contrast, high amounts of menadione, a synthetic form of vitamin K, have resulted in hemolytic anemia, excess bilirubin in the blood, and death in newborns.
