Fundamentals of Human Nutrition/Vitamin K
7.4 Vitamin K
Vitamin K is a group of structurally similar, fat-soluble vitamins that the human body needs for posttranslational modification of certain proteins required for blood coagulation, and in metabolic pathways in bone and other tissue.
Vitamin K refers to a family of compounds , the most common consumed form being phylloquinone (vitamin K1) which is found in plants.
Vitamin K is found in a number of foods, including leafy greens, cauliflower and, if you consider it a food, liver. However, the chief source of vitamin K is synthesis by bacteria in the large intestine, and in most cases, absence of dietary vitamin K is not at all deleterious. Vitamin K is a fat-soluble vitamin and both dietary and microbial vitamin K are absorbed into intestinal lymph along with other lipids. The fetus obtains vitamin K from its mother by transplacental transfer.
Physiologic Effects of Vitamin K
Vitamin K serves as an essential cofactor for a carboxylase that catalyzes carboxylation of glutamic acid residues on vitamin K-dependent proteins. The key vitamin K-dependent proteins include:
•Coagulation proteins: factors II (prothrombin), VII, IX and X
•Anticoagulation proteins: proteins C, S and Z
•Others: bone proteins osteocalcin and matrix-Gla protein, and certain ribosomal proteins
The Vitamin K Cycle
As a cofactor to the carboxylase that generates gamma-carboxyglutamic acid, Vitamin K undergoes a cycle of oxidation and reduction that allows its reuse. The essential details of this cycle are:
•Vitamin K (usually K1) is reduced to vitamin KH2.
•Oxygenation of vitamin KH2 provides the energy to drive the carboxylation reaction, leading to formation of gamma-carboxyglutamic acid residues and vitamin K oxide.
•Vitamin K oxide is reduced by another reductase back to vitamin K, ready to enter another cycle. Anticoagulants such as Warfarin block the reduction of vitamin K oxide to vitamin K, explaining their antagonistic effects on this cycle.
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Vitamin K deficiency can lead to excessive bleeding, which may begin as oozing from the gums or nose. Other things that may lead to vitamin K deficiency include:
•Health problems that can prevent your body from absorbing vitamin K, such as gallbladder or biliary disease, cystic fibrosis, celiac disease, and Crohn's disease
•Taking blood-thinners, such as warfarin (Coumadin)
Conditions that benefit from vitamin K include:
Vitamin K is used to reduce the risk of bleeding in liver disease, conditions where your body doesn' t absorb enough vitamin K, or if you take antibiotics for a long time. Even though vitamin K deficiency in newborns is very rare, it is dangerous enough that doctors give the injections. Newborns at greatest risk for vitamin K deficiency are premature or those whose mother had to take seizure medications during pregnancy. Mothers on seizure medications are often given oral vitamin K for 2 weeks before delivery.
Your body needs vitamin K to use calcium to build bone. People who have higher levels of vitamin K have greater bone density, while low levels of vitamin K have been found in those with osteoporosis. There is increasing evidence that vitamin K improves bone health and reduces risk of bone fractures, particularly in postmenopausal women who are at risk for osteoporosis. In addition, studies of male and female athletes have also found that vitamin K helps with bone health. However, some studies have found that vitamin K didn' t help with bone density.
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