Vitamin K is a group of fat-soluble compounds. It was originally identified for its role in blood clot formation; indeed, the K comes from the German name for the vitamin, Koagulationvitamin, referring to its importance in blood coagulation processes.
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Vitamin K is fat soluble.
Vitamin K is found naturally occurring in two forms: phylloquinone (vitamin K1) and a family of molecules called menaquinones (vitamin K2).
Sources of vitamin K:
Vitamin K1 is the main form of vitamin K in the diet. It is synthesised by plants, and so predominantly found in high concentrations in green leafy vegetables, such as kale, parsley, broccoli, lettuce and spinach. It is also found in some plant oils, including olive, cottonseed, canola and soybean. Its bioavailability from vegetables is lower than from oil and supplements, but is increased with the addition of a fatty source in the meal. The content of vitamin K1 in one cup of spinach and broccoli is 145mcg (micrograms) and 220mcg, respectively, while a quarter of a cup of parsley contains 246mcg.
Vitamin K2, on the other hand, is primarily of microbial origins, and is synthesised by gut bacteria that normally colonise the large intestine (the colon), providing a portion of the dietary requirements. In food, it is commonly found in animal liver and fermented foods, such as fermented cheese, curds and natto (fermented soybeans).
Current research suggests that the contribution of bacterial synthesis is much less than the previously suggested 50% of requirements, and the most common forms of vitamin K2 have approximately 60% of the activity of vitamin K1.
Functions:
This relatively unknown vitamin is an essential nutrient for many chemical reactions in our body. Though mainly involved in blood coagulation, it also plays an important part in bone metabolism and the prevention of vessel calcification. Vitamin K regulates many important cellular functions, and directs calcium into correct areas within the body and its excretion.
Although vitamin K is a fat-soluble vitamin, very small amounts are stored in the body, and without regular dietary intake the vitamin is rapidly depleted. Perhaps because of its limited ability to store vitamin K, the body recycles it through the vitamin K cycle process, and is thus able to reuse it multiple times.
Vitamin K deficiency:
Vitamin K deficiency in healthy children and adults is rare. This is largely due to the wide distribution of phylloquinone in plants, menaquinone production by gut micro-flora, and because the vitamin K cycle allows it to be reused in the body. It is, however, common to see vitamin K deficiency in newborn babies, which has prompted routine administration of a vitamin K1 shot at birth as prophylaxis. In particular, infants who are exclusively breast fed are at increased risk of deficiency, because human milk is relatively low in vitamin K compared to formula. For exclusively breast fed infants low plasma vitamin K levels can be prevented by supplementation of the mother’s diet with 5mg of vitamin K1 throughout the first 12 weeks of life as well as injecting 1 to 2 mg vitamin K1 for prophylaxis at birth. The adequate intake is 400mcg daily in young infants.
Adults at risk of vitamin K deficiency include individuals who are diagnosed with intestinal diseases associated with malabsorption, such as celiac disease, inflammatory bowel disease, short bowel syndrome and cystic fibrosis. As vitamin K dependent coagulation factors are synthesised in the liver, severe liver disease or damage will result in lower blood levels of vitamin K dependent clotting factors, and individuals with these conditions will experience increased risk of uncontrolled bleeding (haemorrhage). High doses of vitamins E or A may contribute to vitamin K deficiency and can increase the risk of bleeding. Low vitamin K levels can also be the result of poor diet or a high alcohol intake.
Symptoms of deficiency:
Signs of vitamin K deficiency include bruising and bleeding easily, which may manifest as nose bleeds, bleeding gums, blood in the urine or stool, or extremely heavy menstrual bleeding. Vitamin K deficiency results in impaired blood clotting, which is usually demonstrated by laboratory tests that measure clotting time. A significant deficiency increases the risk of hemorrhage.
Supplements:
Both vitamin K1 and K2 are available without prescription in multivitamin and other dietary supplements, in doses that generally range from 25-100 mcg per tablet. The adequate daily dietary requirement for vitamin K is 90 mcg for women and 120 mcg for men, an amount that is easily exceeded following the ingestion of half a cup of kale. However, before you take supplements, you must check with your doctor, as vitamin K can interfere with the actions of number of common medications.
One form of vitamin K2 is marketed for osteoporosis treatment in Japan. Although recent clinical trials have examined the use of vitamin K1 and vitamin K2 for the treatment of osteoporosis, the results are conflicting. The effect of supplemental vitamin K in the prevention of abnormal mineralization of blood vessels and cardiovascular events still needs to be evaluated in randomised controlled trials, as the few trials that have been carried out are not conclusive.
Toxicity:
No tolerable upper intake level has been established for vitamin K, and there is no known toxicity associated with high doses of either vitamin K1 or K2. Excessive vitamin K intake is not related with abnormal clotting; however, allergic reactions can occur.
Drug interactions:
Individuals who take drugs that are known to interfere with absorption and metabolism of the vitamin, are at risk of vitamin K deficiency. Conversely, the effect of these drugs may be compromised by a very high dietary or supplemental vitamin K intake.
The anticoagulant effect of vitamin K antagonists, such as Warfarin, will be affected by the intake of vitamin K — the more of the vitamin is introduced to the body, the more Warfarin is needed in order to reach the desired anti-clotting levels. Therefore, if you take an anticoagulant, such as the blood-thinning medication Coumadin, keep your vitamin K intake consistent.
The prescription of anti-vitamin K anticoagulants, anticonvulsants (e.g. phenytoin) and anti-tuberculosis drugs (e.g. rifampicin and isoniazid) to pregnant or breast-feeding women may place the newborn at increased risk of vitamin K deficiency.
Prolonged use of broad-spectrum antibiotics, such as cephalosporins and salicylates, can interfere with the synthesis of vitamin K by intestinal bacteria, which can lead to lower vitamin K absorption. Cholesterol-lowering drugs, such as statins, interfere with the absorption of fat, and can also result in poor absorption of vitamin K, as it is a fat-soluble vitamin.
