(See also Overview of Vitamins.)
Vitamin K has two forms:
Phylloquinone: This form occurs in plants and is consumed in the diet. It is absorbed better when it is consumed with fat. Phylloquinone is not toxic.
Menaquinone: This form is produced by bacteria in the intestine, but only small amounts of it can be absorbed. In some countries, this form is used for supplementation.
Vitamin K is necessary for the synthesis of the proteins that help control bleeding (clotting factors) and thus for the normal clotting of blood. It is also needed for healthy bones and other tissues.
Vitamin K is a fat-soluble vitamin. It dissolves in fat and is best absorbed when eaten with some fat. Good sources of vitamin K include green leafy vegetables (such as collards, spinach, and kale) and soybean and canola oils.
Vitamin K deficiency can cause hemorrhagic disease of the newborn, characterized by a tendency to bleed. A vitamin K injection is usually given to newborns to protect them from this disease. Breastfed infants who have not received this injection at birth are especially susceptible to vitamin K deficiency because breast milk contains only small amounts of vitamin K. Hemorrhagic disease is more likely in infants who are breastfed or who have a disorder that impairs fat absorption or a liver disorder. Formulas for infants contain vitamin K.
In healthy adults, vitamin K deficiency is uncommon because many green vegetables contain vitamin K and bacteria in the intestine produce vitamin K.
If people have vitamin K deficiency, taking warfarin or related anticoagulants can make bleeding more likely or make it worse because these drugs also interfere with the synthesis of clotting factors (which help blood clot). Anticoagulants are given to people with conditions that increase the risk of blood clots. These conditions include having to stay in bed a long time (for example, because of an injury), recovering from major surgery, and having atrial fibrillation (an abnormal, irregular heart rhythm). People who take warfarin need to have blood tests periodically to check how quickly their blood clots.
Vitamin K deficiency can result from the following:
Lack of vitamin K in the diet
A very low fat diet because vitamin K is best absorbed when eaten with some fat
Disorders that impair fat absorption and that thus reduce the absorption of vitamin K (such as blockage of the bile ducts or cystic fibrosis)
Certain drugs, including anticonvulsants, and some antibiotics
Consumption of large amounts of mineral oil, which may reduce the absorption of vitamin K
Newborns are prone to vitamin K deficiency because of the following:
The main symptom of vitamin K deficiency is bleeding (hemorrhage)—into the skin (causing bruises), from the nose, from a wound, in the stomach, or in the intestine. Sometimes bleeding in the stomach causes vomiting with blood. Blood may be seen in the urine or stool.
In newborns, life-threatening bleeding within or around the brain may occur.
Having a liver disorder increases the risk of bleeding because clotting factors are made in the liver.
Vitamin K deficiency may also weaken bones.
Doctors suspect vitamin K deficiency when abnormal bleeding occurs in people with conditions that put them at risk.
Blood tests to measure how quickly blood clots are done to help confirm the diagnosis. Knowing how much vitamin K people consume helps doctors interpret results of the blood test.
A vitamin K injection in the muscle is recommended for all newborns to reduce the risk of bleeding within the brain after delivery.
If the deficiency is diagnosed, vitamin K is usually taken by mouth or given by injection under the skin. If a drug is the cause, the dose of the drug is adjusted or extra vitamin K is given.