Function of vitamin K in the body

Blood clotting
Vitamin K is used to make prothrombin and other proteins which are important in blood clotting. Vitamin K also plays a role in the conversion of prothrombin to thrombin, another protein important in blood clot formation.

Bone metabolism
A bone protein known as osteocalcin regulates the function of calcium in bone turnover and mineralization. Vitamin K is necessary for the conversion of osteocalcin to its active form. It is also necessary for the function of a protein known as MGP which is present in bones, teeth and cartilage.

Kidney function
Vitamin K is necessary for the production of a urinary protein involved in kidney function which inhibits the formation of calcium oxalate kidney stones. This may account for the fact that vegetarians, whose diets are often high in vitamin K, have a low incidence of kidney stones.

Absorption and metabolism
Vitamin K requires the presence of fats and bile in the gut in order to be absorbed. It is absorbed from the upper small intestine and transported to the liver.

Deficiency
Vitamin K deficiency is rare and usually only occurs in people who have prolonged diarrhea, obstructive jaundice, liver disease or malabsorption problems such as celiac disease. It can also occur in the elderly and in newborn and premature babies. Signs of deficiency are prolonged clotting time, easy bleeding and bruising, frequent nosebleeds, blood-stained urine and bleeding from the gut.

Osteoporosis
Vitamin K deficiency may contribute to osteoporosis as low levels have been found in sufferers of the disease. In a Japanese study published in 1997, researchers investigated the relationship between bone mineral density, vitamin K levels and other biological parameters of bone metabolism in 71 post-menopausal women and 24 women with menopausal symptoms receiving hormone replacement therapy. The results showed that women with reduced bone mineral density had lower levels of vitamin K1 and K2 than those with normal bone mineral density. Low levels have also been seen in osteoporotic men.

Sources
Vitamin K is present in a wide variety of foods. Good sources of dietary vitamin K include dark leafy greens Espinach, broccoli, brussels sprouts Eand olive, canola and soybean oils. Cabbage, carrots, avocados, cucumbers, leeks, tomatoes and dairy products such as yogurt are reasonable sources, and meats and cereals also contain some vitamin K. Freezing foods may destroy vitamin K but it remains stable when heated.

Recommended dietary allowances
The RDA for vitamin K is 1 mcg per kg of body weight. It is assumed that half the necessary daily intake comes from intestinal bacteria.

USA

Men 80 mcg

Women 65 mcg

Pregnancy 65 mcg

Lactation 65 mcg

There are no established recommended nutrient intakes for vitamin K in the UK as it is assumed that manufacture by intestinal bacteria provides enough. In

Supplements
Vitamin K is available in some multivitamin supplements. Alfalfa tablets are high in vitamin K.

Toxic effects of excess intake
Reports of toxic effects from natural vitamin K are rare. Large doses of the synthetic form of vitamin K, menadione may cause hemolytic anemia and liver damage and should not be used therapeutically. Side effects of intravenous vitamin K injection include flushing, sweats, chest pain and constricted breathing. Intramuscular injection may cause pain, swelling and eczema. Some research suggests a link between neonatal vitamin K injections and childhood cancer (See below).

Therapeutic uses of supplements

Prevention of hemorrhage
Vitamin K injections may be given to patients before or after surgery to prevent hemorrhage. The injections may be also be used to reduce the risk of excessive bleeding in those with liver disease, jaundice, malabsorption problems or who are taking aspirin or antibiotics.

Hemorrhagic disease of the newborn
Newborn babies are often given a vitamin K injection soon after birth. Vitamin K is not readily transferred from mother to baby, and as newborn babies do not have the intestinal bacteria necessary to produce vitamin K, they are at increased risk of hemorrhage. Premature babies run an especially high risk of brain hemorrhage during delivery because their blood vessels may be too fragile to withstand the surges in blood pressure that can occur. Vitamin K supplements are often given to women who have a high risk of delivering premature babies.

Mothers who are taking anti-epileptic drugs are also given supplements of vitamin K as their babies are at particular risk of vitamin K deficiency. Vitamin K supplements should only be taken on the recommendation of a doctor.

Childhood cancers
In the early 1990s, researchers reported a possible increase in the risk of childhood cancers in children who were given vitamin K injections after birth. These injections are commonly used to reduce the risk of internal bleeding. However, the results of studies are inconclusive. This link was examined in four studies published in the British Medical Journal in 1998. The results of two of the four studies suggest that there is no association between vitamin K injections and cancer, one could not exclude the possibility, and the fourth suggested a possible increase in the risk of leukemia.

In the first study, researchers assessed the risk in over 1000 Scottish children up to 14 years of age. They looked at the links between several types of cancer, including leukemia, and did not find an association. In another study, researchers in Oxford, UK compared the incidence of cancer in groups of children from 94 maternity units with different vitamin K policies. Their results showed that a raised risk was occasionally associated with vitamin K, but the overall results were not significant. The researchers are quoted as saying that "it is unlikely that there is a greatly increased risk of childhood cancer attributable to intramuscular vitamin K given to newborns, if indeed there is any." However, in another study the same researchers drew the conclusion that "the risk..... cannot be large, but the possibility that there is some risk cannot be excluded."

The results of the fourth study by researchers in the north of England indicate that there may be some risk. This study involved 685 children who developed cancer before age 15 and 3442 controls. The results showed a two-fold increase in risk of a type of cancer called acute lymphoblastic leukemia in 1 to 6 year olds who had been given vitamin K injections.

Some research suggests that oral supplements in three doses of 1 to 2 mg, the first given at the first feeding, the second at two to four weeks and the third at eight weeks may be an acceptable alternative to injections.

In general, infants taking formula have a lower risk of hemorrhagic disease than those who are breastfed, as vitamin K levels are higher in formula. Maternal vitamin K supplements can reduce the risk of deficiency in breastfeeding newborn babies. In a study published in 1997, University of Wisconsin researchers showed that maternal vitamin K supplements of 5 mg per day increased the vitamin K content of breast milk to levels comparable with that in infant formula. Plasma vitamin K levels of babies increased compared to the placebo group.

Osteoporosis
Vitamin K supplements may improve bone mineralization in postmenopausal women by boosting blood levels of osteocalcin and also possibly by decreasing calcium excretion through the urine. Research evidence suggests that vitamin K intakes much higher than the current recommendations improve biochemical markers of bone formation as well as bone density. Large supplemental doses of vitamin K have been used to treat osteoporosis.

Interactions with other nutrients
High intake of vitamin E (above 400 mg alpha TE (600 IU)) and vitamin A may antagonize vitamin K action and lead to an increased risk of hemorrhage.

Interactions with drugs
X rays and radiation, aspirin, cholestyramine, the anticonvulsant phenytoin and mineral oil laxatives can raise vitamin K requirements. Some snake venoms act by destroying vitamin K, thus causing uncontrolled bleeding. Vitamin K may be injected to stop the bleeding. Long-term use of antibiotics may produce vitamin K deficiency as these drugs can kill not only harmful bacteria but also the beneficial bacteria which produce vitamin K.

Cautions
Vitamin K supplements can interfere with the action of anticoagulant drugs such as warfarin.