1.5 Vitamin K
Like vitamin E, vitamin K is fat-soluble and composed of a series of related compounds. Vitamin K is widely distributed in the diet (see Table 1) and it is absorbed from the small intestine with the assistance of bile acids. Vitamin K is also manufactured by the bacteria that inhabit the human large intestine and appears to be absorbed there too. The main role of vitamin K is in blood clotting. This process requires the presence of a number of different chemicals, called clotting factors, in the blood. A number of these (including prothrombin and Factors VII, IX and X) require vitamin K in their synthesis. Deficiency could therefore result in an increasing tendency to bleed. Vitamin K also plays a role in the formation of bone and supplements can be effective in increasing bone density in osteoporosis.
How might a course of antibiotics affect the levels of vitamin K in the blood?
Antibiotics kill bacteria, so, as well as destroying those that are causing the infection for which the antibiotics have been prescribed, they also kill many of the useful bacteria in the gut. Since these bacteria synthesise vitamin K, their absence could lead to reduced vitamin K uptake from the gut into the blood for a few days until the normal population of bacteria in the gut is re-established.
There is a rare condition called vitamin K deficiency bleeding which occurs in about 1 in 10 000 babies in the first few weeks of life. Many babies who have this condition die or sustain significant brain damage due to bleeding into the brain. The condition occurs almost exclusively in breastfed babies, since human milk contains very little vitamin K, whereas it is added to formula milk. It is almost completely preventable by giving a single injection of vitamin K soon after birth, and such an injection has been given routinely to UK babies since the 1960s. However, two papers in the early 1990s suggested an association between the vitamin K injection and a very slight increase in the incidence of childhood leukaemia. This discovery led, in some countries, to vitamin K being offered as an oral dose instead. However, vitamin K by mouth was less effective than the injection at preventing vitamin K deficiency bleeding, which still occurred in 1 in 100 000 babies. Other research has not supported the link between vitamin K and leukaemia and a study published in 2004, which looked at 4000 cases of childhood cancers, found no association with the injections of vitamin K. In the UK now, new mothers may be given the choice of a vitamin K injection or a course of oral doses for their newborn baby.
List the vitamins that we have covered so far in this course. Which group remain?
So far we have covered vitamins A, D, E and K – the fat-soluble vitamins. The group that remains is the water-soluble group of vitamins, vitamins B and C.