Vitamin K is the name given to a group of compounds, each of which has a slightly different chemical structure and, therefore, plays a slightly different role within the body. It's one of the fat-soluble vitamins and can be stored by the body so that you don't need to eat it every day.
Vitamin K is an unusual vitamin because, as well as being available in our diet, it's also made by bacteria living in the intestine.
We need vitamin K to help blood to clot - without it, wounds won't heal properly. It also plays a role in building strong bones.
Where can I get it?
One type of vitamin K is produced by plants. The best sources are peas and green leafy vegetables like broccoli, spinach, cabbage and Brussels sprouts.
Another type is found in smaller amounts in meat, poultry and dairy foods.
A form of vitamin K is made by bacteria in the intestine, but it's thought that not enough is produced to manage without any in the diet. Of course, the amount made by these 'friendly' bacteria varies from person to person and it's extremely difficult to conduct research into this. It's known that if you're on antibiotics the intestinal bacteria will be greatly reduced so you should increase the amount of vitamin K-rich foods in your diet.
How much should I eat?
In the UK, vitamin K deficiency is extremely rare. In fact, it's usually only seen in adults who have another health problem.
Long-term use of antibiotics can reduce production of the vitamin by intestinal bacteria and, because it's a fat-soluble vitamin, it can't be used properly if fat isn't absorbed by the body, which can happen when someone has liver, pancreatic or intestinal disease.
Vitamin K is found in a wide range of foods but if overall food intake is exceptionally low - if someone suffers from anorexia nervosa, for example - then there's a risk of deficiency.
Newborn babies are at risk of deficiency because their intestinal bacteria aren't well enough developed yet, and there are low levels of vitamin K in breast milk. This means that a small number of babies are at risk of bleeding - in order to prevent this hospitals give newborns extra vitamin K (with their parents' consent).
Because of the difficulty estimating how much vitamin K is produced by the body, it's been impossible to set a definitive recommendation for intake. Research has been conducted into the amount needed to ensure that blood clots properly, and a guideline has been set, of 1 microgram (0.001mg) per kilogram of body weight per day. For example, a woman who weighs 57kg (approx. 9 stone), should consume around 57 micrograms (0.057mg) per day. This is easily achieved by eating a varied, balanced diet with plenty of vegetables - one portion (80g) of broccoli will provide over a day's worth of vitamin K. Supplements shouldn't usually be necessary, but if you do decide to take them, the Food Standards Agency recommends that you don't exceed 1mg per day, so check the packet.
What does it do?
Most research into vitamin K has focused on its role in blood clotting or coagulation (the vitamin was discovered in Denmark and named for the Danish word ‘koagulation') and it's well known that wounds won't heal properly in vitamin K-deficient people.
People who are using the blood-thinning medication warfarin can still eat vitamin K-containing foods, but should talk to their doctor about their diet.
More recently, research has suggested that vitamin K also helps to build and maintain healthy bones, but it's not yet fully understood how this works. It may also be involved in brain function, but this role has yet to be thoroughly researched.
