Vitamin K

How much do you need daily?

Adults need one microgram a day of vitamin K for each kilogram of their body weight. So, someone who weighs 70 kilograms (around 11 stones) would need 70 micrograms a day of vitamin K, while a person who weighs 83 kilograms (around 13 stones) would need 83 micrograms a day. This guideline is based solely on maintaining normal blood clotting and doesn’t take into account the probable higher requirement for bone and other health benefits.

You should be able to get all the vitamin K you need by eating a varied and balanced diet. Any vitamin K your body doesn't need immediately is stored in the liver, so you don't need it in your diet every day.

Are we getting enough?

The 2000-2001 UK National Diet and Nutrition Survey Dietary (NDNS) looked at vitamin K intake in adults aged 19 to 64. The average intake was 67 micrograms per day. However, 59 per cent of people had intakes below the UK guideline for adequacy (one microgram per kilogram of body weight per day). The situation had worsened since an earlier 1986-1987 NDNS when the average intake was 72 micrograms a day and 47 per cent had intakes below the target. The fall in vitamin K intake is attributed to the lower consumption of cooked leafy green vegetables. 

Why do we need it?

Vitamin K plays an important role in healthy blood clotting. The letter K is derived from the German word, koagulation (blood clotting), and is essential for wound healing when we injure ourselves. Our blood needs to start clotting very quickly otherwise we might bleed to death.

Another important function of vitamin K is in keeping our bones healthy and strong, being crucial to bone cell maintenance and bone protein formation. Low levels have been associated with an increased risk of osteoporosis and arthritis.

There are two types of vitamin K – K1 and K2. Vitamin K1 is found in leafy green and some other vegetables while vitamin K2 is usually found in small amounts in animal-based foods as well as being produced by bacteria. These bacteria convert plant-sourced K1 into K2. A good example is Bacillus natto, which is used in the fermentation of soya beans to produce natto. The bacteria in your gut perform a similar function.

Do I need a supplement?

No, a healthy vegan diet containing the above foods on a regular basis will cover your needs – no need for supplements, just eat your greens!

If you take vitamin K supplements, don't take too much as this might be harmful. The Department of Health says that taking one milligram or less of vitamin K supplements a day is unlikely to cause any harm.

However, if you take prescription medication that affects blood clotting (such as Warfarin), talk to your doctor about it as vitamin K can interfere with its efficacy. In 2012, a man taking anticoagulants (blood thinning medication) was admitted to hospital after doctors could not work out why his medication was not keeping his blood thin until they discovered he had been eating too many sprouts!

The best plant sources

Leafy green vegetables are by far the best source of vitamin K: spring greens, kale, spinach, cabbage, watercress, broccoli and Brussels sprouts all contain substantial amounts. Herbs and other vegetables (parsley, lettuce, asparagus, coriander, green beans, peas, cauliflower, runner beans, mustard and cress and leeks) provide an excellent source too. Plant oils are also a rich source of vitamin K (soya oil and rapeseed oil).

The Japanese food natto (made by fermenting cooked soya beans with Bacillus subtilis natto) has a very high content of K2 of about 1,000 micrograms per 100 grams. Meat, cereal and dairy products contain much lower amounts. Vitamin K is quite resilient and can withstand both cooking and freezing without huge losses.

Signs of deficiency

reduced blood clotting which may lead to easy bruising and prolonged bleeding, increased bone fragility and infections.

Vitamin K deficiency is very rare and usually only develops in people with severe liver or digestive tract diseases. People who take antibiotics for extended periods of time can experience a lack of vitamin K because antibiotics tend to kill the good gut bacteria that produce vitamin K2.