Vitamin K

Vitamin K is a fat-soluble vitamin sometimes called the Blood Coagulation Vitamin. It is also known as phytomenadione, phyloquinone, phytylmenadione and the anti-haemorrhagic vitamin. The general term vitamin K is used to describe the group of quinone compounds which all have anti-haemorrhagic effects and are numbered 1, 2 and 3. A derivative of vitamin K, called acetomenaphthone has the same biological activity.

Vitamin K is measured in micrograms (mcg or g).

Vitamin K1 is fairly widely distributed in foods, the richest sources being fresh leafy green vegetables like broccoli, lettuce, cauliflower, sprouts, kale and spinach. Liver (ox and pig) and lean meat also contain useful amounts of vitamin K. Milk is a poor source with only trace quantities of the vitamin.

Whilst the major source of this vitamin is food, it is also synthesised by bacteria in the large intestine, in the form of K2. This is readily absorbed and utilised by the body and is thought to supply some 50% of the daily requirements of most people.

Like other oil-soluble substances, vitamin K is absorbed in association with fats and requires the assistance of bile salts. Absorption takes place in the intestine following which the vitamin is used by the liver. Only a small amount is retained and stored in the liver and the rest is metabolised. Since so little is stored, if there are any problems of malabsorption, depletion will occur rapidly: deficiency symptoms can be apparent within a week. Absorption of vitamin K can be impaired by conditions such as colitis or sprue or if large doses of liquid paraffin are consumed.

Vitamin K is destroyed by acids, alkalis, oxidising agents, light and ultraviolet irradiation. This nutrient appears to survive storage and cooking methods quite well. The water used for cooking green vegetables contains large amounts of vitamin K and should be used in gravy or sauces wherever possible. Commercial processing and freezing is more destructive and frozen foods have lower values of the vitamin than the fresh equivalent. 

No dietary deficiencies of vitamin K have been demonstrated in normal adults. Infants are particularly prone to a sterile intestinal tract without the bacteria necessary to produce the vitamin. In addition human breast milk is a poor source of vitamin K. Studies conducted in a major London Teaching Hospital demonstrated that vitamin K is not easily transferred across the placenta from the mother to the unborn baby. This is why babies may be prone to a bleeding disorder in the first few days of life. Consequently it is a widespread practice to give the new-born an injection of vitamin K to protect them against possible bleeding in the brain.

Any condition that impairs fat absorption may also drastically reduce the amount of vitamin K available to the body. Patients with an obstruction of the bile duct for instance are routinely injected with vitamin K before they undergo an operation to prevent haemorrhage at the time of surgery. 

The most common reason for vitamin K deficiency is prolonged use of antibiotics. This is because when antibiotics destroy bacteria they cannot differentiate between harmful and harmless strains and so kill all types. If prolonged intake of antibiotics is combined with problems of malabsorption or insufficient dietary intake, the net result is likely to be a tendency to bleed.

Prothrombin and Factors VII and IX are activated only in the presence of calcium, which bonds the pro-enzymes to the activator. When there is a lack of vitamin K, large doses of vitamin E may exacerbate effects of the deficiency.

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