Vitamin K

Vitamin K is a fat-soluble vitamin, the name for a group of compounds: phylloquinone (vitamin K1) and a series of menaquinones (vitamin K2). It is needed to make proteins involved in blood clotting and bone metabolism.

Forms and sources

Phylloquinone is found mainly in green leafy vegetables and is the main dietary form of vitamin K; vegetable oils and some fruits also contain it. The menaquinones, designated MK-4 to MK-13 by the length of their side chain, are mostly of bacterial origin and occur in modest amounts in meat, dairy foods, eggs and fermented foods; MK-4, MK-7 and MK-9 are the best studied.

In the body

Vitamin K is needed by an enzyme that makes proteins for blood clotting and bone metabolism; prothrombin (clotting factor II), a vitamin K-dependent protein in plasma, is directly involved in blood clotting.

How much

In the United States, the adequate intake for adults is 120 µg a day for men and 90 µg for women. Newborn babies are at risk of vitamin K deficiency, because little vitamin K crosses the placenta; in the first weeks of life, deficiency can cause vitamin K deficiency bleeding.

Safety and medicines

No upper limit has been set for vitamin K, because it has a low potential for toxicity, and no adverse effects of vitamin K from food or supplements have been reported. Vitamin K can, however, interact seriously with anticoagulants such as warfarin, which work against vitamin K; people taking them need a consistent intake of vitamin K from food and supplements.

Products containing it

• All Day Fit A-Z Compact tablets (S 06)
• Viva K2 capsules (D 224)