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Vitamin K is a substance that plays a key role in helping blood to clot. Without clotting, a person can bleed to death, even after a minor injury. Vitamin K also plays an important role in helping bones develop normal structure.

Vitamin K deficiency resulting from dietary factors is rare. However, deficiency may develop when conditions such as bile-production failure prevent proper fat absorption of the vitamin. Use of certain drugs also can prevent proper absorption of vitamin K. A deficiency can lead to failure of the blood to clot and tendency to bruise easily.

Excessive intake of vitamin K does not appear to cause illness or symptoms on its own. However, people who take anticoagulants need to avoid eating excessive amounts of food rich in vitamin K because the vitamin can prevent the drug from working properly. 

Bacteria in the body’s intestines produce about half of the body’s needed vitamin K. The rest must come from dietary sources. The best sources include:

• Green, leafy vegetables (e.g., spinach)
• Other cruciferous vegetables (e.g., broccoli, cauliflower)
• Liver

Vitamin K is also available in supplement form. People who have bruising and bleeding disorders may benefit from taking supplements. Supplements also may be recommended for women who are at risk of osteoporosis. However, people should not take vitamin K supplements without first consulting a physician.