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Vitamin K is a fat soluble substance that plays a key role in helping blood to clot. This is the action that takes place when cells in the blood act like glue and stick together at the surface of a cut. Clotting is necessary following an injury that causes bleeding. Without clotting, a person easily can bleed to death. Clotting, or coagulation, is the end of a complex process involving numerous proteins, enzymes and vitamin K.

There are three forms of vitamin K:

• Phylloquinone (K1). Found in plant sources of food, including oils.
  
  
• Menaquinone (K2). Produced by the body and by animal sources of vitamin K (e.g., liver, egg yolks).
  
  
• Menadione (K3). Manufactured, or "synthetic," form.
  

The Food and Nutrition Board of the National Academy of Sciences has issued the following adequate intakes (AIs) for vitamin K. The figures are expressed in micrograms (mcg):

For years, it has been believed that most Americans get enough vitamin K through their diets to meet these recommendations. However, recent studies have brought this belief into question. In particular, surveys have indicated that children and young adults may not be getting as much vitamin K from their diets as they should.

fat soluble vitamins such as vitamin K can be stored in the liver and the fatty tissues of the body. However, the body stores very little vitamin K. 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. For example, prolonged use of antibiotics can lead to destruction of the bacteria in intestines that create vitamin K. Meanwhile, use of anticoagulant drugs can prevent proper absorption of vitamin K.

Various diseases can lead to vitamin K deficiency, including cystic fibrosis, celiac disease, Crohn’s disease and liver disease. Other causes of vitamin K deficiency include:

• Ingesting too much mineral oil
  
  
• Ongoing diarrhea
  
  
• Long-term use of total parenteral nutrition (TPN), which is nutrition provided intravenously
  
  
• Ongoing hemodialysis
  
  
• Serious burns

Elderly individuals may also be at an increased risk of vitamin K deficiency. Many older people take many medications, eat inadequate amounts of vegetables and may have bacterial overgrowth which impacts the production of vitamin K in the gut.

People with a deficiency of vitamin K may have blood that does not properly coagulate. Deficiency also may lead to easy bruising. 

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 foods rich in vitamin K because the vitamin can prevent these drugs from working properly.