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What is vitamin K?

Vitamin K is a nutrient that the body requires in small, regular amounts. It is essential for the formation of several substances called coagulation factors as well as protein C and protein S that work together to clot the blood when injuries to blood vessels occur and to prevent excessive clotting. Insufficient vitamin K can lead to excessive bleeding and easy bruising. Vitamin K is also thought to play an important role in the prevention of bone loss. Low blood levels of vitamin K have been associated with low bone density, and there is some evidence that adequate levels of vitamin K can improve bone health while reducing the risk of fractures.

There are three different types of vitamin K:

  • Vitamin K1 (phylloquinone or phytomenadione) is the natural from of vitamin K that comes from foods, especially green leafy vegetables but also dairy products and vegetable oils. K1 is considered as the “plant form” of vitamin K, but it is also produced commercially to treat some conditions associated with excess bleeding.
  • Vitamin K2 (menaquinones) is made by bacteria, the normal flora in the intestines. Bacteria in the intestines can also convert K1 into K2. Vitamin K2 supplements K1 from the diet to meet the body’s requirements.
  • Menadione is a non-natural, man-made chemical compound that is used in some countries as a nutritional supplement because of its vitamin K activity. It is sometimes called K3. Menadione supplements are currently banned in the U.S. because of their potential toxicity in human use.

Since the body is not able to produce a sufficient amount of vitamin K, a certain amount must be taken in through the diet. It is present in a wide variety of foods, and the normal diet in the U.S. typically supplies enough. Examples of different foods that contain high amounts of vitamin K per serving, often more than the Recommended Daily Allowance (RDA), include leafy green vegetables such as kale, collards, spinach, green leaf lettuce, and other vegetables such as broccoli, asparagus, brussels sprouts, and cabbage. Other sources include dairy products, cereals, vegetable oils, and soybeans. K1 and K2, the types provided by the diet and produced by the body, are both fat-soluble and are stored in the body’s fat tissue and in the liver. An adult typically stores about a week’s worth of vitamin K.


About Vitamin K Deficiency

Causes of Vitamin K Deficiency

The most common causes of vitamin K deficiency are insufficient dietary intake, inadequate absorption, and decreased storage of the vitamin due to liver disease, but it may also be caused by decreased production in the intestines.

  • In the U.S., dietary deficiency of vitamin K is rare in healthy individuals but is relatively common in those who are severely ill or who have certain chronic conditions. For example, it is often seen in patients with serious illnesses, including people with cancer who are receiving chemotherapy, chronic dialysis patients, and people who are at risk for malnutrition, such as those with a poor diet associated with alcoholism or drug abuse.
  • Malabsorption, especially impaired absorption of fats due to diseases such as cystic fibrosis, celiac disease, chronic pancreatitis or Crohns disease, may cause vitamin K deficiency. Cholestatic liver diseases such as a bile duct obstruction or primary biliary cirrhosis can also lead to malabsorption and a deficiency in vitamin K.
  • Some medications, such as antibiotics, antacids, and anti-seizure medications can interfere with the absorption of vitamin K1, decrease the quantity of K2 produced in the intestines, or cause degradation of vitamin K. High doses of aspirin may increase vitamin K requirements.
  • Newborns may be deficient in vitamin K because their intestines do not yet have normal flora, and breast milk does not provide them with much vitamin K. In addition, if the newborn’s mother takes certain drugs during pregnancy, such as anti-seizure medications, then the infant may be vitamin K-deficient at birth. Vitamin K deficiencies in newborns are associated with vitamin K deficiency bleeding or VKDB (also known as hemorrhagic disease of the newborn). This can cause excessive bleeding and bruising and, in severe cases, can lead to fatal bleeding into the brain. VKDB used to be a relatively common occurrence but it has become more rare because of the practice of administering a vitamin K injection to all newborns shortly after birth, as recommended by the American Academy of Pediatrics. When surgeries are necessary, the infants may also be given vitamin K before the procedure to prevent excessive bleeding.

A common question is whether people who are treated with the anticoagulant drug warfarin (Coumadin®) should avoid eating foods that are rich in vitamin K and whether limiting these foods can cause an increased risk of vitamin K deficiency.

Warfarin is prescribed for people with a variety of conditions such as deep vein thrombosis (DVT) and some cardiovascular diseases (CVD) to “thin” their blood and prevent inappropriate clotting. A healthcare practitioner will typically prescribe warfarin and measure how well that dose “thins” the blood using a test called PT/INR. The dose may be adjusted up or down depending on the result and to ensure the dose is sufficient in preventing clots without causing excessive bleeding.

Warfarin works by reducing the liver’s ability to use vitamin K to make blood clotting factors. Thus, warfarin and vitamin K are antagonists—they work against each other. Significant increases or decreases in the amount of vitamin K a person consumes can affect how well their dose of warfarin works in preventing blood clots without causing excess bleeding. So rather than avoiding foods rich in vitamin K, it is more important for people taking warfarin to consume a consistent amount of those foods each day. These individuals can get the vitamin K they need as long as they are consistent about the amount they consume.

Signs and Symptoms

The signs and symptoms associated with vitamin K deficiency may include:

  • Easy bruising
  • Oozing from nose or gums
  • Excessive bleeding from wounds, punctures, and injection or surgical sites
  • Heavy menstrual periods
  • Bleeding from the gastrointestinal (GI) tract
  • Blood in the urine and/or stool
  • Increased prothrombin time (PT/INR)

In vitamin K deficiency bleeding in newborns, signs and symptoms may be similar to those listed above but, in more serious cases, may also involve bleeding within the skull (intracranial).

A deficiency of vitamin K may be suspected when symptoms listed above appear in someone who is at an increased risk, such as:

  • Those who have a chronic condition associated with malnutrition or malabsorption
  • Those who have been on long-term treatment with antibiotics; the antibiotics can kill the bacteria that aid in the production of vitamin K2 in the small intestine.
  • Seriously ill patients such as cancer or dialysis patients


A deficiency of vitamin K is usually discovered when unexpected or excessive bleeding occurs. In such cases, a prothrombin time (PT/INR) is the main laboratory test performed to investigate the bleeding. If the result is prolonged and is suspected to be due to low levels of vitamin K, then vitamin K will often be given by injection. If the bleeding stops and the PT returns to normal, then a vitamin K deficiency is assumed to be the cause.

Other coagulation tests may occasionally be performed to evaluate someone with symptoms of excessive bleeding and bruising, such as partial thromboplastin time (PTT), thrombin time, platelet count, platelet function tests, coagulation factor tests, fibrinogen, von Willebrand factor, and d-dimer.

Measurements of the level of vitamin K in the blood are rarely used to determine if a deficiency exists. Since this is not a routine test, it is usually performed in a reference laboratory and results may take several days.


Short-term treatment for vitamin K deficiency usually involves either oral supplementation or injections. Long-term or lifetime supplementation may be necessary for those with underlying chronic conditions. The action of vitamin K typically requires 2 to 5 days after it is given to show treatment effect.

Problems with high levels of natural forms of vitamin K (K1 and K2) have not been reported. These forms have low toxicity, even at high concentrations. However, water-soluble vitamin K3 can be toxic if administered in large quantities. Also, K3 is known to cause hemolytic anemia in infants, so it is not used to treat the very young.

Vitamin K-dependent clotting factors are produced by the liver. If a person has chronic liver disease, that person may not be able to produce sufficient clotting factors even when adequate vitamin K is available. Vitamin K supplementation may not be effective in those with seriously damaged livers.

View Sources

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