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Vitamin K
What is it?
Vitamin K is a fat-soluble vitamin that comes in many forms. It consists of a group of compounds called vitamin K1, K2, K3 or K4.

 These compounds occur in a variety of natural and laboratory environments.

  • K1 (phylloquinone) is present naturally in plants.
  • K2 (menaquinone) is made by bacteria in the intestinal tract of humans and animals. It is about 75 percent as active as K1. (K2 is a whole family of compounds including menaquinone-4 and MK5 through MK13.)
  • K3 (menadione) is man made.
  • K4 (menadiol) is man made. It can be used by intestinal bacteria to make K2. Unlike the fat-soluble compounds K1, K2 and K3, K4 is water-soluble.

What does it do?
Vitamin K protects us from bleeding excessively due to cuts and wounds or due to internal bleeding. It is used by the body in blood clotting and in the formation of prothrombin and other blood-clotting proteins. Vitamin K is also required for the biosynthesis of some proteins found in plasma, bone and kidneys.

Where do you get it?
The vitamin K in most foods is low -- less than 10 micrograms per 100 grams. Most vitamin K from food sources is obtained from leafy green vegetables (such as those in the accompanying chart) and from four vegetable oils, including olive oil and soybean oil. Beef liver and green tea are also sources of vitamin K.

Leafy Green Vegetables: Sources of Vitamin K


Micrograms of Vitamin K*



Swiss chard






Red leaf lettuce


*based on 1/2 cup servings

Even though the absorption of vitamin K from food is relatively low, a typical mixed diet in the U.S. provides about 400 micrograms a day.

How much do we need?
The current Recommended Dietary Allowance (RDA) for vitamin K is one microgram per kilogram of bodyweight. For adults, this represents about 65 to 80 micrograms a day; young adults usually take in around 80 micrograms a day, and older people consume about 150 micrograms a day. Recent research suggests that getting 420 micrograms a day increases the bone-building proteins in the blood.

There is no reason for healthy people to self-prescribe oral supplements of vitamin K.

Vitamin K deficiency is a risk for:

  • adults with long-term antibiotic intakes
  • people with lipid malabsorption syndromes
  • newborn infants (which is why they are given injections of vitamin K after birth)

Is it safe?
No upper limit has been set for safe ingestion of vitamin K, and no toxic effects have been observed due to large amounts of vitamin K being ingested over long periods of time. However, under certain circumstances, vitamin K can be problematic:

  • People who take coumadin need to be carefully monitored.
  • Vitamin K combined with high intakes of vitamin E can produce hemorrhage, especially for people who also take anticoagulants.
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