Understanding Vitamin K


In the 1920’s and early 1930’s, Danish researchers discovered a substance that was essential for forming blood clots. They called it Vitamin K, for the Danish word koagulation, which is like the English word coagulation and refers to blood clotting.

The Role It Plays

Vitamin K is essential for making the blood clots that quickly stop the bleeding whenever we injure ourselves. All vitamin K ends up in the liver, where it’s used to make some of the substances that make our blood clot. Blood normally has a number of different clotting factors – substances that help it form clots to stop bleeding from cuts, bruises, and other injuries. Vitamin K is needed to help the liver make prothrombin, the most important of the clotting factors. Some of the other factors are also made in the liver and depend on Vitamin K. Without clotting factors, blood clots very slowly or not at all, so even a small cut can bleed for a long time and even a minor bang can cause a big bruise.

Vitamin K is mostly needed to help stop bleeding but it has some other jobs too. The most important role is the one it plays in building bones. It is needed to help our bones hold on to calcium and to make sure the calcium gets to the right place. Vitamin K helps the bones grab onto calcium, put it in the right place, and hold onto it once it’s there. If there isn’t enough Vitamin K new bone won’t form very well. In the long run, a shortage of vitamin K can easily lead to osteoporosis.

There has also been some interesting research on vitamin K and cancer. Vitamin K seems to slow down or kill tumor cells in the lab just as well as powerful drugs. There are currently some studies looking at combining Vitamin K with standard anticancer drugs to help them work better.

Types of Vitamin K

This fat-soluble vitamin actually comes in three different forms.

First, there’s vitamin K1, or phylloquinone. This is the form of vitamin K that is found in plant foods.

Next, there’s vitamin K2, called menaquinone. This is the form friendly bacteria make in our intestines.

The last form is vitamin K3, called menadione which is an artificial form.

There is also a water soluble version of vitamin K1, called  phytonadione.


Age                                        Vitamin K

0 – 0.5   years                     5 mcg

0.5-1 year                            10 mcg

1-3 years                              15 mcg

4-6 years                              20 mcg

7-10 years                           30 mcg

Men 11-14 years              5 mcg

Men 15-18 years              65 mcg

Men 19-24 years              70 mcg

Men 25+ years                  80 mcg

Women 11-14 years        45 mcg

Women 15-18 years        55 mcg

Women 19-24 years        60 mcg

Women 25+ years           65 mcg

Pregnant Women            65 mcg

Nursing Women               65 mcg

Safe Dosage

The RDA for vitamin K is pretty new. Until 1989 there was no RDA because researchers thought that all the vitamin K we needed was made by friendly bacteria. In fact, that bacteria makes only half or less of what we need. Most of the vitamin K we need comes from green leafy vegetables.

The RDA is set for a mythical average person. If you are heavier than average than you’ll need more vitamin K, if you are smaller than average you will need less vitamin K.

Make It Work Better

If you’re advised to take Vitamin K supplements, you will probably have to take only 100 mcg a day. Any more than that in the synthetic form could be toxic and might cause liver damage. Talk to your doctor about supplements made with phytonadione, a water-soluble version of vitamin K1.

Good Sources

The best food source of Vitamin K is seaweed. A lot of foods haven’t ever been analyzed to find out how much Vitamin K they have. And the ones that have been studied, the K amounts are variable. In general though, all the dark-green leafy vegetables, like kale, broccoli, and cabbage, are good sources. Strawberries are also good. Very little Vitamin K is lost in cooking.


If you want to make sure you’re getting enough vitamin K, the best approach is to eat your vegetables.

If you’re low on vitamin K because of an intestinal problem that makes it hard to digest fats, you may need supplements to treat or prevent clotting problems.

Sometimes newborn babies don’t have enough Vitamin K because they don’t yet have any bacteria to make it in their intestines. To make up for that, most newborns are given an injection of Vitamin K soon after birth.

If an adult gets Vitamin K deficiency, it’s generally because they eat very few green leafy vegetables or because they have been taking oral antibiotics for a long time. The antibiotics kill off the intestinal bacteria that make Vitamin K.

Sometimes Vitamin K deficiency is caused by liver disease or a problem digesting fat.

Some drugs block the absorption of Vitamin K and other fat-soluble vitamins: cholestyramine (Cholybar or Questran) or colestipol (Colestid) to lower cholesterol.




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