Vitamin K refers to a group of fat-soluble vitamins that play a role in blood clotting, bone metabolism, and regulating blood calcium levels.

The body needs vitamin K to produce prothrombin, a protein and clotting factor that is important in blood clotting and bone metabolism. People who use blood-thinning medications, such as warfarin, or Coumadin, should not start consuming additional vitamin K without first asking a doctor.

Deficiency is rare, but, in severe cases, it can increase clotting time, leading to hemorrhage and excessive bleeding.

Vitamin K1, or phylloquinone, comes from plants. It is the main type of dietary vitamin K. A lesser source is vitamin K2, or menaquinone, which occurs in some animal-based and fermented foods.

Kale and other cruciferous vegetables are good sources of vitamin K.Share on Pinterest
Kale and other cruciferous vegetables are good sources of vitamin K.

Phylloquinone, also known as vitamin K1, is found in plants. When people eat it, bacteria in the large intestine convert it to its storage form, vitamin K2. It is absorbed in the small intestine and stored in fatty tissue and the liver.

Without vitamin K, the body cannot produce prothrombin, a clotting factor that is necessary for blood clotting and bone metabolism.

Most Americans are not at risk of a vitamin-K deficiency. It is most likely to affect newborns and those with a malapsorption problem, due, for example, to short-bowel syndrome, cystic fibrosis, celiac disease, or ulcerative colitis.

Newborns normally receive a vitamin K injection to protect them from bleeding in the skull, which could be fatal.

The recommended adequate intake for vitamin K depends on age and gender. Women aged 19 years and over should consume 90 micrograms (mcg) a day, and men should have 120 mcg.

Vitamin K benefits the body in various ways.

Bone health

There appears to be a correlation between low intake of vitamin K and osteoporosis.

Several studies have suggested that vitamin K supports the maintenance of strong bones, improves bone density and decreases the risk of fractures. However, research has not confirmed this.

Cognitive health

Increased blood levels of vitamin K have been linked with improved episodic memory in older adults.

In one study, healthy individuals over the age of 70 years with the highest blood levels of vitamin K1 had the highest verbal episodic memory performance.

Heart health

Vitamin K may help keep blood pressure lower by preventing mineralization, where minerals build up in the arteries. This enables the heart to pump blood freely through the body.

Mineralization naturally occurs with age, and it is a major risk factor for heart disease. Adequate intake of vitamin K has also been shown to lower the risk of stroke.

Vitamin K1 occurs in high amounts in leafy green vegetables, such as kale and Swiss chard. Other sources include vegetable oils and some fruits.

Sources of menanoquines, or K2, include meat, dairy products, eggs, and Japanese "natto," made from fermented soy beans.

Here are sample some food sources of vitamin K:

  • 10 sprigs of parsley contains 90 micrograms (mcg)
  • a 3-ounce serving of natto contains 850 mcg
  • a half-cup serving of frozen and boiled collard greens contains 530 mcg
  • one cup of raw spinach contains 145 mcg
  • 1 tablespoon of soybean oil contains 25 mcg
  • a half-cup serving of grapes contains 11 mcg
  • a hard-boiled egg contains 4 mcg

Most adults in the U.S. are believed to consume enough vitamin K.

Recipe tips

These healthy recipes have been developed by a registered dietitian. They can increase your vitamin K intake.

Baked halibut with garlicky kale & toasted cashews

Powered-up spinach lasagne

Roasted Brussels sprouts with toasted pecans & avocado

Spinach-pesto salad

Dietary fat enhances the absorption of vitamin K, so a salad of green leaves drizzled olive oil would both provide vitamin K and help the body absorb it.

No tolerable upper limit has been determined for vitamin K. Toxicity is rare and unlikely to result from eating foods containing vitamin K.

However, taking any type of supplement can lead to toxicity.

Vitamin K can interact with several common medications, including blood-thinners, anticonvulsants, antibiotics, cholesterol-lowering drugs, and weight-loss drugs.

Blood thinners, such as warfarin are used to prevent harmful blood clots that may block blood flow to the brain or heart. They work by decreasing or delaying vitamin K's clotting ability. Suddenly increasing or decreasing vitamin K intake can interfere with the effects of these drugs. Keeping vitamin K intake consistent from day to day can prevent these problems.

Anticonvulsants, if taken during pregnancy or while breastfeeding, can increase the risk of vitamin K deficiency in a fetus or a newborn. Examples of anticonvulsants are phenytoin and dilantin.

Cholesterol-lowering medications interfere with fat absorption. Dietary fat is necessary for absorbing vitamin K, so people who are taking this medication may have a higher risk of deficiency.

Anyone who is taking any of these medications should speak to their doctor about their vitamin K intake.

The best way to ensure the body has sufficient nutrients is to consume a balanced diet, with plenty of fruit and vegetables. Supplements should only be used in case of deficiency, and then, under medical supervision.