Vitamin K is a family of chemically related substances called naphthoquinones. There are two major types of vitamin K that are important for humans: K1 and K2. Vitamin K1 (phylloquinone or phytonadione) is a natural nutrient found in green leafy vegetables. Vitamin K2 (menaquinones or MKs) in foods are mostly in the form of MK4 and MK7.  You can convert vitamin K1 directly into MK4 within certain tissues in your body, and as long as you are getting enough vitamin K1, you are likely to be getting enough vitamin K2 (MK4 type). MK7 is a natural product of bacteria that live in your lower intestine. MK7 is also found in large amounts in the fermented Japanese soy product known as natto.  The Food and Nutrition Board considers 90 micrograms per day for women and 120 micrograms per day for men to be an adequate intake (1 milligram is equal to 1,000 micrograms). Foods usually provide your body with about half of the normal supply of the vitamin, while intestinal bacteria produce the rest.

Newborns lack the bacteria in their intestines to produce vitamin K and may be at risk of serious bleeding. Newborns are usually given vitamin K supplements, either as a shot or by mouth, while in the hospital. Babies who get supplements in the hospital do not need any more after they leave unless they have intestinal problems.

Vitamin K is necessary for a wide variety of bodily functions, including:

1. Normal blood clotting: the letter “K” in the name of this vitamin originally came from the German word koagulation. Blood clotting is a process that is essential for your health. Whenever you get a wound, you need sufficient clotting ability to close the wound and prevent excessive bleeding. However, you don’t want too much blood clotting, because you don’t want a clot to block a blood vessel. Vitamin K is one of the key nutrients for keeping your blood clotting ability at the correct level. For clotting factors to successfully close a wound, they need some way to stick onto the nearby tissue surfaces. Two enzymes catalyze a chemical process called carboxylation that provides glumatic acid, an amino acid in the clotting factors, with “stickiness.” One of these enzymes, (vitamin K epoxide reductase) helps recycle and recharge vitamin K to keep the carboxylation going. For people with an excessive tendency to form blood clots, anticoagulant drugs like warfarin can be life saving. Likewise, too much vitamin K can pose problems for anyone taking anticoagulants such as warfarin, and they should speak with a physician about dietary restrictions.
2. Bone health: in adequate amounts, it protects against bone fractures, especially in post-menopausal women. People who are deficient in vitamin K have a greater risk of fracture. Vitamin K protects your bones in at least two ways. Vitamin K1 helps prevent excessive activation of osteoclasts, the cells that help take minerals out of your bones and make them available for other body functions. While this activity is important for your health, too many osteoclasts (or too much activity by osteoclasts) would mean too much bone demineralization. Vitamin K helps your body keep this process in check. One of the menaquinone forms of vitamin K (MK-4, or menatetrenone) blocks the formation of too many osteoclasts and may initiate their self-destruction (apoptosis).  Additionally, friendly bacteria in your intestines convert vitamin K1 into vitamin K2, which activates carboxylation, the same process discussed above that affects the stickiness of clotting factors. Vitamin K is required for proper activity of the carboxylase enzyme that allows carboxylation of the osteocalcin proteins in your bones, restoring these bone proteins to their proper place in your bone structure and strengthening the composition of your bones. Osteocalcin is the major non-collagen protein in bone, and it anchors calcium molecules inside of your bones.  When too few of the osteocalcin proteins in your bones are carboxylated, you have increased risk for fracture, especially hip fracture.
3. Cardiovascular health: Sometimes calcium builds up inside a tissue that is normally soft, causing the tissue to harden and stop functioning properly. When calcium builds up inside the arteries, it is called hardening of the arteries. Carboxylated MGP, or matrix Gla protein, directly blocks the formation of calcium crystals inside the blood vessels, and vitamin K is required for this carboxylation process. In other words, the heart-protective benefits of MGP in prevention of calcification depend upon vitamin K, and people with vitamin K deficiency are at greater risk for hardening of the arteries than individuals with healthy vitamin K intake.
4. Cancer protection: Eating a diet rich in the powerful antioxidant vitamin K can reduce the overall risk of developing or dying from cancer, according to a study in the American Journal of Clinical Nutrition.

Vitamin K continues to be studied for its role in:

1. Protection against oxidative damage: Vitamin K does not function directly as an antioxidant (like vitamin E and vitamin C do), but both phylloquinone and menaquinone forms of vitamin K appear to protect cells—especially nerve cells – from oxidative damage.
2. Proper regulation of inflammatory response:  several markers of pro-inflammatory activity—including, for example, release of interleukin-6 (IL-6)—are significantly lowered by healthy vitamin K levels.
3. Support of brain and nervous system structure: Vitamin K is required for synthesis of important brain and nervous system fats called sphingolipids. These fats are critical in the formation of the myelin sheath that forms an outer wrapping around the nerves, and both phylloquinone and menaquinone forms of vitamin K have been found effective in supporting synthesis of these key nervous system components.

Excellent sources of vitamin K include kale, spinach, collard greens, chard, turnip greens, mustard greens, Brussels sprouts, parsley, Romaine lettuce, broccoli, okra, asparagus, cabbage, basil, thyme, leeks, celery, blueberries, kiwifruit, sage, oregano, natto, cucumber, cauliflower, green beans, tomatoes, and sea vegetables.

Vitamin K Content of Selected Foods

*Sometimes the outer leaves of a plant can be more concentrated in vitamin K than the inner leaves. Carefully and gently rinse vegetables like cabbage and Brussels sprouts (you can use a soft-bristle brush) to clean the outer leaves and keep them in your meal rather than discarding them.

**On an ounce-per-ounce basis, Romaine lettuce may be about four times higher in vitamin K than iceberg lettuce.

***Fermenting foods can increase their vitamin K content. Bacillus subtilis, when used in fermenting soybeans into natto, creates a K2 form of the vitamin called menaquinone-7 (MK-7). Higher levels of MK-7 in the blood correspond to lower risk of hip fracture in older women, and higher MK-7 levels correspond to increased intake of soy foods fermented with Bacillus bacteria. In addition, these bacteria can potentially stay alive in your lower intestine for up to six days after you eat these foods continue providing you with vitamin K2 (in the form of MK-7).

****Because vitamin K fat-soluble, foods tend to become higher in vitamin K as they become more concentrated, so, on an ounce-for-ounce basis, tomato paste is higher in vitamin K than fresh tomatoes.

To maintain optimal health, make sure that your diet includes plenty of fruits and vegetables that are rich in vitamin K.