If you've read about vitamins A, B, C, D, and E, you might feel like we've missed a few vitamins as we jump over to vitamin K. But there are no vitamins F through J (at least not yet).
Vitamin K is named after the German word for blood clotting (koagulation). In fact, this is probably the most common connection that people make with vitamin K—they associate this vitamin with the process of blood clotting. We'll explain more about this function of vitamin K in our "Role in Health Support" section below. However, it's important to know that vitamin K makes a variety of unique contributions to our health, and our knowledge about these contributions has been expanding in new and unexpected ways.
There are three basic types of vitamin K. Their common names are K1, K2, and K3.
The K1 form of vitamin K is found in plant foods, and 44 of our WHFoods are plant foods that serve as excellent, very good, or good sources of vitamin K! Many of our best sources of this vitamin are green vegetables (including 16 excellent sources); this makes good sense since K1 is required for green plants to conduct the process of photosynthesis. The K2 form of vitamin K is made from K1 and K3 by bacteria and other microorganisms. It can also be made in the human body through a conversion process involving K1 and K3.
In plant foods, you won't find much preformed K2, unless those plant foods have been fermented or otherwise transformed by bacteria or other microorganisms. Certain microorganisms can convert K1 into K2. A great example is Bacillus natto. This bacterium can convert K1 into K2 and it is often used in the production of fermented soy products. In fact, this practice is so common that you will sometimes find the word "natto" being used to refer to these foods. Fermented soyfoods on our WHFoods list—including tempeh and miso—can contain significant amounts of K2. (And as plant foods, they also naturally contain K1.) Most of our WHFoods animal foods also contain K2, although the amounts are relatively small and insufficient to qualify them as excellent, very good, or good sources of vitamin K.
A third type of vitamin, found preformed in food but in very small amounts, is menadione, or vitamin K3. We don't yet have good research on the health role of these small of K3 amounts in food.
Role in Health Support
As mentioned in the Description section, vitamin K is perhaps best known for its role in the blood clotting process. When people hear the term "blood clot," they might sometimes jump to the conclusion that a blood clot is bad. But there are many times when it is very important for our blood to clot. For example, blood clots are necessary to stop bleeding when our skin gets punctured.
Yet at the same time, people are correct when they say that blood clotting can cause problems. For example, if the inside of a blood vessel has become too narrow due to the buildup (over time) of plaque, this plaque can sometimes rupture and our body may form a blood clot in order to seal off the ruptured plaque. However, this blood clot might also end up stopping the flow of blood through the blood vessel since the blood vessel had become overly narrowed from the buildup of plaque.
Regardless of the specific situation, vitamin K is necessary for blood clots to form. The clotting process is very complex, requiring at least 12 proteins to function before the clotting process can be completed. Four of these protein clotting factors require vitamin K for their activity.
Luckily, we rarely see vitamin K deficiency lead to impairment in the clotting process in adults. We see it in newborns because vitamin K does not efficiently cross the placenta to the fetus, and it can take several weeks for the fetus to build up dietary stores. We also occasionally see clotting problems related to vitamin K deficiency in persons with severe liver or gastrointestinal diseases. But vitamin K deficiency basically never causes insufficient clotting disorders in healthy adults.
In contrast to insufficient clotting in healthy adults, we do see vitamin K deficiency becoming involved in unwanted clotting. This process once again involves the activity of multiple vitamin K-dependent enzyme systems, most importantly a system called matrix Gla protein.
It is currently somewhat of an open question how important vitamin K is to the progression of clot formation and heart disease. Researchers have sometimes, but not consistently, been able to correlate low vitamin K intake with increased risk of heart disease.
One problem in interpreting this research, however, is separating out the effect of healthy foods from the nutrients they contain. Even casual readers of this site are probably aware that the same green leafy vegetables that are our richest sources of vitamin K1 are also among the best sources of many other heart-protecting nutrients. Included in this heart-protective list from green leafy vegetables would be the vitamins A (in the form of carotenoids), C, E and B6, the minerals potassium and magnesium, and dietary fiber.
Researchers have attempted to answer this question by giving vitamin K in pill form at amounts similar to those found in the diet. Over a three-year period, 500 mcg of vitamin K—about the amount found in one serving of mustard greens—was associated with slightly slower progression of hardening of the arteries of the heart.
Given the preliminary and somewhat contradictory nature of this research, we would characterize the association between diets high in vitamin K and protection against coronary artery disease to be plausible, but still unproven.
Vitamin K is a fascinating nutrient with respect to bone health, and unlike some of the open-ended questions related to clotting, knowledge about the role of vitamin K nourishment in bone support is fairly well-established. Individuals who are vitamin K deficient have repeatedly been shown to have a greater risk of fracture. In addition, for women who have passed through menopause and have started to experience unwanted bone loss, vitamin K has clearly been shown to help prevent future fractures.
Bone support involves different forms of vitamin K
Research has shown that our bone cells take up vitamin K in the form of K1 as well as K2, suggesting that these forms of the vitamin may play different roles in the health of our bone. In the case of K2, researchers have also become interested in two particular subtypes of K2 called MK-4 and MK-7, which appear to be uptaken by our bone cells in preference to other subtypes. In fact, research on bone health is partly responsible for getting researchers more and more interested in the whole issue of vitamin K2 subtypes. Vitamin K2 contains a chemical "tail" composed of repeating units called prenyl units. The most common forms of K2 contain either 4,5,7,8, or 9 prenyl units, and are therefore referred to as MK-4, MK-5, MK-7, MK-8, and MK-9. (The letter "M" in "MK" refers to "menaquinone"—the scientific name for the K2—and the "K" refers to the common name of vitamin K.) While human diets usually consist of about 10-25% K2, the proportion of these different K2 forms can vary widely. Fermented soy foods (mentioned earlier in this article as an important source of K2) tend to have greater amounts of MK-7. Cheese may have greater amounts of MK-8 and MK-9. However, in the average U.S. diet, MK-4 typically accounts for about one-third or more of all K2 due to its presence in eggs and meats.
How bone support works
The bone-related benefits of vitamin K appear to depend on at least two basic mechanisms. The first of these mechanisms involves a type of bone cell called osteoclasts. Osteoclasts are bone cells in charge of bone demineralization—they help take minerals out of the bone and make them available for other body functions. While the activity of these cells is important for proper health, we do not want too many osteoclasts (or too much activity by osteoclasts) since those imbalances would mean too much demineralization of bone. Vitamin K helps our body keep this process in check. The MK-4 form of vitamin K2 (also called menatetrenone) is known to block formation of too many osteoclasts, and perhaps also to initiate their programmed cell death (a process called apoptosis).
A second mechanism involves the role of vitamin K in a process called carboxylation. (This process is the same one discussed earlier in relationship to the stickiness of clotting factors required for proper blood clotting.) For our bones to be optimally healthy, one of the proteins found in bone—a protein called osteocalcin—needs to be chemically altered through the process of carboxylation. (Osteocalcin is not just any typical bone protein. It is a protein especially linked to our bone mineral density (BMD), and for this reason, it often measured in our blood when doctors are seeking to determine the health of our bone.) When too few of the osteocalcin proteins in our bone are carboxylated, our bones have increased risk for fracture. This unwanted risk appears to be particularly important with respect to hip fracture. Scientists refer to this bone problem as a problem involving "undercarboxylated osteocalcin" and they have determined that vitamin K can greatly improve the situation. Since vitamin K is required for proper activity of the carboxylase enzyme that allows carboxylation of the osteocalcin proteins in our bone, vitamin K can help restore these bone proteins to their proper place in our bone structure and strengthen the composition of the bone. In clinical studies, both K1 and K2 forms of vitamin K appear to play a role in osteocalcin carboxylation. Some studies show the K2 form (and specifically MK-4) to be especially helpful in postmenopausal bone protection.
Whether provided by the diet in the form of K1 or K2, this vitamin is becoming more and more focal in research on bone protection. Low levels of vitamin K intake are emerging as dietary risk factors for osteoporosis. Researchers have shown that increasing dietary vitamin K intake by 100 mcg per day—roughly doubling the average American adult intake for a time period of one full year—can lead to a significant increase in bone density in post-menopausal women. Low levels of vitamin K have also been associated with increased risk of arthritis. Low activity of vitamin K-dependent proteins inside the joints has been suggested as a likely mechanism for this increased risk.
Other Potential Health Benefits
Not suprisingly based on its role in photosynthesis and movement of electrons to generate energy, vitamin K may function as an important antioxidant nutrient especially in certain chemical forms (called "reduced" forms). In older men, vitamin K has been shown to help improve insulin resistance. In preliminary lab and animal studies, vitamin K has been investigated as a critical nutrient for protecting cells that line blood vessels, including both veins and arteries.
Summary of Food Sources
Both plant and animal foods can provide us with significant amounts of vitamin K. Fresh green vegetables are our most steadfast source of K1. At WHFoods, 16 of our green vegetables rank as excellent sources for this vitamin. Many of our Herbs & Spices—including parsley, basil, cilantro, sage, oregano and black pepper—also provide excellent amounts of vitamin K.
One serving of any food noted above will provide you with at least 10% of your daily vitamin K needs. In the case of kale—our top source of vitamin K—a 1-cup serving will provide you with over 1,000 micrograms, which is approximately 10 times the recommended minimum daily amount! Since the National Academy of Sciences has chosen not to set a maximal recommended intake level (Tolerable Upper Limit, or UL) based on available research, you will not be exceeding a recommended maximum amount even with ten times the minimum requirement (or greater amounts).
Outside of the vegetable family, you will find kiwifruit, blueberries, prunes, and grapes amount the most vitamin K-rich fruit sources, and soybeans and miso as two good legume sources. As mentioned earlier, most of our featured animal foods—including pasture-raised eggs, pasture-raised chicken, grass-fed beef, grass-fed lamb, grass-fed cheese, and grass-fed cow's milk—contain measurable amounts of vitamin K, as do shrimp, sardines, tuna, and salmon.
As you can see, nearly half of our WHFoods (44/100) rank as good, very good, or excellent sources of vitamin K. Your meal combinations for achieving ample vitamin K here are extensive. Still, as a fallback source for vitamin K1, you would most likely want to turn to dark green leafy vegetables since they typically provide 500-1,000 micrograms per serving. For vitamin K2, you would mostly likely want to turn to fermented plant foods (like miso or tempeh) or animal foods. As mentioned earlier, fermented plant foods and animal foods feature different subtypes of vitamin K2. Remember, however, that researchers know of no hard and fast requirement for consuming any set amount of preformed K2 from your meal plan since the cells of your body are able to take K1 and convert it into K2. This provides you with a lot of flexibility in choosing among the 40+ WHFoods that are ranked sources of this vitamin.
|Common name||Vitamin K1||Vitamin K2||Vitamin K3|
|Food sources||plant foods, especially dark green leafy vegetables||meats, eggs, dairy, fish, fermented plant foods, fermented animal foods||not known to be provided in substantial, naturally occurring amounts in food|
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