Of the 13 vitamins essential for human health, vitamin K is among the least familiar to most people. But that may change as researchers report new findings about the vitamin’s key roles in the body, especially for bone health.
Like vitamin E, vitamin K is actually a group of structurally similar fat-soluble vitamins. It is necessary for the production of proteins needed for the coagulation of blood and other functions, and it works synergistically with vitamin D for bone health and possibly cardiovascular health. The K comes from Koagulation, German for coagulation, meaning the ability of blood to clot and thus prevent hemorrhage.
Getting to know K
There are two main types of the vitamin:
Vitamin K1 (phylloquinone). Well known for its role in blood clotting, K1 is found in many plant foods, notably leafy greens, such as spinach, kale, and collards, as well as broccoli, Brussels sprouts, cabbage, and other cruciferous vegetables. Consuming some fat with these foods improves absorption of vitamin K. Smaller amounts are found in soybeans, avocados, asparagus, kiwifruit, and a few vegetable oils (notably soybean and canola). The synthetic form of K1 used for medical injections and in many supplements is called phytonadione.
Vitamin K2 (menaquinones, designated as MK-1 to MK-13, depending on the length of their molecular structure). Important for calcium regulation in bones, cartilage, and blood vessels, K2 has been gettingincreasing attention from researchers. It is synthesized by bacteria, including various types in the intestinal tract. Moderate amounts are found in animal foods, such as dairy, chicken, egg yolks, and meat. Certain types of K2 (notably MK-7) are found in fermented foods such as cheese and particularly natto (a fermented soy product); the types and amounts of K2 depend on the bacterial strains used to make the foods and the fermentation conditions. Since vitamin K is fat-soluble, full-fat dairy products (such as milk, yogurt, cheese, and cottage cheese) contain much more K than reduced-fat ones; nonfat dairy products contain little or no K.
Vitamin K2 accounts for about 10 to 25 percent of average vitamin K intake, coming mostly from dairy in Western countries. Intestinal bacteria make some vitamin K2, but it’s unclear how much of this can be utilized by the body.
How much do you need?
The recommended Adequate Intake for vitamin K, set by the Institute of Medicine, is 90 micrograms a day for women, 120 micrograms for men (a microgram is one-thousandth of a milligram). This recommendation does not distinguish between K1 and K2 and is based on adequate K1 intake for coagulation. One-quarter cup of cooked kale or spinach, one-half cup of chopped broccoli or Brussels sprouts, or one cup of dark leaf lettuce supplies enough vitamin K to meet the recommended daily intake. Vitamin K is relatively heat-stable, so cooking does notreduce it (in fact, cooking may increaselevels of K1 by helping to release it from the plants’ cell walls). No safe Upper Limit has been set because there are no known risks from even very high doses of the vitamin, though long-term data are lacking.
Most Americans consume adequate amounts of vitamin K from food, based on their intake of K1. True vitamin K deficiency, resulting in bleeding problems, is rare, except in newborns. (Because babies are born with little stored K, the American Academy of Pediatrics recommends that they be given an injection of K at birth to prevent potentially life-threatening bleeding.)
Factors that can reduce or prevent vitamin K absorption and lead to insufficiency include chronic malnutrition, alcoholism, extremely low-fat diets, certain gastrointestinal disorders, and chronic kidney disease. Certain drugs can reduce levels of vitamin K or interfere with its action, including warfarin, broad-spectrum antibiotics (especially when taken for more than several weeks), bile acid sequestrants (cholesterol-lowering drugs such as cholestyramine and colestipol), and the diet drug orlistat (Alli and Xenical).
Multivitamins typically contain 25 to 60micrograms of K1. Most separate vitamin K supplements and many bone-health formulas contain K2 (usually MK-4, also called menatetrenone) in a wide range of doses. Vitamin K2 in the form of MK-7 (often derived from natto) is being increasingly used in supplements.
When K may not be ok
Vitamin K can be a problem for people taking the anticoagulant drug warfarin (Coumadin), which decreases blood clotting by inhibiting vitamin K’s role in the production of certain clotting factors. Thus, consuming high amounts of K can defeat the anticlotting action of the drugs. If you take warfarin, you should not avoid foods rich in K, since becoming deficient is not desirable, but you’ll need to follow professional advice about how much you can consume. What’s important is keeping your intake relatively consistent and moderate-that is, don’t eat huge servings of K-rich foods one day and none the next. Be sure to tell your doctor if you take any supplement (even a multi) containing vitamin K. Newer types of anticoagulants-such as dabigatran (Pradaxa), rivaroxaban (Xarelto), and apixaban (Eliquis)-are not affected by vitamin K.
By the way, despite what some nutrition textbooks and websites may say, people taking warfarin needn’t worry about drinking tea. While the leaves contain vitamin K, brewed tea contains virtually none.
For bones and heart?
While the roles of vitamin K1 in coagulation have been well established, scientistsare still learning about vitamin K2, which has been the subject of much research during the past two decades, especially in Japan, Korea, and the Netherlands. This research has primarily focused on the vitamin’s effects on bone and cardiovascular health, but its potential roles in osteoarthritis, cancer, diabetes, and cognition are also being studied. One complication: Some clinical trials testing vitamin K2 have co-administered other nutrients (such as vitamin D, calcium, and magnesium), making it even harder to pinpoint the effect of K alone.
Bone health. Many (but not all) observational studies have found that people who have a high dietary intake or blood level of vitamin K2 (less often K1) are at decreased risk for low bone mineral density (BMD), osteoporosis, and fractures. For instance, some Japanese studies have found that postmenopausal women who regularly eat natto maintain BMD better than those who do not consume it. (Natto is by far the richest source of vitamin K2, specifically MK-7. It’s a traditional Japanese food with a strong cheese-like flavor that can be eaten alone or with rice, added to miso soup, or used to make sushi.) Results of animal and test-tube studies also support the bone benefits of K2.
Clinical trials testing vitamin K supplements (usually K2, often in conjunction with vitamin D) for bone health in postmenopausal women have had inconsistent results, however. Some of these trials, especially those done in Japan, used very high daily doses of K2 (as high as 15 to 45 milligrams-that is, 15,000 to 45,000 micrograms), which would be considered pharmacological doses. In some studies, people taking vitamin K had improvements in BMD only at some measurement sites, and some trials suggest thatvitamin K may improve bone quality rather than bone quantity (as measured by BMD).
In Japan and some other Asian countries, a pharmacological dose of vitamin K2 (MK-4, 45 milligrams) is approved as a treatment for osteoporosis. But the vitamin is not part of standard treatment of the disorder in the U.S. or Canada. And while the European Food Safety Authority has approved a bone-health claim for vitamin K on the labels of foods and supplements, the FDA has not authorized such a medical claim for the vitamin (though it allows vague structure/function claims such as “promotes bone health”).
The National Osteoporosis Foundation has concluded that the research so far does not support the use of vitamin K supplements to prevent osteoporosis and fractures. “Taking a supplement doesn’t always have the same effects as eating whole foods that contain that same nutrient. . . . More research will help us to determine the amount and type of vitamin K that is necessary for bone health.”
Cardiovascular health. Researchers have been studying the effects of vitamin K (usually K2, sometimes combined with vitamin D) on cardiovascular health, notably via its ability to improve arterial function and reduce arterial calcification. Many large observational studies, notably some done in the Netherlands, have found that low dietary intake of vitamin K2 is associated with increased coronary calcification, atherosclerosis, and cardiovascular risk. Clinical trials using supplements have been limited, however. According to the NIH’s vitamin K fact sheet for health professionals, “At this time, the role of the different forms of vitamin K on arterial calcification and the risk of coronary heart disease is unclear, but it continues to be an active area of research.”
BOTTOM LINE: Get vitamin K from food: leafy greens and other vegetables for K1, and dairy products or fermented foods for K2. Because clinical trials on vitamin K supplementation for bone or cardiovascular health have been limited, have varied in dose and type of vitamin K, and have had inconsistent results, we don’t recommend supplements, except for people whose doctors have advised them (because of a malabsorption problem, for example). More research is needed, especially on the interplay between vitamins K and D in the body.