vitamin k2

Vitamin K2, also called menaquinone, is one of the two main families of vitamin K, a fat-soluble nutrient (the other is K1, or phylloquinone). The NIH Office of Dietary Supplements describes vitamin K as a coenzyme for an enzyme the body uses to make proteins involved in blood clotting (hemostasis) and bone metabolism. Menaquinones are found in modest amounts in some fermented and animal-based foods — fermented soybean natto is notably rich in them — and are also produced by bacteria in the human gut. Among the menaquinones, MK-4 and MK-7 are the forms most commonly used in dietary supplements.

Vitamin K's role in blood clotting and in making bone proteins is well established, but the NIH cautions that evidence for K2/menaquinone supplements treating specific diseases is not settled. For bone health, the NIH health-professional fact sheet states "it is unclear whether supplementation with any form of vitamin K reduces the risk of osteoporosis." Some randomized trials — mostly using high-dose (15–45 mg/day) MK-4 in postmenopausal Japanese women — improved bone mineral density and reduced fractures, while other randomized trials found vitamin K supplementation had "no effect on bone mineral density" in older adults; the NIH notes that co-administered vitamin D and calcium may partly explain the inconsistent results. For heart disease, observational data link higher dietary menaquinone intake to less arterial calcification, but the NIH concludes that "the role of the different forms of vitamin K on arterial calcification and the risk of coronary heart disease is unclear" and remains an active research area. The FDA has not authorized a health claim for vitamin K in the United States, and for both bone and heart outcomes the NIH consumer fact sheet says "more research is needed."

There is no Recommended Dietary Allowance for vitamin K; the NIH sets only an Adequate Intake (AI), which for adults is 120 mcg/day for men and 90 mcg/day for women. These values cover all forms of vitamin K together, not K2 alone. Vitamin K deficiency is very rare in the US and most people get enough from food, so a supplement is not routinely needed. Supplement doses range widely — from micrograms in a multivitamin to milligram-level MK-4 used pharmacologically for osteoporosis in Japan and parts of Asia — so talk to a doctor, pharmacist, or registered dietitian before starting vitamin K2, especially if you take any medication.

The NIH reports that vitamin K (both K1 and K2/menaquinone) has a low potential for toxicity: the Food and Nutrition Board did not set a Tolerable Upper Intake Level, and stated in its report that "no adverse effects associated with vitamin K consumption from food or supplements have been reported in humans or animals." This favorable profile does not extend to menadione (sometimes called vitamin K3), a different synthetic form that was found to harm liver cells in laboratory studies decades ago and is no longer permitted in dietary supplements or fortified foods. Anyone taking a blood thinner should not change their vitamin K intake without clinician guidance (see interactions).

The NIH does not list specific side effects for vitamin K1 or K2 (menaquinone) and states in its consumer fact sheet that vitamin K "has not been shown to cause any harm." The principal safety concern is not a side effect of the vitamin itself but its interaction with anticoagulants such as warfarin. As with any supplement, discuss new or unexpected symptoms with a health care provider.

Vitamin K can interact seriously with the anticoagulant warfarin (Coumadin) and similar drugs: because these medications work by blocking vitamin K, a sudden change in how much vitamin K you take in can throw off their effect, raising the risk of either excess bleeding or clotting. People on these drugs should keep their daily vitamin K intake steady and consult their clinician before changing diet or supplements. The NIH also notes that prolonged antibiotic use can lower the vitamin K made by gut bacteria; that bile acid sequestrants (cholestyramine, colestipol) and the weight-loss drug orlistat (Alli, Xenical) can reduce vitamin K absorption. Tell your doctor and pharmacist about every supplement and medicine you take.