Vitamin K exists in several forms with different dietary sources and physiological roles. Vitamin K1 (phylloquinone) is found primarily in green leafy vegetables. Vitamin K2 (menaquinone) is more difficult to obtain from the diet and is found in foods such as egg yolks, organ meats, and natto, a fermented soy product commonly consumed in Japan that is especially rich in menaquinone. Populations in regions of Japan where natto is regularly consumed tend to have notably high vitamin K2 levels.
Vitamin K2 plays an important role in both bone health and cardiovascular health, largely through its involvement in calcium regulation. Vitamin K–dependent proteins, such as matrix Gla protein, help inhibit inappropriate calcium deposition in the arteries. Low vitamin K status has been associated with increased risk of atherosclerosis and heart disease. A large observational study published in the Journal of Nutrition (November 2004; Volume 134:3100–3105) followed 4,807 individuals with no history of myocardial infarction at baseline between 1990 and 1993 through January 1, 2000. Vitamin K2 intake was inversely associated with the incidence of heart disease. Participants with the highest vitamin K2 intake experienced a 57% lower risk of death from heart disease compared with those with the lowest intake.
Vitamin K2 is also important for maintaining bone strength and may help reduce fracture risk. A two-year study published in the Journal of Bone and Mineral Research (March 2000; Volume 15, Issue 3, Pages 515–521) examined 241 patients with osteoporosis. Participants were divided into two groups, with one group receiving 45 mg per day of vitamin K2 and the other serving as a control. The control group experienced a higher incidence of fractures and lower bone density compared with the group receiving vitamin K2.
Vitamin K2 has also been compared with pharmaceutical osteoporosis treatments. Research published in the Yonsei Medical Journal (October 30, 2003; Volume 44, Issue 5, Pages 751–756) found that women receiving 45 mg per day of vitamin K2 had a lower fracture rate than women treated with etidronate, a commonly used osteoporosis medication.
