Vitamin K is necessary for normal blood clotting, and it regulates calcium in the body, keeping it in the bones and out of the arteries. Adequate vitamin K intake also appears to reduce the risks of both heart disease and cancer.
There are two main forms of vitamin K. Phylloquinone or vitamin K1 comprises about 90 percent of the K we get from fruits and vegetables. (Good sources include cauliflower, parsley, kiwifruit, avocadoes, olives, Brussels sprouts, broccoli, and dark leafy greens). Vitamin K2 comes in part from the conversion of K1 in the gastrointestinal tract by gut flora as well as from dietary sources, including some types of meat, egg yolks, fermented dairy products (yogurt and some natural, ripened cheeses) and fermented soy foods, like miso and natto.
You ask about taking 45 mg of vitamin K2 daily to keep bones strong. That's way too much - intake of vitamin K1 and K2 should be in micrograms (mcg), not milligrams (mg).
The Adequate Intake (AI) for vitamin K1 established by the Food and Nutrition Board of the U.S. Institute of Medicine is 120 mcg for men age 19 and older and 90 mcg for women age 19 and older. Regarding bone health: the Nurses' Health Study that followed more than 72,000 women for 10 years found that those whose vitamin K intakes were lowest had a 30 percent higher risk of hip fracture than those with the highest vitamin K intakes (at least 110 mcg). Another study involving more than 800 elderly men and women participating in the Framingham Heart Study found that those with the highest dietary vitamin K intakes had a 65 percent lower risk of hip fracture than those with the lowest intakes. Even though there were fewer broken bones, the investigators found no association between dietary vitamin K intake and bone mineral density.
If you're taking a daily multivitamin, it should contain some vitamin K2, typically 10 to 25 mcg, which should be adequate in combination with dietary sources if your bone density is normal. If you have osteoporosis or osteopenia, you should take a daily supplement providing 50 to 100 mcg of this vitamin. The Nurses Study found that eating a serving of lettuce or other green, leafy vegetable daily cut the risk of hip fracture in half when compared with eating one serving a week.
If you're taking anticoagulant drugs, don't take vitamin K supplements or increase your dietary intake without consulting your doctor.
Andrew Weil, M.D.