How vitamin K functions in the body
First discovered in 1929, vitamin K actually consists of a group of related compounds-vitamin K1 (also known as phylloquinone), vitamin K2 (also known as menaquinone) and vitamin K3 (also known as menaphthone). Found in a variety of foods, vitamin K1 is most abundant in leafy, dark green vegetables (see table).

Vitamin K serves two basic purposes. First, it plays a major role in both the production and conversion of certain proteins that are integral to blood clotting. (In fact, its name is derived from the Danish word for clotting). Second, vitamin K plays an integral role in bone metabolism, both by helping to transport calcium throughout the body and by assisting in the formation and saturation of osteocalcin, a protein necessary for bone formation.

The recommended dietary allowance (RDA) of vitamin K is 60 micrograms for women and 80 micrograms for men. This is based on the amount of vitamin K necessary for blood clotting.

But the initial results of some recently conducted studies suggest that an intake much higher than the current RDA may be necessary if vitamin K is to be effective in bone formation.

What the research says
One study conducted in France focused on osteocalcin, bone density and the risk of hip fractures in older women. That study found that when osteocalcin remains highly unsaturated, the risk of hip fractures is greatly increased.

Recognizing that vitamin K plays an integral role in the saturation of osteocalcin, researchers tried to learn whether greatly increasing the amount of vitamin K in the diet would lead to a greater amount of saturated osteocalcin in the body, and thus, better bone formation. And, in a pilot study conducted at Tufts University with nine healthy men and women, researchers found just that. Increasing the intake of vitamin K to four times the current RDA did result in a significant increase in the subjects' percentage of saturated osteocalcin protein.

Finally, a third study of the effect of vitamin K intake on bones-this one conducted at Harvard Medical School-suggested that, at least in some women, increasing the intake of vitamin K to an amount much greater than the current RDA is necessary to reduce the risk of hip fractures.

How much should you consume?
Because these are only preliminary studies, it's unclear how much, if any, extra vitamin K people should include in their diets. Sarah Booth, PhD, a scientist at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University points out that the current data suggest that there is a correlation between vitamin K intake and age-related bone loss, but she says that before it can be definitively stated that this correlation exists, much more research needs to be done.

Booth goes on to say that increasing your consumption of vitamin K can't hurt, and may very well prove beneficial. There is however, one very important caveat: Because of the role vitamin K plays in blood clotting, people who take warfarin (Coumadin), a widely prescribed anticoagulant medication, should never increase their intake of vitamin K without first consulting their physician.

For people who don't take warfarin, though, the best way to increase your consumption of vitamin K is to add dark green, leafy vegetables, such as spinach, broccoli, and alfalfa and Brussels sprouts, to your diet, since these are high in vitamin K.