NUTRITION NEWS

Featured Nutrient: Vitamin K

Vitamin K? What, you might well ask, ever happened to vitamins F, G, H, I and J?

In fact, when Danish researcher Henrik Dam discovered a substance essential to blood clotting in 1929, he jumped right over the intervening letters of the alphabet and named his new find vitamin K, for koagulation. For years afterwards, coagulation seemed to be the vitamin’s only function. But now researchers are turning up evidence that vitamin K plays other crucial roles in the body, prompting some scientists to think the optimal intake may be higher than current guidelines recommend.

Why It’s Essential

Vitamin K is used by the body to produce an array of different proteins. Some of them are used to create factors that allow blood to oagulate—critical in stemming bleeding and allowing cuts and wounds to heal. Other vitamin K-dependent proteins are used for maintaining healthy bones and keeping arteries unclogged. “Vitamin K allows a protein called osteocalcin to bind to calcium in bone, for example, which helps maintain bone density,” explains Sarah L. Booth, Ph.D., a scientist at the USDA Human Nutrition Research Center on Aging at Tufts University and one of only a small group of vitamin K nutrition experts in the world. There’s also preliminary evidence that vitamin K-dependent proteins may have a role in preventing hardening of the arteries, which can constrict blood flow and trigger heart attacks.

How Much You Need

The current recommended daily intake of vitamin K is 90 micrograms for women and 120 for men, based on the levels that are needed to maintain normal coagulation. Booth and other experts suspect that the most effective level may be higher, based on what is being discovered about K’s other roles. “At the moment, though, we don’t have the data to say exactly what optimal intake should be,” she admits.

Luckily, vitamin K deficiency is extremely uncommon. And since the main dietary sources of vitamin K are dark green vegetables, which are healthy for many reasons, the best advice is to help yourself to as much as you can. In a 10-year study of 72,000 women enrolled in the Nurses’ Health Study, researchers found that those who consumed the most foods containing vitamin K had a 30 percent lower risk of hip fractures than those whose diets contained the least vitamin K. Whether the vitamin itself or other nutrients in the foods were responsible for the benefit isn’t known. But in a small 2002 study at Osaka Medical College in Japan, researchers found that vitamin D and K supplements increased bone density in postmenopausal women.

Where to Get It

Leafy green vegetables of all kinds, from spinach and kale to arugula and green leaf lettuce, are especially rich in the vitamin. “In plants, vitamin K is involved in photosynthesis, so an easy rule of thumb is the greener the leaf, the more vitamin K it contains,” says Booth. Spinach has a lot, for example; pale iceberg lettuce much less. Vitamin K is also abundant in many vegetable oils, including soybean, canola, and olive oil.

A word of caution: If you are taking an anticoagulant medication such as warfarin, your doctor may advise you to avoid overdoing foods rich in vitamin K, since the vitamin can interfere with the drug’s action. If you have any questions, talk to your doctor before making changes to your diet.

—Peter Jaret