Vitamin K

Claims, Benefits: Helps keep bones strong, essential for blood clotting.

Bottom Line: You don't need supplements of vitamin K. In most people, intestinal bacteria make much of the K needed. But to make sure you get enough K, especially to keep bones strong, you should eat vegetables rich in K.

Full Article, Wellness Letter, July 1999:

Greens for bones

Even Martians know that broccoli, cabbage, Brussels sprouts, spinach, kale, dark lettuce, and other leafy greens are nutritious—:rich in vitamin C, folic acid, carotenoids (such as lutein and beta carotene), and fiber. But did you know that these vegetables may help keep bones strong? And this isn't merely because some of them contain calcium, but because they are the best sources of vitamin K, according to a study that appeared in the American Journal of Clinical Nutrition in January. This vitamin is needed to make a protein that's essential for bone formation.

Vitamin K is best known for its crucial role in blood clotting. In the great majority of people, beneficial intestinal bacteria make much of the K needed for this purpose, which is why you hear so little about this vitamin. But to get enough K, especially to keep bones strong, you need to eat some of these K-rich vegetables.

The new research comes from the ongoing Nurses' Health Study and followed 72,000 middle-aged women for 10 years. It found that those who consumed moderate or high amounts of vitamin K (nearly all from vegetables) had a 30% lower risk of hip fractures than women consuming little or no K. This held true even when other factors affecting bone health, such as calcium and vitamin D, were factored in.

It didn't take much vitamin K—about 100 to 150 micrograms a day—to achieve this protective effect. That is more than the RDA (65 to 80 micrograms), but can be supplied by the following: one-quarter cup of cooked kale or collards; one-half cup spinach, Swiss chard, chopped broccoli, or Brussels sprouts; or one cup green cabbage or leaf lettuce. You don't need K supplements.

The only people who should avoid large servings of green vegetables are those taking anticoagulant drugs such as Coumadin, which decrease blood clotting by inhibiting the action of vitamin K. Thus consuming high amounts of this vitamin can defeat the anti-clotting action of the drugs. Despite what some nutrition textbooks and doctors say, brewed tea contains virtually no K.

Second thoughts: Large doses of vitamin E (such as the doses the Wellness Letter recommends) impair the absorption of vitamin K somewhat. So it is important to make sure you eat some K-rich foods.

UC Berkeley Wellness Letter, July 1999