More evidence that vitamin K helps maintain strong bones comes from a new look at data from 888 elderly men and women participating in the Framingham Heart Study between 1988 to 1995. The study is reported in the May issue of the American Journal of Clinical Nutrition.
The risk of hip fractures among these elderly decreased as intakes of vitamin K increased, according to study leader Sarah Booth. She heads vitamin K research at the Jean Mayer USDA Human Nutrition Research Center on Aging in Boston, funded by the Agricultural Research Service, USDA's chief scientific agency. Booth collaborated with researchers from the Hebrew Rehabilitation Center for Aged Research and Training Institute, Harvard Medical School and others.
Men and women who reported the lowest daily vitamin K intakes--averaging 56 micrograms--in 1988 had experienced significantly more hip fractures by the 1995 examination than those reporting the highest intakes--averaging 254 micrograms.
Dark green leafy vegetables, like spinach and broccoli, are rich in vitamin K-- known chemically as phylloquinone. One serving of spinach or two servings of broccoli provide four to five times the Recommended Dietary Allowance, now set at 65 to 80 micrograms.
Vitamin K activates at least three proteins involved in bone health, according to Booth, but exactly how it all works is still a mystery. That may partly explain why the study found no relationship between bone mineral density and vitamin K intakes.
Booth and colleagues estimated vitamin K intakes from food frequency questionnaires the volunteers fill out at each examination. But the estimated intakes are misleading. They overestimate intakes by 50 percent or more, explains Booth, because people report eating more vegetables than they actually do. So the estimates aren't a good ruler for setting recommendations.
The new findings support others reported in 1999. Analysis of data from more than 72,000 women in the Nurses' Health Study showed that low vitamin K intakes increased risk of hip fracture. Booth provided vitamin K levels for the foods reported in the nurses study.