CHICAGO — Too much vitamin A may increase the risk of hip fractures in older women, according to a study researchers say suggests the need to reevaluate the levels in supplements and fortified food.
Vitamin A is important for such things as healthy skin and hair and bone growth. But in the study published in Wednesday's Journal of the American Medical Assn., researchers found that women with the highest total intake--from food and vitamin supplements--had double the risk of hip fractures compared with women with the lowest intakes.
One theory is that too much vitamin A inhibits the ability of vitamin D to help the body absorb calcium, said lead author Diane Feskanich, an epidemiologist at Brigham and Women's Hospital in Boston. She said previous studies also suggest vitamin A affects cells that work in the breakdown and rebuilding of bone.
The adverse effects appear to be caused only by too much retinol--the true form of vitamin A, found in such things as liver, fish oils and supplements--and not by foods rich in beta carotene, such as dark, leafy vegetables. Beta carotene is converted by the body to vitamin A as needed.
Researchers analyzed dietary questionnaires from more than 70,000 post-menopausal women--all nurses from 34 to 77 years old. From 1980 to 1998, there were 603 hip fractures from such things as falling from the height of a chair or tripping.
The risk was almost twice as high among women with retinol intake of 2,000 micrograms or more per day, compared with those with intakes of less than 500 micrograms daily. And women taking a vitamin A supplement had a 40% greater risk of hip fracture than women not taking the supplement, Feskanich said.
The Institute of Medicine, a private organization that sets the nation's recommended daily allowances for nutrients, recommends that women get 700 micrograms a day of the vitamin. But multivitamins typically contain about 1,500 micrograms because the Food and Drug Administration has not updated vitamin supplement labels, Feskanich said.
She said that, between multivitamin supplements and fortified food, it is not difficult for women to attain vitamin A levels high enough to cause problems. Some increased risk was even seen at the recommended levels.
"If you're taking a multivitamin and consuming fortified milk and cereal . . . after a while, there are just too many sources," she said.
She said the FDA should consider lowering labeling standards for vitamin A. Also, some foods fortified with the vitamin might not need to be or could be fortified with beta carotene instead of retinol, she said.