Too much vitamin A may increase the risk of hip fractures in older women. Vitamin A is important for such things as healthy skin and hair and bone growth. Researchers found that women with the highest total intake - both from food and vitamin supplements - had double the risk of hip fractures compared with women with the lowest intake.
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 bone remodeling.
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.
The Institute of Medicine -- a private science 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 U.S. Food and Drug Administration has not updated vitamin supplement labels, Feskanich said.
She said the FDA should consider lowering labeling standards for vitamin A. Also, some foods currently fortified with the vitamin might not need to be, or could be fortified with beta carotene instead of retinol. Researchers cautioned that people should not stop taking multivitamins, which help lower the risk of other diseases. Instead, consumers might consider a multivitamin that supplies some of its vitamin A from beta carotene.