CHICAGO (AP) -- Too much vitamin A may increase the risk of hip fractures in older women, according to a new study that 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 Association, 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 breakdown and rebuilding of bone.
''There is a biologic reason to support what we're observing, but we don't know what those reasons are,'' said Feskanich, adding that the research confirms the findings of an earlier Swedish study.
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 postmenopausal woman -- 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 about 2,000 micrograms or more per day, compared with those with intakes of less than 500 micrograms daily. And women specifically taking a vitamin A supplement had a 40 percent greater risk of hip fracture than women not taking the supplement, Feskanich said.
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 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 old 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 currently fortified with the vitamin might not need to be, or could be fortified with beta carotene instead of retinol, she said.
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, said co-author, Dr. Walter Willett, professor of epidemiology and nutrition at the Harvard School of Public Health.
The Council for Responsible Nutrition, a trade group for the dietary supplement industry, played down the significance of the study, saying another recent survey found no connection between high vitamin A intake and bone density problems. The study of nurses was ''observational'' only, the trade group said, and ''by its very nature, never provides proof'' but only suggests further study.
But Dr. John Anderson, a nutrition professor at the University of North Carolina at Chapel Hill, said the study appears to show a strong link between high vitamin A levels and hip fractures.
But, ''I think we kind of need to wait and see,'' said Anderson, who was not involved in the study. ''There is enough suggested evidence from previous studies and this one that it's very possible we're overfortifying and oversupplementing in the U.S. But it's unclear what a safe upper limit is, and the issue needs to be resolved more with clinical studies.''
In an accompanying editorial, Dr. Margo A. Denke of the Center for Human Nutrition at the University of Texas Southwestern Medical Center said the findings support the need to set a safe upper limit to dietary retinol only.
She noted that the study participants were white U.S. women of high socioeconomic status, and said further study is needed on populations with diets less rich in vitamin A, ''to avoid confronting the other, even more hazardous side of the sword -- vitamin A deficiency.''
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