CHICAGO -- 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 re-evaluate 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 today'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.
Twice the risk
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.