Too much of a good thing: excess vitamin A may hike bone-fracture rate.
New evidence bolsters these findings. Researchers have now correlated men's blood concentrations of vitamin A with a later incidence of broken bones--a comparison that avoids the vagaries that plague diet-recall studies.
Taken together, the new work and the diet studies raise knotty questions about the maximum amount of vitamin A that a person can safely ingest each day, says study coauthor Karl Michaelsson, an orthopedic surgeon at University Hospital in Uppsala, Sweden. He and his colleagues report the new findings in the Jan. 23 New England Journal of Medicine.
In the United States, the average, daily intake of vitamin A through food--especially fish, eggs, and meat--is roughly 2,600 international units (I.U.) for men, and many multivitamins contain 5,000 I.U. The U.S. Institute of Medicine recommends that people get 2,300 to 3,000 I.U. of vitamin A each day and sets the safe upper limit around 10,000 I.U.
"I believe this upper level should be lowered," Michaelsson says. When he and his colleagues gave the men dietary questionnaires, they learned that men ingesting as little as 5,000 I.U. of vitamin A per day were more prone to fractures than were men getting less.
Manufacturers should lower the amount of vitamin A in multivitamin tablets and fortified foods, such as cereals, says Michaelsson.
The new study began in the early 1970s when researchers stored blood samples from 2,047 men about 50 years old. Since then, 266 of the men have had at least one bone fracture.
After dividing the men into five equal groups according to their blood vitamin A concentrations, the researchers found that men in the top group were nearly twice as likely as those in the middle group to have broken a bone. The correlation was particularly strong with fractures of the hip.
"I think it's pretty conclusive now that there's a bad effect of [vitamin A] supplementation," says Margo A. Denke, an endocrinologist at the University of Texas Southwestern Medical Center in Dallas. Elderly people may be at special risk because they're slow to clear the vitamin from their bodies. Studies of animals have established that excess vitamin A stimulates the formation of cells that dissolve bone.
However, since some vitamin A is necessary to maintain good eyesight and general health, Denke and Michaelsson agree that fully fortified foods and supplements should remain available in countries where poor nutrition puts people at risk of a vitamin A deficiency.
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|Date:||Jan 25, 2003|
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