Too Much of a Good Thing: Excess vitamin A may hike bone-fracture rate

Nathan Seppa

Dietary studies have suggested that people who consume large amounts of vitamin A in foods, multivitamins, or both are more likely to suffer hip fractures than are people who ingest modest amounts.

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 Michaëlsson, 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," Michaëlsson 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 Michaëlsson.

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 Michaëlsson 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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Letters:

I wonder whether researchers offer any recommendations based on a difference in effect between ingesting vitamin A and beta-carotene. Do people who consume large amounts of yellow vegetables, for example, put themselves at risk?

Mary A. Turzillo
Berea, OH

Yellow fruits and vegetables contain large amounts of the vitamin A precursor beta-carotene. However, says Margo Denke of the University of Texas Southwestern in Dallas, only a fraction of precursors are converted to vitamin A in the body, so eating them is unlikely to cause a vitamin A overdose and weaken bone.—N. Seppa

References:

Panel on Micronutrients, Subcommittees on Upper Reference Levels of Nutrients and of Interpretation and Use of Dietary Reference Intakes, and the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. 2002. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, D.C.: National Academy Press. Available at http://www.nap.edu/catalog/10026.html.

Feskanich, D., et al. 2002. Vitamin A intake and hip fractures among postmenopausal women. Journal of the American Medical Association 287(Jan. 2):47–54. Abstract available at http://dx.doi.org/10.1001/jama.287.1.47.

Michaëlsson, K., et al. 2003. Serum retinol levels and the risk of fracture. New England Journal of Medicine 348(Jan. 23):287–294. Abstract available at http://content.nejm.org/cgi/content/short/348/4/287.

Further Readings:

Denke, M.A. 2002. Dietary retinol—a double-edged sword. Journal of the American Medical Association 287(Jan. 2):102–104. Available at http://dx.doi.org/10.1001/jama.287.1.102.

Lips, P. 2003. Hypervitaminosis A and fractures. New England Journal of Medicine 348(Jan. 23):347–349.

Melhus, H., et al. 1998. Excessive dietary intake of vitamin A is associated with reduced bone mineral density and increased risk for hip fracture. Annals of Internal Medicine 129(Nov. 15):770–778. Abstract available at http://www.annals.org/cgi/content/full/129/10/770.

Promislow, J.H.E., et al. 2002. Retinol intake and bone mineral density in the elderly: The Rancho Bernardo Study. Journal of Bone Mineral Research 17(August):1349–1358. Abstract available at http://dx.doi.org/10.1359/jbmr.2002.17.8.1349.

Sources:

Margo R. Denke
University of Texas Southwestern Medical Center
5324 Harry Hines Boulevard
Dallas, TX 75390-9052

Karl Michaëlsson
Department of Surgical Sciences
Section of Orthopedics
University Hospital
S-751 85 Uppsala
Sweden

From Science News, Volume 163, No. 4, January 25, 2003, p. 52.