Women who consume a diet that includes plenty of vitamin A from foods and supplements may not be doing themselves much of a favor, says a group of Harvard researchers. Their study, published in the Journal of the American Medical Association, suggests that a high intake of vitamin A may increase an older woman's risk of hip fracture.

Tracking vitamin A intake

The researchers analyzed the diets of more than 72,000 women and tallied their intake of vitamin A from both food and dietary supplements. They then tracked the women for 18 years, noting the number who suffered a hip fracture during this time.

They found that postmenopausal women who consumed more than 3,000 micrograms (µg) of vitamin A from foods and supplements were about 40% more likely to suffer a hip fracture when compared with women who consumed less than 1,250 µg per day. This effect was most pronounced in those who didn't receive hormone replacement therapy (HRT).

The increased risk appeared to come from intake of retinol - the form of vitamin A from animal sources - rather than from carotenoids, which are plant compounds, like beta-carotene, that are converted by the body into vitamin A.

Results will take time to unravel

About 85% of the women were consuming the recommended amount of at least 700 µg vitamin A per day. More than 20% of the women, though, were consuming more than 3,000 µg per day, which is the "tolerable upper limit," the amount that most healthy adults can consume without risk of adverse effects. This is apparently fairly easy to do for those who routinely take vitamin supplements and eat a large amount of vitamin A-rich foods, including liver, eggs, dairy foods, and fish oils. In the United States, foods fortified with vitamin A - low-fat milk, margarine, and many cereals - add to the total.

The possible explanation

Scientists already know that sustained high doses of vitamin A (far above a normal intake) can cause bone abnormalities in growing animals, and that high doses of vitamin A used to treat skin conditions have been tied to an increased risk of osteoporosis. Studies in animals show that high levels of vitamin A interfere with the flow of minerals into and out of bones, an effect that may result in decreased formation of the minerals in bone.

The best advice for now

The study's authors stress that older women who use vitamin A preparations for therapeutic reasons should not discontinue using them, nor should they stop taking multivitamins if they require them to meet their nutrient needs. They should, however, talk to their doctor about steps they can take to monitor changes in bone density, a wise precaution for all older women regardless of how much vitamin A they consume.

Note: The authors conclude their article with a call to US health officials to reconsider the amount of vitamin A that is added to foods, suggesting that consumption of vitamin A-fortified foods is harmful to bone health in older women. But basic foods - milk, margarine, and cereal - are fortified with vitamin A to guard against deficiency primarily in children, a group not considered in this study.