In general vitamins are good for us, but can too much of some vitamins actually be bad for your health? A recent article in the Journal of the American Medical Association suggests that too much vitamin A over a lifetime may increase a woman's risk of hip fracture from osteoporosis.

About the study

Researchers at Harvard University and Roche Vitamins Inc. studied 72,337 women aged 34 to 77 years old who participated in the Nurses' Health Study (NHS). The NHS began in 1976 when 121,700 female nurses aged 30 to 55 were enrolled in the study. At the time of enrollment and every two years thereafter, the women provided information about medical history, behavior, and lifestyle (e.g., smoking, exercise) via a mailed questionnaire. Dietary information was added to the questionnaire in 1980 and questions regarding hip fractures were added in 1982. At least 90% of the women have answered the questionnaire in each two-year follow-up cycle.

This recent study included NHS women who were postmenopausal as of 1980 and had answered the dietary questionnaire in 1980. As other women who had answered the 1980 dietary questionnaire became postmenopausal, they too were entered into the analysis. Women were excluded from the study if they had a previous hip fracture or a diagnosis of heart disease, cancer, stroke, or osteoporosis—conditions that might have led to a change in dietary habits.

Researchers assessed the women's dietary intake of various nutrients based on their responses to food frequency questionnaires (FFQ) in 1980, 1984, 1986, 1990, and 1994. For the years 1980-1998, they compared the women who reported low or moderate trauma hip fractures with their dietary intakes of vitamin A, retinol, and beta carotene. Low and moderate trauma hip fractures are generally considered to be the result of weakened bones caused by osteoporosis.

The findings

Analysis of the data showed that women who consumed more than 3000 micrograms (mcg) of vitamin A per day had a 48% increased risk of hip fracture compared to women who consumed less than 1250 mcg per day. This data included vitamin A intake from food plus vitamin supplements. Specifically, the intake of retinol (the active compound in vitamin A) in the high- intake group increased fracture risk by nearly 90% compared with the low intake group.

Beta carotene, a compound that is converted to vitamin A in the body, is another source of vitamin A from food. However, beta carotene intake did not contribute to fracture risk.

Many other dietary and lifestyle factors can contribute to a woman's risk of hip fractures, including calcium and vitamin D intake, smoking, and postmenopausal hormone use. Since data on these factors were also collected, researchers accounted for the effects of these factors when calculating the risk statistics listed above.

There are several limitations in the design of this study that should be considered along with the study's results. Although the sample of women was large, 98% were white, so these findings may not be applicable to women of other racial and ethnic groups. In comparison with other large study populations, it appears that women in this study were of higher socioeconomic status and consumed more vitamin A, retinol and beta carotene. In addition, the information collected from the women was entirely self-reported with no corroboration from laboratory tests or medical records. And finally, information on weight-bearing exercise, which helps strengthen bones, was not factored into the analysis.

How does this affect you?

Retinol is available naturally and most concentrated in animal liver, fish liver oils, eggs, and whole-milk products. In addition, foods fortified with retinol include low-fat dairy products, margarine, breakfast cereals, and meal replacement beverages. Vitamin A in the form of beta carotene and other carotenoids can be found in fruits and vegetables, such as carrots, tomatoes, vegetable soups, greens, cantaloupe, and spinach.

Should you stop eating foods that are rich in vitamin A, and especially those high in retinol? Absolutely not. These findings suggest there is a point at which too much vitamin A in a woman's diet may increase her risk for osteoporotic fractures. But alternatively, vitamin A deficiency—an even more hazardous condition according to Dr. Margo Denke in her accompanying editorial—is also something to be avoided. Therefore, moderation is the key. If you fit the profile of the women in this study, eat a balanced diet with plenty of fruits and vegetables. Check the vitamin A content of your multivitamins, and don't take stand-alone vitamin A supplements without talking first with your health care provider.

Vitamin A is essential for many body functions including vision, growth, reproduction, and the immune system. The Institute of Medicine's current recommendations for vitamin A intake are 800 mcg per day for men and 700 mcg per day for women, with a safe upper limit of 3000 mcg of retinol. Although the findings of this study suggest that 3000 mcg of retinol may be too high for a safe upper limit in white women, they do not provide any evidence that moderate intake of vitamin A increases a woman's risk of fractures.