You've probably heard that not eating enough calcium and vitamin D, as well as not getting enough weight-bearing exercise, can increase your risk for weakened bones and subsequent fractures. But could getting too much of a nutrient also increase your risk for fracture? Could be, according to a study in the New England Journal of Medicine, which found that high levels of vitamin A were associated with an increased risk of fractures.

Studying serum vitamin A in Swedish men

Researchers in Sweden recruited 2,322 men who were between the ages of 49 and 51 at baseline, and followed them for 30 years to determine what, if any, relationship existed between the men's serum levels of vitamin A and their risk for subsequent fracture.

During the course of the study, 266 men had bone fractures. For purposes of statistical analysis, the men were categorized into groups based on their serum vitamin A levels. Those in the highest-level group had more than 1.6 times the risk of fracture compared with men in the middle group. And among the men in the highest-level group, those with the very highest serum vitamin A levels had more than 7 times the risk of fracture than men in the lower groups.

How vitamin A affects bones

Scientists are still studying the possible mechanisms by which raised vitamin A levels may weaken bones. It's likely that several complex metabolic processes within the body, including the interaction between serum vitamin A levels and serum calcium levels, play a role.

Because this study was conducted in a group of very similar subjects (middle-aged Swedish men), the results cannot necessarily be applied to other ethnic or age groups, or to women (although previous data from women enrolled in the Nurses' Health Study indicate an association between high vitamin A intake and increased risk for hip fracture).

How much vitamin A should you eat?

The results of this study do not mean that you should avoid vitamin A; it's a vital nutrient that, when consumed in recommended amounts, plays an important role in maintaining health. But, like many nutrients, more is not necessarily better.

The recommended daily intake of vitamin A is 700 micrograms (µg) for women and 900 µg for men. Vitamin A is found primarily in animal foods, such as liver, fish, eggs, and milk, as well as in some fortified foods, such as cereals and energy bars. Beta-carotene, which is found primarily in plant foods, is converted to vitamin A in the body, but, according to research, does not appear to increase fracture risk. An antioxidant nutrient, beta-carotene may help protect against cancer and preserve eye health. It is found in dark green and orange fruits and vegetables, such as sweet potatoes, carrots, cantaloupe, and leafy greens.