Bone density and breast cancer

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Two recent reports have linked high bone density with an increased risk of breast cancer in postmenopausal women. The association isn't too surprising, given the current state of knowledge about osteoporosis and breast cancer and the influence of hormone replacement therapy (HRT) on the risk of developing either condition. Taking HRT lowers the risk of the former and may raise the risk of the latter. However, HRT use wasn't a factor in these two studies.

Although earlier studies indicated that women who suffered hip and arm fractures had a lower rate of breast cancer than the general population, previous investigations had not included measurements of bone density. Both new reports, from the multi-center Study of Osteoporotic Fractures and the Framingham Study, did so. The first investigation, which followed 6,854 women for 3 years, determined that the risk of breast cancer for women in the highest 25% on the bone-density chart was 2.5 times as great as for those in the lowest 25%. The second, which tracked 1,373 women between ages 47 and 80 for as long as 26 years, indicated that breast cancer risk was 3.5 times higher for women in the top 25% than for women in the bottom 25%.

In both studies, none of the other factors that influence bone density varied significantly with breast cancer risk. Neither investigation found a significant difference in levels of physical activity (which increases bone density) or in alcohol consumption or cigarette smoking (both of which contribute to bone loss) among women who developed breast cancer and those who did not. Thus, the researchers were able to rule out these habits as independent influences.

There was no direct evidence that estrogen was responsible for the increased breast cancer risk among women in either investigation. In the Framingham study, HRT use wasn't significantly different among women who developed breast cancer than among those who didn't (The study of Osteoporotic Fractures excluded those who were taking HRT at the time.) Moreover, other factor that increase one's native exposure to estrogen -- the onset of menstruation before-age 12, menopause after age 55, and having children later in life or not at all -- were no more prevalent among the women who developed breast cancer than among the women who didn't, in either study. Levels of circulating estrogen were not measured in either study.

The message these reports bring to women approaching menopause is anything but clear. The results seem to be saying that merely having strong bones is a sign of increased risk of breast cancer. Because it has been demonstrated that circulating estrogen promotes bone formation, both teams of researchers have postulated that the women with denser bones may have had higher cumulative levels of circulating estrogen throughout their lifetimes. They theorize that bone density is only a sign of estrogen exposure, and that estrogen is in some way responsible for the increased risk of breast cancer. However, some other still-unidentified factor may be responsible.

Of course estrogen isn't the only way to strengthen one's skeleton. There are other approaches to bolstering bone mass that do not influence breast cancer risk. Calcium supplementation, exercise, and drugs such as alendronate (Fosamax) and calcitonin (Miacalcin) have been shown to be effective to varying degrees.

This research underscores the incomplete state of medical knowledge, particularly when it comes to factors that affect the risk of disease in post-menopausal women. Certainly, additional research is required to identify the underlying connection between bone density and breast cancer. At present, these two studies shouldn't form the basis for action. They are no more than interesting observations -- pieces of the menopause puzzle that haven't yet fallen into place.

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