A study of 6,854 white women aged 65 or older found that those who had the strongest, healthiest bones and thus were at the lowest risk of getting osteoporosis - had the highest risk of breast cancer.
This is the first major study to correlate directly bone mineral density and breast cancer. The researchers’ underlying hypothesis is that the female hormone estrogen is what links the two, said the chief author, Jane Cauley of the University of Pittsburgh.
A lack of estrogen is associated with an increased risk of osteoporosis as well as an increased risk of heart disease. And lifetime exposure to estrogen is believed to play a pivotal role in breast cancer.
There is little a woman can do - or should do, Cauley and others say - to change her natural lifetime exposure to estrogen. But the study - which did not include women on hormone replacement therapy - raises questions for those who might be taking HRT.
Women who had the highest bone mineral density were at 2.0 to 2.5 times the risk of breast cancer compared with women with the lowest bone mineral density, according to the study to be published today in the Journal of the American Medical Association.
In fact, the association between a high bone mineral density and breast cancer was similar to “other strong predictors of breast cancer,” including a family history of the disease or radiation exposure.
“The observation that BMD (bone mineral density) predicts breast cancer suggests a linkage between two of the most common conditions affecting a woman’s health,” said the researchers from the University of Pittsburgh and the University of California, San Francisco.
In the study, the researchers say that a woman who has osteoporosis presumably has low estrogen levels and would be at little increased risk of breast cancer if she took HRT. But they raise the question of whether a woman with normal or high estrogen levels who takes hormone replacement therapy to, say, cut her risk of heart disease, could increase her risk of breast cancer.
However, in an interview Cauley said it was too early to draw any conclusions. “I think it (the study) identifies the need for further research. First of all, we need to know what is the common denominator. It may not be estrogen. It could be other hormones. We need to verify these findings,” she said.
Other experts agreed. “This is the first time we’ve seen this relationship … It sounds biologically plausible, but we still need more study,” said Dr. Sherry Sherman, director of the hormone, bone and muscle group at the National Institute of Aging, part of the National Institutes of Health, which funded the study.
“This gives us another indication that these diseases share a powerful connection through estrogen … I think women who have low bone density may feel somewhat fortunate, but I wouldn’t want women with good bone density to be worried,” said Dr. Joan McGowan, chief of the musculoskeletal diseases branch of the National Institute of Arthritis and Musculoskeletal Diseases, also part of NIH.