Higher hip bone mineral density may be a reflection of greater lifetime exposure to estrogen; many (but not all) breast cancers are estrogen-sensitive.
Of course, if bone mineral density drops too low, that brings different health risks -- osteoporosis and fractures.
Cancer is published by the American Cancer Society. An American Cancer Society news release states that the study's findings don't change "the need to treat osteoporosis in order to reduce the risk of fractures."
Bone Mineral Density and Breast Cancer Risk
The new study is based on 9,941 postmenopausal women who took part in the Women's Health Initiative, a long-term women's health study.
When the study started, the women were 63 years old, on average. They got a checkup that included a hip bone mineral density scan using dual-energy X-ray absorptiometry (DXA). They also completed questionnaires about their medical history and lifestyle every six months.
The women were followed for about eight years; during that time, 327 women were diagnosed with breast cancer.
Women with high hip bone mineral density at the study's start were more likely to develop breast cancer during the study, report the researchers, who included Zhao Chen, PhD, MPH, of the University of Arizona College of Public Health in Tucson, Ariz.
BMI (body mass index) or hormone replacement therapy when the study started didn't explain the results. Race didn't, either, but because most of the women were white, that needs to be checked in other studies.
Hip bone mineral density also helped predict breast cancer risk beyond the Gail risk model, which is for women who are at least 35 years old. The Gail risk model estimates a woman's odds of developing breast cancer based on her current age, her age when she first menstruated, her age when she first gave birth, family history of breast cancer, past biopsies, and race.
In Chen's study, the hip bone mineral density score and the Gail score were "similar" in predicting breast cancer. Using both of those scores together may further help predict breast cancer in postmenopausal women, the researchers note. Women with high hip bone density and high Gail scores were particularly likely to develop breast cancer during the study.
Chen's study, which was funded by the drug company Lily, doesn't make new recommendations about getting DXA bone density tests. The study doesn't change the use of bone mineral density testing to diagnose osteoporosis, states the American Cancer Society news release.
A 2005 study published in the Journal of the National Cancer Institute showed no relationship between bone mineral density and breast cancer risk. That study, conducted in San Francisco, mainly included postmenopausal women, but premenopausal women were also included.