Previous research has suggested that the long-term use of female hormone therapy is associated with an increased risk of developing breast cancer, according to background information in the article. However, most of the women in the largest studies have been white, and few studies have looked at the risks specifically in black women.
Lynn Rosenberg, Sc.D., of Slone Epidemiology Center, Boston University, and colleagues examined the association in 23,191 women age 40 years or older who were part of the Black Women's Health Study, conducted by investigators at Boston University and Howard University, Washington, D.C. The participants filled out an initial questionnaire about medical history, menopausal status and hormone use when they enrolled in the study in 1995. Follow-up questionnaires that also included questions about the development of breast cancer were completed every two years through the year 2003. The women's body mass index (BMI) was calculated by dividing their weight in kilograms by the square of their height in meters.
During the course of the study, 615 cases of breast cancer were reported. Breast cancer was more likely to occur in women who had taken hormones, including estrogen or estrogen plus progestin, in the previous two years than in women who had never taken hormones. The longer a woman took hormones, the more likely she was to develop breast cancer, and the risk was slightly higher for women taking estrogen with progestin than for those taking progestin alone. The association between hormone use and breast cancer was strongest for women who were lean, defined as having a BMI of less than 25.
"Associations of exogenous [supplemental] female hormone use with increased risk of breast cancer are biologically plausible," the authors write. Estrogens produced by the body are associated with the development of breast cancer, and progestin is known to induce cell division in breast tissue, which could lead to a higher risk of cancer. The relationship may be stronger in lean women because fat tissue produces estrogen, so the extra hormones added by taking pills may not have as much of an effect on estrogen levels in heavier women.
"These results based on data from U.S. black women strengthen the evidence that use of estrogen alone and estrogen with progestin increases the risk of breast cancer and that the association is stronger among leaner women," the authors conclude.
(Arch Intern Med. 2006; 166: 760-765. Available pre-embargo to media at www.jamamedia.org.)
Editor's Note: This study was supported by a grant from the National Cancer Institute, Bethesda, Md.