The finding was reported by the Endogenous Hormones and Breast Cancer Collaborative Group. Joanne F. Dorgan, M.P.H, Ph.D., an epidemiologist at Fox Chase Cancer Center, is the lead investigator of one of eight cohorts included in the analysis.
"We've known that postmenopausal women who are overweight have an increased risk of breast cancer, and the risk also is higher in women who have higher levels of estrogens in their blood," said Dorgan. "Our results suggest that obesity increases breast cancer risk in postmenopausal women by increasing serum concentrations of estrogens."
For the study, researchers analyzed the blood donated by women in eight cohorts in the United States, Europe and Asia. All the women were cancer-free and were not using hormone replacement therapy when the blood was collected. The women were followed for two to 12 years and 624 women developed breast cancer. Hormones in their blood were compared with the hormones from 1,640 cancer-free women who were the same age when blood was donated as the women who developed breast cancer. Obesity was measured by body mass index (BMI), a measure of weight that is adjusted for height.
Most of the established risk factors for breast cancer are either fixed (family history and genotype) or not amenable to modification (age at menarche, number of and ages at pregnancy, age at menopause).
"This is an example of a risk factor that a woman can control," Dorgan said. "The effect of obesity on breast cancer risk is important because the prevalence of obesity is high and increasing." According to Dorgan, the estimated prevalence of obesity in U.S. women aged 60 to 74 increased from 29 percent between 1988 and 1994 to 40 percent in 1999-2000.
"Obesity is a risk factor for other diseases such as heart disease and diabetes," Dorgan added. "Women need to know that breast cancer has been added to that ominous list."
The Endogenous Hormones and Breast Cancer Collaborative Group is international consortium that includes epidemiologists from the United States, England, Italy, and Japan. The group conducts research on the relationship of endogenous hormones to breast cancer development. For the current analysis, researchers from eight of the nine cohorts pooled their data so they could evaluate whether higher serum estrogens in heavier women explains the relationship of obesity with breast cancer risk in postmenopausal women.
The nine cohorts and the principal investigators who make up the Endogenous Hormones and Breast Cancer Collaborative Group include the Columbia, Mo., cohort (JF Dorgan); the Guernsey, UK, cohort (TJ Key); Nurses' Health Study cohort (SE Hankinson); New York University Women's Health Study cohort (PG Toniolo); the ORDET, Italy, cohort (F Berrino); the Rancho Bernardo cohort (E Barrett-Connor); the RERF, Japan, cohort (M Kabuto); the SOF cohort (JA Cauley); and the Washington County, Md., cohort (KJ Helzlsouer).
Fox Chase Cancer Center, one of the nation's first comprehensive cancer centers designated by the National Cancer Institute in 1974, conducts basic and clinical research; programs of prevention, detection and treatment of cancer; and community outreach. For more information about Fox Chase activities, visit the Center's web site at http://www.fccc.edu or call 1-888-FOX CHASE.
Journal
JNCI Journal of the National Cancer Institute