Science has already suggested that obesity is associated with an increased risk of breast cancer in postmenopausal women, but a new study now explains why. According to research published in the August 20, 2003, issue of the Journal of the National Cancer Institute obesity increases the risk of breast cancer in postmenopausal women by increasing the amount of estrogens in the blood. High levels of estrogen have been linked as a causative factor for breast cancer.
The finding was reported by the Endogenous Hormones and Breast Cancer Collaborative group. Joanne F. Dorgan, 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 groups 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 US women aged 60 to 74 increased from 29 per cent between 1988 and 1994 to 40 per cent 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."