ATLANTA — In postmenopausal women, an active lifestyle provided about a 10% reduction in the risk of developing breast cancer over a 17-year period in over 36,000 women, according to findings from a prospective cohort study presented at the annual meeting of the American Society of Clinical Oncology.
The protective effect of physical activity was most significant against the risk of developing the more aggressive estrogen receptor (ER)-positive, progesterone receptor (PR)-negative form of breast cancer, providing a 34% risk reduction, said Dr. Aditya Bardia, who was pursuing a degree at the University of Iowa, Iowa City, at the time of the study.
Dr. Bardia and colleagues from the Iowa Women's Health Study mailed questionnaires addressing leisure time physical activity and breast cancer risk factors to postmenopausal women living in Iowa in 1986; 41,837 women (43%) responded.
The participants' levels of physical activity were classified as low, medium, or high based on the frequency and intensity of their exercise.
The investigators determined cancer incidence between 1986 and 2003 using the Iowa Surveillance, Epidemiology, and End Results (SEER) Cancer Registry and mortality information from state and national resources. Periodic follow-up surveys confirmed continued residence in Iowa in greater than 99% of the women each year.
After exclusion of women with cancer, a full or partial mastectomy, and those with incomplete data, the cohort included 36,363 women. During the 17-year follow-up period, 2,548 women were diagnosed with breast cancer at an average age of 71 years.
Overall, 47% of women reported low physical activity, 28% reported medium physical activity, and 25% were highly active. In addition to having a lower body mass index (BMI), active women were more likely to have received education beyond high school and reached menopause at an older age.
A high level of physical activity was associated with a 13% reduction in the risk of developing ER-positive breast cancer and an 8% reduction in ER-negative breast cancer, compared with low physical activity. The risk reduction for PR-positive and PR-negative breast cancer was 5% and 27%, respectively.
After the 34% risk reduction for ER-positive/PR-negative breast cancer, the next greatest benefit was a 20% reduction in the risk of developing ER-negative/PR-negative cancer.
The risk of ER-negative/PR-positive breast cancer was increased by 42% with a high activity level, but Dr. Bardia said the confidence intervals were high on this association due to a small number of women in this group.
Modifiable risk factors such as obesity and a sedentary lifestyle may be contributing to the increasing incidence of breast cancer. However, previous prospective studies have yielded mixed results on the correlation between physical activity and breast cancer risk. Furthermore, such prospective cohort studies have until now evaluated only the link between physical activity and breast cancer risk as a whole. Associations between physical activity and certain types of breast cancer would not have been detected.
Dr. Bardia commented that future studies should investigate the association between physical activity and breast cancer incidence in premenopausal women.
Dr. Banu Arun, of the department of breast medical oncology at the M.D. Anderson Cancer Center, Houston, noted that future studies should evaluate other factors including the use of hormone therapy and genetic risks. “There may be many other differences in lifestyle between women who exercise and who do not which could be a confounding factor as well.”
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