Background: Accumulating evidence suggests that physical activity may protect against the development of breast cancer, but less is known about the role of modest physical activity during the postmenopausal years and in the context of physical function.
Methods: We evaluated this association in the Iowa 65+ Rural Health Study, a population-based, prospective cohort study of elderly adults. The cohort was linked to a population-based cancer registry for the years 1973-93, and the at-risk cohort consisted of 1806 women ages 65 to 102 years with an in-person baseline interview in 1982 and with no documented cancer between 1973 and the baseline interview. Through 1993 (16,857 person-years of follow-up) there were 46 incident cases of breast cancer.
Results: Greater level of physical activity in women with no physical disabilities was inversely associated with breast cancer risk (p for trend = .01). Compared to inactive women with no physical disability, women reporting moderate (age-adjusted relative risk [RR] = 0.5, 95% confidence interval [CI] 0.3-1.1) or high (age-adjusted RR = 0.2, 95% CI .05-0.9) activity levels were at decreased risk of breast cancer. Women with any disability were also at decreased risk of breast cancer compared to inactive women with no disability (age-adjusted RR = 0.4; 95% CI 0.2-0.9). Adjustment for education, body mass index, age at menarche, age at menopause, previous use of hormone replacement therapy, pregnancy history, systolic blood pressure, smoking, and alcohol use did not alter these associations. In addition, these associations were similar after exclusion of cases occurring during the first two years of follow-up, after adjusting for the number of doctor visits, and after stratifying by stage at diagnosis.
Conclusions: These data suggest that postmenopausal activity level, after accounting for physical disability, is inversely associated with breast cancer risk.