Leisure-time physical activity and breast cancer risk defined by estrogen and progesterone receptor status--the Japan Public Health Center-based Prospective Study.

OBJECTIVE: The study aims to investigate the association between leisure-time physical activity and breast cancer risk in consideration of tumor estrogen-receptor/progesterone-receptor status.
METHODS: We conducted a population-based prospective cohort study among 53,578 women in the Japan Public Health Center-based Prospective Study. Leisure-time physical activity was assessed by self-reported questionnaires. A Cox proportional hazards regression model was used to derive relative risks and 95% confidence intervals.
RESULTS: From 1990-1993 to the end of 2007, 652 cases were identified. The breast cancer rates (per 100,000 person-years) in the sedentary groups (≤3 days/month) was 84 in overall, 97 in premenopausal and 75 in postmenopausal women. We observed a statistically significant inverse association between leisure-time physical activity and breast cancer risk (relative risk(≥3 days/week vs. ≤3 days/month)=0.73; 95% confidence interval 0.54-1.00; p(trend) 0.037), particularly in estrogen receptor+progesterone receptor+ (relative risk 0.43; 0.19-1.00; p(trend) 0.022), and this inverse trend was apparent among postmenopausal women (relative risk 0.25; 0.06-1.06; p(trend) 0.041). An inverse trend was also observed between daily total physical activity and postmenopausal estrogen receptor+progesterone receptor+ risk (p=0.046). Among body mass index ≥25 kg/m(2) group, leisure-time physical activity was associated with decreased risk (relative risk(≥1 day/week vs. ≤3 days/month)=0.65; 0.43-0.97; p(trend) 0.033).
CONCLUSION: Active participation in leisure-time physical activity may contribute to a decrease in breast cancer risk, particularly for postmenopausal estrogen receptor+progesterone receptor+ tumors.