Long-term recreational physical activity and risk of invasive and in situ breast cancer: the California teachers study.

Background: Long-term physical activity may affect breast cancer risk. Few prospective studies have evaluated in situ or invasive breast cancer risk, or breast cancer receptor subtypes, in relation to long-term activity.
Methods: We examined the association between recreational physical activity and risk of invasive and in situ breast cancer in the California Teachers Study, a cohort of women established in 1995-1996. Of 110 599 women aged 20 to 79 years with no history of breast cancer followed up through December 31, 2002, 2649 were diagnosed as having incident invasive breast cancer and 593 were diagnosed as having in situ breast cancer. Information was collected at cohort entry on participation in strenuous and moderate recreational activities during successive periods from high school through the current age or age 54 years (if older at enrollment) and in the past 3 years. A summary measure of long-term activity up to the current age, or age 54 years if older, was constructed for each woman.
Results: Invasive breast cancer risk was inversely associated with long-term strenuous activity (>5 vs </=0.5 h/wk per year: relative risk, 0.80; 95% confidence interval, 0.69-0.94; P trend = .02), as was in situ breast cancer risk (>5 vs </=0.5 h/wk per year: relative risk, 0.69; 95% confidence interval, 0.48-0.98; P trend = .04). Strenuous and moderate long-term activities were associated with reduced risk of ER-negative (strenuous: P trend = .003; moderate: P trend = .003) but not ER-positive (strenuous: P trend = .23; moderate: P trend = .53) invasive breast cancer.
Conclusion: These results support a protective role of strenuous long-term exercise activity against invasive and in situ breast cancer and suggest differing effects by hormone receptor status.