Physical activity and breast cancer in the Multiethnic Cohort Study

Purpose Breast cancer remains the most common cancer among women and risk varies by race/ethnicity. Physical activity has been shown to reduce risk by 10-21% in White women but this reduction has not been established in other races/ethnicities. This study explores the association between physical activity and breast cancer risk in a large cohort of multiethnic women. Methods We analyzed data from 93,137 African American, White, Japanese American, Latina, and Native Hawaiian women who participated in the Hawaii-California Multiethnic Cohort Study. Data on physical activity (hours and METs) was based on a self-administered questionnaire at baseline (1993-1996) and a 10-year follow-up questionnaire (2003-2007). Multivariate Cox proportional hazards models provided estimates of hazard ratios (HRs) and 95% confidence intervals (CIs). Results We observed an inverse association between breast cancer risk and METs of total activity per day (HR = 0.92, 95% CI: 0.87-0.98, P _trend =0.01) but not for total hours of moderate/vigorous activity per day (HR = 0.95, 95% CI: 0.89-1.02, P _trend = 0.08) when comparing the highest quartile to the lowest in all women. After stratification, Latina women in the highest quartile of METs of total activity per day had a statistically significant reduction in breast cancer risk (HR = 0.84, 95% CI: 0.72-0.98, P _trend =0.05) compared to the lowest quartile. For others, HRs were below one for the highest METs quartile; there was no evidence of heterogeneity across race/ethnicity (p = 0.76). Conclusion This research confirms an overall protective association between physical activity and breast cancer risk, and suggests protection across races/ethnicities.