A prospective study of physical activity and breast cancer incidence in African-American women.

BACKGROUND: Physical activity has been associated with reduced risk of breast cancer. Evidence on the association in African Americans is limited.
METHODS: With prospective data from the Black Women's Health Study, we assessed vigorous exercise and walking in relation to incidence of invasive breast cancer overall (n = 1,364), estrogen receptor-positive (ER(+), n = 688) cancer, and estrogen receptor-negative (ER(-), n = 405) cancer, based on 307,672 person-years of follow-up of 44,708 African-American women ages 30 years or older at enrollment. Cox proportional hazards models estimated incidence rate ratios (IRR) and 95% confidence intervals (CI).
RESULTS: Vigorous exercise at baseline was inversely associated with overall breast cancer incidence (Ptrend = 0.05): the IRR for ≥7 h/wk relative to <1 h/wk was 0.74 (95% CI, 0.57-0.96). The association did not differ by ER status. Brisk walking for ≥7 h/wk was associated with a reduction similar to that for vigorous exercise. Vigorous exercise at the age of 30 years, 21 years, or in high school was not associated with breast cancer incidence. Sitting for long periods at work or watching TV was not significantly associated with breast cancer incidence.
CONCLUSION: High levels of vigorous exercise or brisk walking may be associated with a reduction in incidence of breast cancer in African-American women. IMPACT: These results provide informative data on a potential modifiable risk factor, exercise, for breast cancer in African-American women. ©2014 American Association for Cancer Research.