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 aged 30 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 (p trend = 0.05): the IRR for ≥7 hour/week relative to <1 hour/week was 0.74 (95% CI 0.57-0.96). The association did not differ by ER status. Brisk walking for ≥7 hours/week was associated with a reduction similar to that for vigorous exercise. Vigorous exercise at age 30, age 21, 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 exerciseor 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.