Background: Physical inactivity is a potentially modifiable breast cancer risk factor. Because few data on this relationship exist for black women, we examined the relationship between breast cancer risk and lifetime and time- or age-specific measures of recreational exercise activity among white women and among black women.
Methods: The Women's Contraceptive and Reproductive Experiences Study was a multicenter population-based case-control study of black women and white women aged 35-64 years with newly diagnosed invasive breast cancer. We collected detailed histories of lifetime recreational exercise activity during in-person interviews with 4538 case patients with breast cancer (1605 black and 2933 white) and 4649 control subjects (1646 black and 3033 white). Control subjects were frequency-matched to case patients on age, race, and study site. We examined associations between exercise activity measures (metabolic equivalents of energy expenditure [MET]-hours per week per year) and breast cancer risk overall and among subgroups defined by race, other breast cancer risk factors, and tumor characteristics by use of unconditional logistic regression. All statistical tests were two-sided.
Results: Among all women, decreased breast cancer risk was associated with increased levels of lifetime exercise activity (e.g., average MET-hours per week per year, P(trend) = .002). An average annual lifetime exercise activity that was greater than the median level for active control subjects was associated with an approximately 20% lower risk of breast cancer, compared with that for inactivity (for 6.7-15.1 MET-hours/week/year, odds ratio [OR] = 0.82, 95% confidence interval [CI] = 0.71 to 0.93; for > or =15.2 MET-hours/week/year, OR = 0.80, 95% CI = 0.70 to 0.92). The inverse associations did not differ between black and white women (for MET-hours/week/year, P(trend) = .003 and P(trend) = .09, respectively; homogeneity of trends P = .16). No modification of risk was observed by disease stage, estrogen receptor status, or any breast cancer risk factor other than first-degree family history of breast cancer.
Conclusions: This study supports an inverse association between physical activity and breast cancer among black women and among white women.