Recreational physical activity and breast cancer risk among women under age 45 years.

To evaluate whether recreational physical activity is associated with breast cancer among young women, the authors analyzed data from a population-based case-control study. Cases (n = 1,668) were women under age 45 years who had been newly diagnosed with breast cancer between 1990 and 1992 in Atlanta, Georgia, central New Jersey, or Seattle, Washington. Controls (n = 1,505) were frequency-matched to cases by 5-year age group and geographic area of residence. Breast cancer was not associated with recreational activity in any of the three time periods assessed (highest quartile of activity vs. lowest: age- and center-adjusted odds ratio (OR) = 0.94 (95% confidence interval (CI) 0.77-1.15) at ages 12-13 years, OR = 1.08 (95% CI 0.88-1.32) at age 20 years, and OR = 1.18 (95% CI 0.97-1.44) during the past year), with the average of the three time periods (OR = 1.02, 95% CI 0.84-1.25), or with daily climbing of at least two flights of stairs (without stopping) during the past year (daily climbing vs. never climbing: OR = 1.03, 95% CI 0.86-1.23). Estimates were not modified or confounded by body mass index, menopausal status, or caloric intake during the past year. These results do not support a protective role for physical activity in the risk of breast cancer among young women.