Strenuous physical activity in young adulthood and risk of breast cancer (United States).

The epidemiologic data on the relation between strenuous physical activity and breast cancer are limited and inconsistent. Because risk of breast cancer may be influenced by ovarian function which, in turn is modulated by physical activity, the hypothesis that exercise may be associated with a reduced risk of breast cancer merits further investigation. We, therefore, conducted a large case-control study in 1988-1991, and interviewed 6,888 women (17 to 74 years of age) with breast cancer in Maine, Massachusetts, New Hampshire, and Wisconsin (United States). Interviewed controls (9,539 women, 18 to 74 years of age) were selected randomly from lists of licensed drivers (for younger women) or from a roster of Medicare enrollees (for older women). We used multivariate adjusted odds ratios (OR) and 95 percent confidence intervals (CI) from logistic regression models to estimate relative risks between self-reported physical activity when 14 to 22 years of age and breast cancer. When compared with sedentary controls, women who reported any strenuous physical during ages 14 to 22 years had a modest reduction in the risk of breast cancer (OR = 0.95, CI = 0.93-0.97). However, those who exercised vigorously at least once a day had a 50 percent reduction in risk of breast cancer (OR = 0.5, CI = 0.4-0.7). These data support the hypothesis that women who are physically active have a reduced risk of breast cancer.