Case-control study of physical activity and breast cancer risk among premenopausal women in Germany.

Important aspects of the recognized inverse relation between physical activity and breast cancer risk are still under discussion. Data on physical activity from sports, occupational activity, household tasks, walking, and cycling by reported frequency, duration, and intensity during adolescence and young adulthood were collected in 1999-2000 from 360 premenopausal breast cancer cases and 886 controls who had previously participated in a German population-based case-control study. In multivariate conditional logistic regression, no association between total physical activity and premenopausal breast cancer was found in two age periods. For women who were active during both periods, the adjusted odds ratio was 0.83 (95% confidence interval: 0.60, 1.14). When both age periods were combined, higher quartiles of total physical activity compared with the lowest quartile showed adjusted odds ratios of 0.97, 0.68, and 0.94. Only the effect of moderately high physical activity was statistically significant. Analyses by type of activity revealed significant protective effects for women who reported the highest levels of cycling activities (adjusted odds ratio = 0.66, 95% confidence interval: 0.45, 0.97). These data do not suggest an inverse monotonic association between total physical activity and breast cancer risk in premenopausal women. The study prevalence of cycling and walking for transportation demonstrated that national habits need consideration in the exposure assessment.