Influence of physical activity in different age and life periods on the risk of breast cancer

We conducted a population-based case-control study of 1,237 incident breast cancer cases and 1,241 controls in Alberta between 1995 and 1997 to examin e the effect of physical activity performed at different ages and life peri ods on breast cancer risk. In this study, we measured all types of physical activity done throughout life with a questionnaire developed and tested sp ecifically for this study. We found that breast cancer risk was most associ ated with a risk reduction for activity done later in life, particularly be tween menopause and the reference year, for which we observed an odds ratio of 0.70 (95% confidence interval = 0.52-0.95). We also stratified the stud y participants into four categories according to their patterns of physical activity performed before and after menopause. For the women who sustained physical activity throughout life vs those who were never active, we found an odds ratio of 0.58 (95% confidence interval = 0.41-0.83). This study su ggests that sustained activity throughout life and particularly activity do ne later in life may have the most benefit in reducing breast cancer risk.