Abstract
The epidemiologic association between physical activity and breast cancer has been corroborated in many studies. Some inconsistencies remain, possibly due to variation in life periods for exposure assessment, confounding and undetected effect modification. In our cohort study, we address some of these questions by taking into account physical activity in different periods of life and by investigating effect modification by birth cohort and body mass index (BMI). Altogether 9,539 same‐sex twin women aged 42–70 years who answered a questionnaire about their work and leisure‐time physical exercise from ages 25 to 50 during 1967 and 1970 were included in our cohort. During follow‐up, 506 breast cancer cases occurred through 1997. We used multivariate Cox models to estimate relative risk (RR) with 95% confidence interval (CI). We found no associations between physical activity and breast cancer overall. Women born between 1901 and 1917 (aged 51–70 at baseline) who reported regular leisure‐time activity had a borderline significant 40% lower risk compared with those who reported no activity (RR 0.6; 95% CI 0.4–1.0; test for trend, p = 0.07). This association appeared to be confined to women with a low BMI after the age of 50 and to women with a high BMI during the premenopausal period. We found no evidence that work activity reduces risk for breast cancer. The importance of physical activity for breast cancer risk seems to depend on birth cohort. The association may be limited to normal‐weight postmenopausal women and overweight premenopausal women. © 2002 Wiley‐Liss, Inc.