A case-control study of lifetime light intensity physical activity and breast cancer risk

Physical activity reduces breast cancer risk, although most evidence is for activity in the moderate-to-vigorous intensity range. The effect of light intensity physical activity (LIPA) is unknown. We aimed to determine the association between self-reported lifetime LIPA and pre- and post-menopausal breast cancer risk. Our secondary objective was to analyze risk stratified by estrogen and progesterone tumor receptor status. Data were from a case-control study of 1,110 incident breast cancer cases (388 pre-menopausal; 722 post-menopausal) and 1,172 controls (442 pre-menopausal; 730 post-menopausal) recruited at two Canadian sites. Lifetime leisure-time, household, and occupational physical activity and covariates were assessed by questionnaire. Mean minutes per day of LIPA for each of the age periods 12-17, 18-34, 35-49, a parts per thousand yen50, and the total lifetime were calculated. Odds ratios were calculated using unconditional logistic regression for overall breast cancer risk and using polytomous logistic regression for estrogen receptor (ER)/progesterone receptor (PR)-defined tumor subtypes and were adjusted for moderate-to-vigorous physical activity and other confounders. LIPA was not associated with breast cancer risk at any age period across the life course: odds ratio (OR) = 0.81; 95 % CI 0.53-1.24 for pre-menopausal women and OR = 0.87; 95 % CI 0.64-1.19 for post-menopausal women in the highest vs. lowest categories of total lifetime LIPA. No heterogeneity in risk by ER/PR tumor status was observed. Our results suggest that light intensity physical activity is not associated with breast cancer risk reduction. This finding is important for physical activity recommendations for breast cancer prevention.