Physical Activity Across the Life Course and Risk of Pre- and Post-Menopausal Breast Cancer
Kobayashi, Lindsay Clare
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Background: Moderate-to-vigourous intensity physical activity (MVPA) is among the few modifiable factors known to reduce breast cancer risk. However, the independent effects of leisure-time, household, and occupational MPVA by age period across the life course remain poorly understood. Whether these effects differ by menopausal status and by tumour subtypes defined by the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER2) is unknown. An understanding of these issues will help advance policy and public health action targeting breast cancer prevention through physical activity. Methods: A case-control study of 1,011 incident breast cancer cases and 1,014 cancer-free controls aged 40-80 years was conducted from 2006-2010 in Vancouver, British Columbia (BC). Lifetime leisure-time, household, and occupational MVPA were assessed by questionnaire and mean MET-hrs/week of each were calculated for age periods 12-17, 18-34, 35-49, and ≥50 years and the total lifetime. Odds ratios for pre- and post-menopausal breast cancer risk associated with each activity domain across age periods were estimated using unconditional logistic regression, and odds ratios for risks of ER/PR-defined and ER/PR/HER2-defined breast tumours were estimated using unconditional polytomous logistic regression. Results: Among post-menopausal women, lifetime leisure-time and household MVPA reduced breast cancer risk by approximately 50% at volumes equal to 3 hours per week of running and 21 hours per week of active household work. MVPA reduced risk at all age periods across the life course, particularly during adulthood. Effects of leisure-time MVPA appeared restricted to HER2- tumours. Household MVPA reduced risk for ER/PR+ tumours, regardless of HER2 status. MVPA was not associated with pre-menopausal breast cancer risk, except occupational MVPA performed during ages 18-34 was associated with a doubling in risk. Conclusions: MVPA is a lifestyle factor women may engage in to reduce post-menopausal breast cancer risk. Results suggest HER2 may be implicated in anti-breast carcinogenic effects of leisure-time MVPA. Increased risk associated with occupational MVPA may be due to occupational exposures related to job intensity. Further research on specific aspects of weekly MVPA energy expenditure dose required to reduce breast cancer risk will aid in refining physical activity recommendations for breast cancer prevention.