Multinational pooled analysis of leisure-time physical activity and premenopausal breast cancer

There is strong evidence that leisure-time physical activity is protective against postmenopausal breast cancer risk but the assocation with premenopausal breast cancer is less clear. A pooled analysis of 19 cohort studies with 547,601 premenopausal women investigated this association. Over a median of 11.5 years of follow-up (IQR 8.0-16.1) incident breast cancer was diagnosed in 10,231 participants. Self-reported high (90^th percentile) versus low (10^th percentile) leisure-time physical activity was associated with a 6% lower risk of breast cancer (HR 0.94; 95% CI 0.89-0.99) before adjustment for body mass index and a 10% lower risk (0.90; 0.85-0.95) after adjustment for body mass index. Tests of nonlinearity suggested an approximately linear relationship. The inverse association was particularly strong for HER2-enriched breast cancer (HR 0.57; 95% CI 0.39-0.84). The authors conclude that this analysis adds to evidence that engagement in higher levels of leisure-time physical activity may lead to reduced breast cancer risk.