The Association between Cardiorespiratory Fitness and the Risk of Breast Cancer in Women.

INTRODUCTION:Studies have shown an inverse association between the risk of breast cancer in women and physical activity. However, information on the association between cardiorespiratory fitness (CRF) assessed objectively by a standardized test and the risk of developing breast cancer is limited. PURPOSE:To examine the CRF-breast cancer risk association in healthy females. METHODS:This retrospective study was derived from the ETHOS cohort (n = 750,302). Female participants (n = 44,463; mean age ± SD; 55.1 ± 8.9 years) who completed an exercise treadmill (ETT) test evaluation (Bruce protocol) at the Veterans Affairs (VA) Medical Centers nationwide from 1999 to 2020 were studied. The cohort was stratified into four age-specific CRF categories (Least-fit, Low-fit, Moderate-fit, and Fit), based on the peak metabolic equivalents (METs) achieved during the ETT. RESULTS:During 438,613 person-years of observation, 994 women developed breast cancer. After controlling for covariates, the risk of breast cancer was inversely related to exercise capacity. For each 1-MET increase in CRF the risk of cancer was 7% lower (HR 0.93; 95% CI, 0.90-0.95; P < .001). When risk was assessed across CRF categories with the Least-fit group as the referent, the risk was 18% lower for Low-fit women (HR 0.82; 95% CI, 0.70-0.96; P = .013), 31% for Moderate-fit (HR 0.69; 95% CI, 0.58-0.82; P < .001) and 40% for Fit (HR 0.60; 95% CI, 0.47-0.75; P < .001). CONCLUSIONS:We observed an inverse and graded association between CRF and breast cancer risk in women. Thus, encouraging women to improve CRF may help attenuate the risk of developing breast cancer.