Physical activity and breast cancer survival: Results from the Nurses’ Health Studies


 
 
 Physical activity is generally associated with better outcomes following diagnosis; however, few studies have evaluated change in pre- to post-diagnosis activity, and repeated measures of activity by intensity and type.
 
 
 
 We evaluated physical activity and survival following a breast cancer diagnosis in the Nurses’ Health Study (NHS) and NHSII (n = 9,308 women, n = 1,973 deaths). Physical activity was evaluated as updated cumulative average of metabolic equivalent of task (MET)-hours/week (assigned per activity based on duration and intensity) and change in pre-to-post diagnosis activity. Cox proportional hazards models were used to estimate hazard ratios (HR) and 95% confidence intervals (CI).
 
 
 
 Higher post-diagnosis activity was inversely associated with breast cancer-specific mortality in categories from ≥9 MET-hours/week (vs. <3 MET-hours/week, ≥9-<18 HR = 0.74 (0.55-0.99); ≥27 HR = 0.69 (0.50-0.95), ptrend=0.04) and all-cause mortality from ≥3 MET-hours/week (≥3-<9 = 0.73 (0.61-0.88); ≥27 = 0.51 (0.41-0.63), ptrend<0.001)). Associations were predominantly observed for estrogen receptor-positive tumors and in postmenopausal women. Walking was associated with lower risk of all-cause mortality (≥9 vs. <3 MET-hours/week = 0.69 (0.57-0.84)), as was strength training. Relative to stable activity pre- to post-diagnosis (±3 MET-hours/week), increases from ≥3-9 MET-hours/week were associated with lower all-cause mortality risk (ptrend<0.001). Results were robust to adjustment for pre-diagnosis physical activity.
 
 
 
 Physical activity was associated with lower risk of death following diagnosis. Increased pre-to-post-diagnosis activity corresponding to ≥ 1-3 hours/week of walking was associated with lower risk of death. These results provide further impetus for women to increase their activity after a breast cancer diagnosis, though reverse causation cannot be fully excluded.
