Increased Duration of Exercise Decreases Rate of Nonresponse to Exercise but May Not Decrease Risk for Cancer Mortality.

INTRODUCTION: Previous studies have observed an inverse relationship between exercise and breast cancer risk. However, there is interindividual variability in response to exercise training interventions. We investigated whether increasing the dose of aerobic exercise (150 or 300 min·wk-1), while keeping intensity of exercise constant (70%-80% HRmax), decreases the number of exercise nonresponders and further decreases associated risk for cancer mortality in our study population of women genetically predisposed for breast cancer.
METHODS: Healthy premenopausal women at elevated risk of breast cancer were randomized into control (<75 min·wk-1, n = 47), low-dose exercise (150 min·wk-1, n = 39), and high-dose exercise groups (300 min·wk-1, n = 39) for approximately 6 months. We assessed 1) clinical effectiveness (CE), defined as an improvement in predicted V˙O2max of ≥1 mL·kg-1·min-1, and twice the typical error (2× TE) of V˙O2max as thresholds to classify exercise "nonresponders"; 2) CE and 2× TE relative to exercise adherence levels; and 3) related changes in V˙O2max to predicted cancer mortality risk.
RESULTS: After our 6-month intervention, we observed that 23.5% of women in the low-dose group and 5.6% of women in the high-dose group were clinical nonresponders (P = 0.04). Clinical nonresponder status was independent of adherence level. Associated reduction in risk for cancer mortality was observed among 87.2% of women in the low-dose group and 94.9% in the high-dose group (P = 0.43).
CONCLUSION: Increasing volume (not intensity) of exercise via time spent exercising significantly decreases the number of "nonresponders." True nonresponders were observed as some women did not improve their fitness capacity despite high exercise adherence levels. Lastly, it appears 150 min·wk-1 is sufficient to decrease the predicted risk of cancer mortality.