Long-term Effects of Moderate versus High Durations of Aerobic Exercise on Biomarkers of Breast Cancer Risk: Follow-up to a Randomized Controlled Trial.

Background: The optimal lifestyle for breast cancer prevention over the long term is unclear. We aimed to determine whether or not the amount of exercise prescribed in a year-long exercise intervention influences breast cancer biomarker levels 1 year later.

Methods: We conducted a 24-month follow-up study (2012-2014) to the Breast Cancer and Exercise Trial in Alberta (BETA), a 12-month, two-armed (1:1), two-center randomized controlled trial of exercise in 400 cancer-free, postmenopausal women. The exercise prescription was moderate-vigorous aerobic exercise, 5 days/week (3 days/week supervised) for 30 minutes/session (MODERATE) or 60 minutes/session (HIGH). Participants were asked not to change their usual diet. We used linear mixed models to compare biomarker concentrations (C-reactive protein, insulin, glucose, HOMA-IR, estrone, sex hormone binding globulin, total estradiol, and free estradiol) over time (0, 12, and 24 months) by group (MODERATE, HIGH), using group-time interactions.

Results: After 12 months of no intervention, 24-month fasting blood samples were available for 84.0% and 82.5% of MODERATE and HIGH groups, respectively (n = 333/400). We found no evidence that 0 to 24- or 12 to 24-month biomarker changes differed significantly between randomized groups (HIGH:MODERATE ratio of mean biomarker change ranged from 0.97 to 1.06, P values >0.05 for all). We found more favorable biomarker profiles among participants who experienced greater than the median fat loss during the trial.

Conclusions: Prescribing aerobic exercise for 300 versus 150 minutes/week for 12 months to inactive, postmenopausal women had no effects on longer-term biomarkers. Impact: Exercise may lead to larger improvements in breast cancer biomarkers after intervention among women who also experience fat loss with exercise.