Exercise Dose Effects on Insulin Resistance Indicators in Postmenopausal Women: A Randomized Trial

Background: While >= 150 minutes/week of moderate-vigorous physical activity is recommended for cancer prevention, the optimal amount for breast cancer prevention is unknown. Insulin resistance is a candidate biomarker of breast cancer risk. The objective of the study was to determine if 300 versus 150 minutes/week of moderate-vigorous aerobic exercise produces stronger improvements in insulin resistance indicators. Methods: The Breast Cancer and Exercise Trial in Alberta (BETA) was a year-long, two-center, two-armed, randomized controlled exercise trial between June 2010 and June 2013. The setting was a volunteer sample from Calgary and Edmonton, Canada. Participants were 400 inactive, non-diabetic, postmenopausal women with body mass index of 22 - 40 kg/m(2). Interventions were 5 days/week aerobic exercise (3 days/week supervised) for 60 minutes/session (high) or 30 minutes/session (moderate) at 65-75% heart rate reserve for >= 50% of each session. The main outcome measures were fasting blood concentrations of insulin, glucose, leptin, adiponectin, and resistin measured at baseline, 6 and 12 months. Results: Dose effects were evaluated using linear mixed models adjusted for baseline biomarker concentrations. High/moderate ratios of geometric mean biomarker concentrations over 12 months ranged from 0.93 to 1.00 (all P-values >= 0.05), indicating no dose effect. Among adherent women (n = 137), ratios for insulin, homeostasis model assessment of insulin resistance (HOMA-IR), and leptin were 0.77 - 0.89 (P-values < 0.05), indicating more benefit from the high prescription. There was a significant dose effect (P-value < 0.05) on resistin and leptin in non-obese and younger women, respectively. Conclusions: Prescribing 300 versus 150 minutes/week of moderate-vigorous aerobic exercise to inactive, non-diabetic, postmenopausal women may not produce stronger changes in insulin resistance indicators. Among exercise-adherers, however, a 300 minutes/week prescription might provide some additional benefit for breast cancer prevention.