Purpose: Accumulating data suggest that exercise may affect breast cancer risk and outcomes. Studies have demonstrated that high levels of insulin, often seen in sedentary individuals, are associated with increased risk of breast cancer recurrence and death. We sought to analyze whether exercise lowered insulin concentrations in breast cancer survivors.
Methods: One hundred one sedentary, overweight breast cancer survivors were randomly assigned either to a 16-week cardiovascular and strength training exercise intervention or to a usual care control group. Fasting insulin and glucose levels, weight, body composition, and circumference at the waist and hip were collected at baseline and 16 weeks.
Results: Baseline and 16-week measurements were available for 82 patients. Fasting insulin concentrations decreased by an average of 2.86 microU/mL in the exercise group (P = .03), with no significant change in the control group (decrease of 0.27 microU/mL, P = .65). The change in insulin levels in the exercise group seemed greater than the change in controls, but the comparison did not reach statistical significance (P = .07). There was a trend toward improvement in insulin resistance in the exercise group (P = .09) but no change in fasting glucose levels. The exercise group also experienced a significant decrease in hip measurements, with no change in weight or body composition.
Conclusion: Participation in an exercise intervention was associated with a significant decrease in insulin levels and hip circumference in breast cancer survivors. The relationship between physical activity and breast cancer prognosis may be mediated, in part, through changes in insulin levels and/or changes in body fat or fat deposition.