Whereas observational studies have suggested that increased physical activity is linked to lower breast cancer risk, Canadian researchers explored how sex hormones were positively influenced by aerobic exercise.  Specifically, the team examined how an aerobic exercise intervention influenced circulating estradiol, estrone, sex hormone binding globulin (SHBG), androstenedione, and testosterone levels, which may be involved in the association between physical activity and breast cancer risk.  The researchers enrolled 320 postmenopausal women, ages 50 to 74 years, in a year-long trial, with 160 women randomly assigned to 225 minutes a week of aerobic exercise, and the remaining study subjects maintaining their normal level of activity.  At the end of the study, the team found statistically significant reductions in estradiol and free estradiol, as well as increases in SHBG, among the exercising women (as compared to controls).  While these hormonal changes were modest, the researchers urge that: “This trial found that previously sedentary postmenopausal women can adhere to a moderate- to vigorous-intensity exercise program that results in changes in estradiol and SHBG concentrations that are consistent with a lower risk for postmenopausal breast cancer.”