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Patrick Maguire, MD, Oncology, answeredBecause it's widely known that female hormones play a key role in the development of breast cancer for many women, researchers have investigated how physical activity affects sex hormone levels. Prior studies had shown that increased estradiol levels and decreased sex hormone binding globulin levels, for instance, were associated with an increased risk of breast cancer. Two recent trials which randomized sedentary (relatively inactive) women to vigorous exercise for a few hours per week versus no increased activity or only stretching found that the exercise significantly affected the levels of these important sex hormones. The investigators concluded that the protective effect of exercise against breast cancer risk is likely due, at least in part, to its effects on sex hormone levels.