Background: It is hypothesized that exercise can lead to a decrease in breast cancer risk through several hormonal and nonhormonal mechanisms. The WISER (Women In Steady Exercise Research) study investigated the effects of aerobic exercise on premenopausal sex hormone levels. Methods: Three hundred ninety-one sedentary, healthy, young eumenorrheic women were randomized either into an exercise intervention of 30 minutes of aerobic exercise 5 times a week for approximately 16 weeks (n = 212) or into a control group (n = 179). Serum levels of estradiol, estrone sulfate, testosterone, and sex hormone-binding globulin (SHBG), all in the midfollicular phase, and of progesterone, in the midluteal phase, were measured at baseline and at the end of the 16-week period. Results: Compared with the controls (n = 153), exercisers (n = 166) experienced significant increases in aerobic fitness, lean body mass, and decreases in percent body fat. There were no significant changes in body weight and menstrual cycle length between or within groups. Progesterone decreased significantly in exercisers; however, this reduction was similar to that of the control group. No significant changes between or within groups were found for any of the other sex hormones or SHBG. Conclusions: In premenopausal women, 16 weeks of 150 minutes per week of moderate aerobic exercise in young women did not significantly alter sex hormone or SHBG levels. Impact: Any favorable effects that moderate aerobic exercise without an associated weight change may have on breast cancer risk in premenopausal women are unlikely to be a consequence of changes in levels of sex hormones or SHBG.
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