Background: The protective effect of physical activity on breast cancer risk might be mediated by sex hormone levels. In this study, we examined the association between usual physical activity and plasma levels of estrogens, androgens, dehydroepiandrosterone sulphate (DHEAS), and sex hormone binding globulin (SHBG) in postmenopausal women.
Methods: We conducted a cross-sectional study among 806 postmenopausal women participating in the Prospect-EPIC study. Usual physical activity was assessed using a short questionnaire and summarized into a simple, validated four-level index. This index combines occupational physical activity with time spent on cycling and sporting. Levels of estrone, estradiol, androstenedione, DHEAS, testosterone, and SHBG were measured in plasma. General linear models were used to examine the association between usual physical activity and sex hormone levels, adjusted for confounders.
Results: We observed an inverse association between physical activity and estradiol levels (free: inactive, 0.26 pg/mL; active, 0.23 pg/mL; P-trend = 0.045; total: inactive, 8.8 pg/mL; active, 8.0 pg/mL; P-trend = 0.08) and a positive association between physical activity and SHBG (inactive, 15.1 nmol/L; active, 19.3 nmol/L; P-trend = 0.05). These associations could largely be explained by the effects of sporting. We also observed a positive association between physical activity and DHEAS (inactive, 352.4 ng/mL; active, 460.3 ng/mL; P-trend = 0.01).
Conclusions: Our results are in accordance with the hypothesis that usual physical activity decreases estradiol levels and increases SHBG. We also found that high levels of physical activity are associated with high levels of DHEAS. Furthermore, our results suggest that vigorous forms of physical activity influence sex hormone levels most.