Abstract
IntroductionExposure to high levels of endogenous estrogens is a main risk factor for breast cancer in women, and in observational studies was found to be inversely associated with physical activity. The objective of the present study is to determine the effect of physical activity interventions on sex hormone levels in healthy women.MethodsElectronic databases (MEDLINE, EMBASE, CENTRAL), from inception to December 2014, and reference lists of relevant reviews and clinical trials were searched, with no language restrictions applied. Randomized controlled trials (RCTs) were included if they compared any type of exercise intervention to no intervention or other interventions, and assessed the effects on estrogens, androgens or the sex hormone binding globulin (SHBG) in cancer-free women. Following the method described in the Cochrane Handbook for Systematic Reviews of Interventions, data on populations, interventions, and outcomes were extracted, and combined using the inverse-variance method and a random-effects model. A pre-established protocol was drawn up, in which the primary outcome was the difference in circulating estradiol concentrations between the physical activity (experimental) and the control groups after intervention. Pre-specified subgroup analyses and sensitivity analysis according to the risk of bias were conducted.ResultsData suitable for quantitative synthesis were available from 18 RCTs (1994 participants) for total estradiol and from 5 RCTs (1245 participants) for free estradiol. The overall effect of physical activity was a statistically significant decrease of both total estradiol (standardized mean difference [SMD] -0.12; 95 % confidence interval [CI] -0.20 to −0.03; P = 0.01; I2 = 0 %) and free estradiol (SMD −0.20; 95 % CI −0.31 to −0.09; P = 0.0005; I2 = 0 %). Subgroup analyses suggest that this effect is independent of menopausal status and is more noticeable for non-obese women and for high intensity exercise. Meta-analysis for secondary outcomes found that physical activity induces a statistically significant decline of free testosterone, androstenedione, dehydroepiandrosterone–sulfate and adiposity markers, while a significant increase of SHBG was observed.ConclusionsAlthough the effect is relatively modest, physical activity induces a decrease in circulating sex hormones and this effect is not entirely explained by weight loss. The findings emphasize the benefits of physical activity for women.Electronic supplementary materialThe online version of this article (doi:10.1186/s13058-015-0647-3) contains supplementary material, which is available to authorized users.
Highlights
Exposure to high levels of endogenous estrogens is a main risk factor for breast cancer in women, and in observational studies was found to be inversely associated with physical activity
Physical activity is associated with a 25 % reduction in the average risk of breast cancer among women [4], and this protective effect appears to be independent of menopausal status [5]
Ten studies were conducted on premenopausal eumenorrheic women [22,23,24,25,26,27,28,29,30,31,32], one study on perimenopausal women [33], 13 studies on postmenopausal women [21, 34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53], and 12 studies on women with polycystic ovary syndrome (PCOS) [54,55,56,57,58,59,60,61,62,63,64,65,66,67]
Summary
Exposure to high levels of endogenous estrogens is a main risk factor for breast cancer in women, and in observational studies was found to be inversely associated with physical activity. Physical activity is associated with a 25 % reduction in the average risk of breast cancer among women [4], and this protective effect appears to be independent of menopausal status [5]. Several observational studies have found an inverse association between physical activity and circulating estrogen levels [11, 12]. This effect may be mediated by the decrease in fat mass [13], the main source of estrogens in postmenopausal women. The assessment of the exposure to physical activity remains imprecise, because it is generally only possible to measure it in naturally living subjects using questionnaires [4, 15]
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