Alcohol intake and endogenous sex hormones in women: meta-analysis of cohort studies and Mendelian randomization.

Background The mechanisms underlying alcohol-induced breast carcinogenesis are not fully understood but may involve hormonal changes. Methods We investigated cross-sectional associations between self-reported alcohol intake and serum or plasma concentrations of oestradiol, oestrone, progesterone (in pre-menopausal women only), testosterone, androstenedione, DHEAS (dehydroepiandrosterone sulphate) and SHBG (sex hormone binding globulin) in 45 431 pre-menopausal and 173 476 post-menopausal women. We performed multivariable linear regression separately for UK Biobank, EPIC (European Prospective Investigation into Cancer and Nutrition) and EHBCCG (Endogenous Hormones and Breast Cancer Collaborative Group), and meta-analysed the results. For testosterone and SHBG, we also conducted two-sample Mendelian Randomization (MR) and colocalisation using the ADH1B (Alcohol Dehydrogenase 1B) variant (rs1229984). Results Alcohol intake was positively, though weakly, associated with all hormones (except progesterone in pre-menopausal women), with increments in concentrations per 10 g/day increment in alcohol intake ranging from 1.7% for luteal oestradiol to 6.6% for post-menopausal DHEAS. There was an inverse association of alcohol with SHBG in post-menopausal women but a small positive association in pre-menopausal women. MR identified positive associations of alcohol intake with total testosterone (difference per 10 g/day increment: 4.1%; 95% CI: 0.6%, 7.6%) and free testosterone (7.8%; 4.1%, 11.5%), and an inverse association with SHBG (-8.1%; -11.3%, -4.9%). Colocalisation suggested a shared causal locus at ADH1B between alcohol intake and higher free testosterone and lower SHBG (PP4: 0.81 and 0.97 respectively). Conclusions Alcohol intake was associated with small increases in sex hormone concentrations, including bioavailable fractions, which may contribute to its effect on breast cancer risk.