Melatonin and Sex Hormones Among Rotating Shift Nurses

Abstract

O-30B8-5 Background/Aims: The causes of breast cancer are not fully understood. Global discrepancies in breast cancer incidence suggest environmental and societal influences on this disease—long-term shift work was recently classified as a “probable carcinogen” by the International Agency for Research on Cancer. A proposed pathway is from nighttime light exposure, which leads to decreases in melatonin, a cancer-protective hormone. Irregular melatonin levels may influence patterns of sex hormone production that in turn influence breast cancer risk. This study examined relationships between melatonin and sex hormones among shift-working women, with the goals of better understanding this occupational exposure, and ultimately, to minimize health effects. Methods: A longitudinal study was conducted among 82 premenopausal shift-working nurses. Epidemiological data and biological specimens were collected over 2 time periods (summer/winter). Melatonin metabolite levels were measured in urine, and estradiol, estrone, progesterone, and prolactin concentrations from fasting blood. Information regarding personal, reproductive, and lifestyle factors was collected from questionnaires and physical examinations. We assessed melatonin-sex hormone relationships both cross-sectionally (within each of 2 seasons), and longitudinally (across seasons), to investigate possible latency effects. Multivariate linear regression was used to explore these relationships, with adjustment for confounders including age and body mass index (BMI). Results: Our sample of premenopausal nurses has a mean age of 35.8(±8.2) and BMI of 27.1 kg/m2(±6.7), are mostly nonsmokers (85.7%) and nonusers of oral contraceptives (77.1%). Melatonin levels were suggested to be inversely associated with estradiol levels in winter only (regression coefficient, β = −0.16; P = 0.07). No association was observed between melatonin and estrone, progesterone or prolactin. In longitudinal analyses, melatonin in the first season was not associated with sex hormones in the second season. Conclusion: Our findings suggest, but do not confirm, that melatonin exerts a regulatory influence on estradiol production; thus, shift-work could contribute to increased breast cancer risk via increased estradiol.