Melatonin and sex hormones among rotating shift nurses
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Background: In 2007, the International Agency for Research on Cancer classified shift-work involving circadian disruption as a “probable carcinogen.” One proposed pathway for this relationship involves nighttime light exposure and subsequent decreases in melatonin production. It is postulated that melatonin, a cancer-protective hormone, may influence patterns of sex hormone production that in turn influence breast cancer risk. The purpose of this study was to investigate the relationships between night shift-work history, melatonin and sex hormone levels among shift-working women. Methods: 82 pre-menopausal nurses who work a rotating shift pattern of two days (7AM-7PM), two nights (7PM-7AM), followed by five days off participated in two study periods approximately six months apart (in summer and winter), each taking place during a day shift of the normal rotating shift pattern. Creatinine-adjusted melatonin metabolite concentrations were measured from morning void urine samples, and estradiol, estrone, progesterone and prolactin concentrations were measured from fasting blood samples taken at the same time. Other pertinent information was collected by measurement (weight, height) and by self-report via questionnaire. We examined melatonin-sex hormone relationships within each of two seasons, and across seasons, to investigate two hypothesized latency periods for influences of melatonin levels on sex hormones. Multivariate linear regression was used to explore relationships, with adjustment for confounders including age and body mass index. Results: An inverse relationship between melatonin and estradiol was suggested in winter (β = -0.13, p = 0.11), and a positive relationship was suggested for increasing estrone with increasing melatonin tertile in summer (p = 0.07), after multivariate adjustments. Melatonin was not associated with other hormones in either season. On investigation of a longer latency period, melatonin in the first season was not associated with sex hormones in the second season. While those working night shifts for 20 years or more had higher mean levels of estradiol, estrone and progesterone, results were not statistically different from those with a shorter history of night work. Conclusions: The results of this study do not provide evidence to support the proposed biological pathway involving altered melatonin and sex hormone levels as intermediates between shift-work and breast cancer risk.