Background: Melatonin seems to play a role in breast cancer etiology, but data addressing the association between melatonin levels and breast cancer risk in postmenopausal women is sparse.
Methods: We conducted a nested case-control study in the Nurses' Health Study cohort. First spot morning urine was collected from 18,643 cancer-free women from March 2000 through December 2002. The concentration of the major metabolite of melatonin, 6-sulfatoxymelatonin (aMT6s), was available for 357 postmenopausal women who developed incident breast cancer through May 31, 2006, along with 533 matched control subjects. We used multivariable conditional logistic regression models to investigate associations. All statistical tests were two sided.
Results: An increased concentration of urinary aMT6s was statistically significantly associated with a lower risk of breast cancer (odds ratio for the highest versus lowest quartile of morning urinary aMT6s, 0.62; 95% confidence interval, 0.41-0.95; P(trend) = 0.004). There was no apparent modification of risk by hormone receptor status of breast tumors, age, body mass index, or smoking status.
Conclusion: Results from this prospective study add substantially to the growing literature that supports an inverse association between melatonin levels and breast cancer risk.