Original Contribution Urinary Melatonin Concentration and the Risk of Breast Cancer in Nurses ’ Health Study II

Experimental and epidemiologic data support a protective role for melatonin in breast cancer etiology, yet studies in premenopausal women are scarce. In a case-control study nested within the Nurses ’ Health Study II cohort, we measured the concentration of melatonin’s major urinary metabolite, 6-sulfatoxymelatonin (aMT6s), in urine sam-ples collected between 1996 and 1999 among 600 breast cancer cases and 786 matched controls. Cases were predominantly premenopausal women who were diagnosed with incident breast cancer after urine collection and before June 1, 2007. Using multivariable conditional logistic regression, we computed odds ratios and 95 % confi-dence intervals. Melatonin levels were not significantly associated with total breast cancer risk (for the fourth (top) quartile (Q4) of aMT6s vs. the first (bottom) quartile (Q1), odds ratio (OR) = 0.91, 95 % confidence interval (CI): 0.64, 1.28; Ptrend = 0.38) or risk of invasive or in situ breast cancer. Findings did not vary by body mass index, smoking status, menopausal status, or time between urine collection and diagnosis (all Pinteraction values ≥ 0.12). For exam-ple, the odds ratio for total breast cancer among women with ≤5 years between urine collection and diagnosis was 0.74 (Q4 vs. Q1; 95 % CI: 0.45, 1.20; Ptrend = 0.09), and it was 1.20 (Q4 vs. Q1; 95%CI: 0.72, 1.98; Ptrend = 0.70) for women with>5 years. Our data do not support an overall association between urinary melatonin levels and breast cancer risk. breast cancer; melatonin; 6-sulfatoxymelatonin Abbreviations: aMT6s, 6-sulfatoxymelatonin; BMI, body mass index; CI, confidence interval; ER, estrogen receptor; NHS II, Nurses ’ Health Study II; OR, odds ratio; ORDET, Hormones and Diet in the Etiology of Breast Cancer Risk; PR, progesteron