A protective role of melatonin in the etiology of breast cancer has been suggested.
The down-regulation of estrogen secretion by melatonin is thought to be a main cause of the link between melatonin and breast cancer risk. The present cross-sectional study examined whether the urinary 6-sulfatoxymelatonin (aMT6-s) level is inversely associated with mammographic density, which is regarded as a marker of breast cancer risk. The study subjects were 289 Japanese women (175 premenopausal and 123 postmenopausal women) who were recruited from participants in a mammographic breast cancer screening. The size of the total breast area and that of the dense area were measured quantitatively using an automated mammographic mass detection method. The concentration of aMT6-s was measured using first-void morning urine. In premenopausal women, the urinary aMT6-s level was significantly positively associated with percent density after controlling for covariates (P for trend=0.02). There was no significant association between urinary aMT6-s level and the percent density in postmenopausal women. We found no evidence that the melatonin level is inversely associated with mammographic density.