A high sperm DNA fragmentation index (DFI) is associated with reduced fertility. DFI is influenced by the balance between reactive oxygen species and antioxidants. A circannual variation in melatonin, an antioxidant and free radical scavenger, could thus impact semen quality and fertility. The association between the major melatonin metabolite, urine 6-sulfatoxymelatonin (aMT6s), and DFI was analyzed in 110 Oslo men (south of the Arctic Circle) and 86  Tromsoe men (north of the Arctic Circle). Two semen analyses, summer and winter, and four urine samples (early/late summer; early/late winter), were analyzed. The associations between aMT6s in urine and DFI were characterized in a cross-sectional and longitudinal manner using correlation analysis and linear regression. Regardless of season and location, no significant correlations between aMT6s and DFI were observed. The correlation coefficients for associations between changes over time (early winter-early summer) in aMT6s and DFI were for the total cohort: rho = −0.08 (P = 0.322), for the Oslo cohort: rho = −0.07 (P = 0.485), and for the Tromsoe cohort: rho = −0.14 (P = 0.273), respectively. Similar results were seen when comparing late winter and late summer. There was no any statistically significant correlation between changes over time in aMT6s and DFI for men with DFI below and above the median value (10%), respectively. The seasonal variation in melatonin excretion seems not to have any impact on DFI.

Keywords: male infertility; melatonin; semen quality; sperm DNA index

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