Melatonin negatively correlates with C-peptide after food intake.

Melatonin plays a key role in the circadian timing system. At present, many other functions of melatonin are known. Question remains whether changes in endogenous melatonin may be associated with food intake. Hence, the levels of melatonin, C-peptide and glucose were followed during a daily regimen (16 hours) including standardized food intake using commercial kits. The diurnal profiles of the hormones and serum glucose were evaluated using ANOVA with Period and Subject as independent factors. Pearson's correlations and using a multiple stepwise backward regression model consisting of the time factor as a polynomial, and serum C-peptide and glucose assessed the correlations between melatonin and the remaining parameters. Our results showed a significant negative correlation between melatonin and C-peptide. The profile of melatonin was physiological, decreasing after wake-up, showing minor changes during the daytime and increasing in the evening. As documented, lesser alterations were indicated in the course of the melatonin daytime profile, which may reflect periodic food intake. Food intake is not the primary factor influencing the melatonin course. While previous studies have mostly considered the protective effect of melatonin in diabetic subjects, our study brought the results suggesting food intake as a factor contributing to daytime melatonin variation in humans. However, the physiological role of melatonin association with food intake in daytime remains in question and should be further investigated.