Constant pineal output and increasing body mass account for declining melatonin levels during human growth and sexual maturation.

Twenty-four-h urine samples, divided into two fractions representing night- and daytime melatonin production, were collected from 115 healthy individuals between the ages of 3 and 80, of known height and weight, and assayed for 6-hydroxy melatonin sulphate (SaMT), a major urinary metabolite of melatonin, by gas chromatography mass spectrometry. The population was divided for analytical purposes into children (boys aged 3-10.99, girls aged 3-9.59), adolescents (males aged 11-17.99, females aged 9.60-17.99), and adults (men and women over 18). The results showed approximately the same excretion over 24 h in all 3 groups but that the night/day ratio was considerably greater in children and adolescents compared to adults (P less than 0.001). However, when the results were expressed as a function of body weight (BW), body surface area (BSA), or creatinine excretion (CE), nocturnal SaMT was higher in children than in adults (P less than 0.001 for all 3 parameters) or adolescents (BW, P less than 0.001; BSA, P less than 0.002; CE, P less than 0.001) and was higher in adolescents than in adults (BW and BSA, P less than 0.001). Children also excreted more during the day than adults (BW, P less than 0.01; CE, P less than 0.001) or adolescents (BW alpha CE, P less than 0.02). Our results show that pineal output barely changes during childhood and adolescence. However, there is an age related decrease in SaMT excretion/unit body mass which correlates with an age-related increase in body mass. We therefore conclude that the decrease in circulating levels of melatonin during growth and sexual maturation is brought about by an increase in body mass.