24-hour profiles of melatonin, cortisol, insulin, C-peptide and GIP following a meal and subsequent fasting.

Melatonin, free and total cortisol, insulin, C-peptide and glucose-dependent insulin-releasing peptide (GIP) were measured in the plasma of twelve normal volunteers (eight women and four men), at hourly intervals for 24 h following a meal and subsequent fasting. One volunteer was excluded from calculations due to a possible effect of stress on melatonin secretion. Melatonin and cortisol showed the normal 24-h variation with peak values at 0200-0500 h, and 0900 h respectively. Following post-prandial stimulation, gut hormones remained basal throughout the sampling period. No significant relationship was found between 24-h melatonin secretion and basal, or stimulated gut hormone secretion. Melatonin secretion did relate significantly to body weight, suggesting that data concerning pineal effects in endocrine physiology and pathology, and affective disease, should be reviewed in the light of these observations.