Fasting and reproduction in non-human primates

Recent studies in our laboratory, in monkeys and humans, indicate that reproductive hormone secretion begins to be suppressed within very brief periods of undernutrition. Tn male monkeys, luteinizing hormone and testosterone secretion begin to decline when a single meal is missed, whereas eating a single meal after a brief period of fasting rapidly and dramatically stimulates LH and testosterone secretion. In addition, changing the timing of daily meal intake alters the diurnal pattern of LH and testosterone secretion in male monkeys. These findings suggest that nutritional/metabolic signals that are linked to food intake are part of the normal physiological mechanism regulating the daily activity of the reproductive axis, rather than simply signals that influence the activity of the reproductive axis in pathological conditions of severe undernutrition. Interestingly, mild changes in food intake are only effective in regulating LH secretion in females during the luteal phase of the menstrual cycle. In both males and females, the central neural drive to the reproductive axis is sensitized to modulation by nutritional signals by the presence of physiological levels of gonadal steroid hormones. A key issue at this time is identification of the mechanism(s) by which mild changes in nutrient intake modulate the central neural system regulating the reproductive axis. Recent studies in our laboratory indicate that the metabolic cue transmitting information to the reproductive axis is dependent on calorie intake, but is not dependent on changes in body mass or composition, changes in intake of a specific nutrient, changes in plasma glucose, insulin, thyroid hormone, or cholecystokinin concentrations, or signals emanating from the taste or smell of food or the process of food ingestion. However, it appears that central noradrenergic and neuropeptide Y neural systems play roles in transmitting nutritional information to the reproductive axis.