Predictors of growth hormone secretion in aging

Growth hormone (GH) secretion decreases progressively after mid-puberty in both men and women. This decrease occurs predominantly before age 40-50 and affects both daytime and nocturnal GH secretion. A reduction in the amplitude of GH secretory pulses accounts for the majority of the reduction in GH secretion. With aging, changes in hypothalamic function may occur that result in decreased GH secretion. These changes may include decreased secretion of GH-releasing hormone and/or the putative natural ligand for the GH secretagogue receptor or an increase in somatostatin release. Multiple physiological factors have been reported to regulate GH secretion including sleep, body composition (% body fat and amount of abdominal visceral fat), aerobic physical fitness and serum concentrations of insulin-like growth factor-I (IGF-I), gonadal steroids and insulin. Changes in these factors with aging may contribute to the reduction in GH secretion observed in older adults. However, these physiological predictors of GH secretion are not independent of one another and the relative importance of these factors in the regulation of GH secretion is not known. Preliminary evidence suggests that the amount of abdominal visceral fat and fasting serum concentrations of insulin and IGF-I are the most important predictors of 24-hour GH release in healthy adults, independent of age and gender. Bi-directional feedback between these three factors and GH secretion may account for the strong relationships observed.