Neuroregulatory pathophysiology of impoverished growth hormone (GH) secretion in the aging human

A central enigma in neuroendocrine pathophysiology is the virtually uniform, but mechanistically incompletely explicable, attenuation of secretory and trophic activity of the growth hormone (GH)-insulin-like growth factor type I (IGF-I) axis in healthy aging in mammalian species, including the primate and human. Indeed, in humans, the calculated daily GH secretion rate falls approximately 50% every 7 years beginning at age 18 to 21, but this diminution in GH secretion is approximately twofold less rapid in premenopausal women. In contrast, the magnitude of relative GH deficiency appears to be similar in individuals of older (e.g., postmenopausal) age of either gender. Interpreting the mechanisms that underlie such marked attenuation of secretory activity of the GH-IGF-I axis is confounded by endocrine-metabolic covariates that accompany healthy aging, such as an accumulation of (visceral) adiposity, a decline in physical fitness, a reduction in sex steroids, disruption of slow-wave sleep, and concurrent illness and medications. Available clinical investigations point to a partial endogenous GHRH deficiency state, in this so-called somatopause. An important additional age-related fall in brain cholinergic activity (which regulates somatostatin secretion) is likely, thus arguing for combined hypothalamic somatostatin excess and GHRH, GHRP, or both deficiency. This article also evaluates the novel hypothesis that the GH impoverishment of aging is marked by disrupted network function of the GH-IGF-I feedback axis. Given this background, available and new technologies applied in patient-oriented investigations will likely unravel further the presumptively multiple mechanisms that subserve the hyposomatotropism of healthy aging, and begin to address the relative risks and benefits of restoring secretory activity of the aging GH-IGF-I axis.