Growth hormone (GH) is a major anabolic hormone that exerts important stimulatory effects on protein synthesis. Many of the peripheral tissue effects of GH are mediated by insulin-like growth factor I (IGF-I), produced systemically by the liver or locally in tissues in response to GH stimulation. IGF-I in turn regulates GH secretion by negative feedback mechanisms at the pituitary gland. Several investigators have shown that aging is associated with a decrease in spontaneous GH secretion and IGF-I levels. GH levels decline by 14% for each decade after puberty, and, in healthy 59-98 year old men, IGF-I levels below the 2.5 percentile of younger men are present in 85%. Reduction of GH release in aging is thought to be associated with an increase in somatostatin tone, decrease in hypothalamic GHRH output, and diminished response to GHRH. The fact that aging is accompanied by a decrease in protein synthesis leading to a loss of lean body mass (LBM) and a gain in body fat suggests that a decrease in GH secretion may contribute to these changes. It has been hypothesized that restoration of GH level in the elderly to the levels observed in younger individuals may lead to improvements in body composition.
