Aging-related growth hormone (GH) decrease is a selective hypothalamic GH-releasing hormone pulse amplitude mediated phenomenon

Background. Aging is accompanied by declining growth hormone (GH) and insul in-like growth factor-I (IGF-I) levels. The neuroendocrine mechanisms of th is decline have been studied previously, but the interpretation of the data was confounded by the imprecision in GH measurements and by the intervenin g variables of altered body composition and decreased gonadal steroid milie u in the elderly subjects of both sexes. Methods. To study the contribution of aging per se, we evaluated discrete p arameters of GH pulsatility in young (n = 8 women. n = 8 men) and elderly ( n = ii women, n = 10 men) subjects closely matched for body mass index. Blo od samples for GH were obtained every 10 minutes for 24 hours. Plasma GH wa s measured by a sensitive chemiluminescent assay. GH pulsatility was assess ed using cluster analysis. Results. The elderly subjects had plasma IGF-I levels and integrated GH con centrations that were 32% to -56% of their sex-matched younger counterparts . The age-associated attenuation in GH was due to a decrease in GH pulse am plitude, whereas pulse frequency and nadir levels were unchanged. The major ity of the young subjects (81%) reached their peak GH Juring the "lights of f" period, whereas the majority of the elderly subjects (62%) pecked during the "lights on" period (p = .01). Conclusions. We conclude that aging in both sexes is accompanied by profoun d decreases in GH output and in plasma IGF-I concentrations. This effect is separate from the alterations in body mass index that accompany the normal aging process. Attenuation of GH output associated with aging is related s olely to the lower GH and, by inference. GH-releasing hormone (GHRH) pulse amplitude.