Diminished secretion of growth hormone is responsible in part for the decrease of lean body mass, the expansion of adipose-tissue mass, and the thinning of the skin that occur in old age.

A B S T R A C T Growth hormone (GH) release was studied in adults of normal stature, ages 21-86 yr. The subjects were 85-115% of ideal body weight, between the 5th and 95th percentiles in height, and free ofactive or progressive disease. 9 to 12 individuals in each decade from third to ninth were evaluated. The following criteria of GH status were measured: serum GH concentration, analyzed by radioimmunoassay at halfhour intervals for 4 h after onset of sleep, and at 1-h intervals from 8 a.m. to 4 p.m. in 52 subjects; daily retention of N, P, and K in response to 0.168 U human (h)GH/kg body wt314/day in 18 subjects; and plasma somatomedin C (SmC) level before and during exogenous hGH treatment in 18 subjects.All 10 individuals, 20-29 yr old, released substantial amounts of endogenous GH during both day and night (average peak serum GH obtained during day and night was 7.3 and 20.3 ng/ml, respectively); average plasma SmC was 1.43 U/ml (95% tolerance limits, 0.64-2.22 U/ ml). There was no significant effect of exogenous hGH on elemental balances or on plasma SmC. In contrast, 6 of 12 individuals 60-79 yr old showed the following evidences of impaired GH release: peak waking and sleeping seruim GH < 4 nglml; plasma SmC < 0.38 U/ ml; a significant retention in N, P, and K; and a significant rise in plasma SmC, in response to exogenous hGH.Plasma SmC, serum GH during sleep, serum GH during the day, retentions of N, P, and K in response to exogenous hGH, and rise in plasma SmC in response to hGH were all intercorrelated (P < 0.05). Plasma SmC < 0.38 U/ml corresponded to peak nocturnal serum GH < 4 ng/ml. The prevalence of plasma SmC < 0.38 U/ml

For many years, it has been known that the composition of the human body changes with advancing age. In healthy youth, about 10% of the body weight is bone, 30% is muscle, and 20% is adipose tissue. After about age 50, these ratios change progressively. At age 75, a typical composition is 8% bone, 15% muscle, and 40% adipose tissue. The loss of bone mass predisposes to fractures of the spine and extremities. Loss of muscle mass reduces strength and endurance. Simultaneously, the functional capacities of most organ systems decline.We do now know the cause for these age-related, undesirable changes in body composition and in physiologic functions. Recently, a new aspect of the geriatric endocrine system was discovered that may provide both a partial explanation and a treatment. After about age 50, the secretion of growth hormone (GH) gradually declines and ultimately stops in many individuals. About half of our citizens over age 65, therefore, are partially or totally deficient in GH.Growth hormone has powerful effects on body composition and on the functions of most organs except the nervous system. Many of these somatotropic effects are opposite in direction to the geriatric declines in structure and function. Thus, GH causes enlargement of muscles, liver, kidneys, bones, and lymphoid organs; shrinkage of adipose mass; and increases in renal blood flow and glomerular filtration rate. The hypothesis arises, therefore, that several of the geriatric changes in structure and function result at least in part from G H deficiency, and could be reversed by continuing treatment with human GH (hGH) at physiologic or replacement doses.

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