Body composition changes progressively in mid and late adulthood. Though the lean body mass in men over 50 years old falls at an average rate of -0.6% per year, the total body weight tends to remain stable because of a reciprocal expansion of adipose mass. The shrinkage of the lean body mass reflects the atrophy of skeletal muscles, skin and visceral organs. It is said that geriatric hyposomatotropism is the main cause for these symptoms as growth hormone causes expansion of the lean body mass and contraction of the adipose mass, and because growth hormone secretion tends to diminish in late adulthood.
The authors have tested this hypothesis by recruiting 45 independent men over 61 years old with plasma somatomedin C level below 0.35 U/ml, indicating little or no detectable growth hormone secretion. The 21-month protocol was as follows: baseline period 0-6 months, experimental period 6-18 months and post-experimental period 18-21 months. During the experimental period, 26 men (group I) received approximately 0.03 mg/kg of biosynthetic human growth hormone (HGH) subcutaneously 3 times a week, while 19 men (group II) received no treatment. Plasma somatomedin C was measured monthly. The following outcome variables were measured at 0, 6, 12 and 18 months: lean body mass, adipose mass, skin thickness (dermis plus epidermis), sizes of the liver, spleen and kidneys, the cross sectional areas of ten muscle groups, and bone density at 9 skeletal sites. Lean body mass and adipose mass were also measured at 21 months. In group I, hGH treatment raised the plasma somatomedin C level and maintained it in the range 0.5-1.5 U/ml.(The declining activity of the growth hormone–insulin-like growth factor I (IGF-I) axis with advancing age may contribute to the decrease in lean body mass and the increase in mass of adipose tissue that occur with aging.
METHODS: To test this hypothesis, we studied 21 healthy men from 61 to 81 years old who had plasma IGF-I concentrations of less than 350 U per liter during a six-month base-line period and a six-month treatment period that followed. During the treatment period, 12 men (group 1) received approximately 0.03 mg of biosynthetic human growth hormone per kilogram of body weight subcutaneously three times a week, and 9 men (group 2) received no treatment. Plasma IGF-I levels were measured monthly. At the end of each period we measured lean body mass, the mass of adipose tissue, skin thickness (epidermis plus dermis), and bone density at nine skeletal sites.
RESULTS: In group 1, the mean plasma IGF-I level rose into the youthful range of 500 to 1500 U per liter during treatment, whereas in group 2 it remained below 350 U per liter. The administration of human growth hormone for six months in group 1 was accompanied by an 8.8 percent increase in lean body mass, a 14.4 percent decrease in adipose-tissue mass, and a 1.6 percent increase in average lumbar vertebral bone density (P less than 0.05 in each instance). Skin thickness increased 7.1 percent (P = 0.07). There was no significant change in the bone density of the radius or proximal femur. In group 2 there was no significant change in lean body mass, the mass of adipose tissue, skin thickness, or bone density during treatment.
CONCLUSIONS: 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.
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