Growth Hormone and Testosterone: Effects on Whole Body Metabolism and Skeletal Muscle in Adolescence

Abstract

The characteristic changes in human puberty – increased muscle bulk, increased linear growth, and the mineralization of the skeleton – are due, in part, to the combined effects of growth hormone (GH), insulin-like growth factor I (IGF-I), sex hormones and insulin. GH and testosterone, but not oestrogen, are highly protein-anabolic in vivo, and the combined administration of GH and testosterone has synergistic effects on these measures, both in children and adults. GH has no significant effect on enhancing skeletal muscle strength in humans; however, testosterone is highly ergogenic and is largely responsible for the differences in muscle bulk apparent in late puberty between the genders. The results of recent detailed metabolic studies suggest that boys have higher rates of protein synthesis and IGF-I generation than girls, given comparable doses of GH. In patients with a constitutional delay in growth and maturation, relative deficiencies in testosterone and lower IGF-I concentrations are associated with greater rates of total energy expenditure, suggesting that this relatively hormone-insufficient state is associated with a hypermetabolic state. Whether added nutritional supplements, alone or in combination with GH, could improve the growth pattern and final height of these children deserves further study. In conclusion, the metabolic effects of GH and testosterone are complex and they synergize in puberty. These hormones could be of use as protein-anabolic agents in selected catabolic conditions.