Short-Term Effects of Growth Hormone on Fuel Oxidation and Regional Substrate Metabolism in Normal Man

Abstract

Primarily by increasing the availability of lipid intermediates, GH is likely to have profound effects on substrate consumption rates. To examine the short term actions of GH on glucose turnover, fuel oxidation and regional forearm metabolism, six normal volunteers were each studied twice for 5.5 h after having received a 4-h infusion of GH (20 ng/kg .cntdot. min) or saline. GH induced slightly falling plasma glucose levels, acute 40-50% decreases in forearm glucose uptake, and no change in glucose turnover. Furthermore, substantial increases in circulating concentrations and forearm uptake of nonesterified fatty acids and 3-hydroxybutyrate were recorded. Although GH infusion was followed by a 50% reduction of forearm alanine release hepatic nitrogen excretion seemed unaffected. Energy expenditure was not influenced by GH, but the non-protein respiratory exchange ratio decreased from a basal value of 0.778 .+-. 0.008 to 0.732 .+-. 0.007 after GH treatment (P < 0.05). Correspondingly, lipid oxidation increased from 1.20 .+-. 0.06 to 1.48 .+-. 0.09 mg/kg .cntdot. min, and glucose oxidation decreased from 0.97 .+-. 0.12 to 0.39 .+-. 0.06 mg/kg .cntdot. min (P < 0.05). Nonoxidative glucose utilization tended to increase. These data indicate that GH, by promoting lipid utilization and decreasing glucose oxidation, diminishes the need for gluconeogenesis and, therefore, could be protein preserving in the long term. Overall, we found no evidence of GH having acute insulin-like effects on glucose metabolism. GH appears to increase glucose storage, leaving total energy expenditure unaffected.