The effects of GH replacement therapy on energy metabolism are still uncertain, and long-term benefits of increased muscle mass are thought to outweigh short-term negative metabolic effects. This study was designed to address this issue by examining both short-term (1 wk) and long-term (6 months) effects of a low-dose (9.6 μg/kg body weight·d) GH replacement therapy or placebo on whole-body glucose and lipid metabolism (oral glucose tolerance test and euglycemic hyperinsulinemic clamp combined with indirect calorimetry and infusion of 3-[³H]glucose) and on muscle composition and muscle enzymes/metabolites, as determined from biopsies obtained at the end of the clamp in 19 GH-deficient adult subjects.

GH therapy resulted in impaired insulin-stimulated glucose uptake at 1 wk (−52%; P = 0.008) and 6 months (−39%; P = 0.008), which correlated with deterioration of glucose tolerance (r = −0.481; P = 0.003). The decrease in glucose uptake was associated with an increase in lipid oxidation at 1 wk (60%; P = 0.008) and 6 months (60%; P = 0.008) and a concomitant decrease in glucose oxidation. The deterioration of glucose metabolism during GH therapy also correlated with the enhanced rate of lipid oxidation (r = −0.508; P = 0.0002). In addition, there was a shift toward more glycolytic type II fibers during GH therapy.

In conclusion, replacement therapy with a low-dose GH in GH-deficient adult subjects is associated with a sustained deterioration of glucose metabolism as a consequence of the lipolytic effect of GH, resulting in enhanced oxidation of lipid substrates. Also, a shift toward more insulin-resistant type II X fibers is seen in muscle. Glucose metabolism should be carefully monitored during long-term GH replacement therapy.