Short-term low-dose growth hormone administration in subjects with impaired glucose tolerance and the metabolic syndrome: Effects on β -cell function and post-load glucose tolerance

Objective: Modest elevations in circulating IGF-I levels have been suggested to protect against the development of glucose intolerance in insulin-resistant subjects. To further understand the interactions of GH and IGF-I on β-cell function and post-load glucose tolerance in glucose-intolerant subjects predisposed to diabetes, we performed a pilot study in 12 subjects with impaired glucose tolerance and the metabolic syndrome using a low GH dose (1.7 μg/kg per day) known to increase endogenous IGF-I production. Design: Fourteen daily GH or placebo injections in a double-blind cross-over study. Methods: Baseline and post-treatment oral glucose tolerance tests were performed. The homeostasis model assessment and the insulinogenic index was used to estimate fasting insulin sensitivity (S_I) and β-cell function respectively, whereas changes in the incremental area under the curve were used to estimate post-load glucose tolerance (ΔAUC_glu) and post-load insulin levels (ΔAUC_ins). Results: GH increased total IGF-I (P < 0.02), free IGF-I (P < 0.04) and fasting insulin (P < 0.04) levels, but did not modify plasma IGF-binding proteins (IGFBPs)-1 and -3, fasting glucose, non-esterified fatty acid and C-peptide levels, and fasting S_I. After oral glucose intake, glucose tolerance improved (P < 0.03), but post-load insulin levels and β-cell function remained unchanged. Conclusion: Short-term low-dose GH administration induced fasting hyperinsulinaemia possibly by reducing insulin clearance but improved post-load glucose tolerance, suggesting that increased bioavailable IGF-I enhanced post-load S_I without altering β-cell function. Longer-term studies are required to ascertain whether these positive effects on post-load glucose tolerance and the preservation of β-cell function can be sustained by this GH dose in these high-risk subjects.