Compared with NGT, subjects with IFG and IGT manifested a decrease in β-cell glucose sensitivity; IFG subjects, but not IGT subjects, had decreased β-cell rate sensitivity.
In IFG subjects, the defect in β-cell glucose sensitivity was time dependent, began to improve after 60 min, and was comparable to NGT after 90 min.
The incremental area under the plasma C-peptide concentration curve during the first 12 min of the hyperglycemic clamp (ΔC-pep[AUC]0-12) was inversely related with the increase in FPG concentration (r=-36, r=0.001), whereas ΔC-pep[AUC]15-120 positively correlated with FPG concentration (r=0.29, r<0.05). When adjusted for the prevailing level of insulin resistance, first-phase insulin secretion was markedly decreased in both IFG and IGT, whereas second-phase insulin secretion was decreased only in IGT. These results demonstrate distinct defects in β-cell function in IFG and IGT.