Predominant role of reduced beta-cell sensitivity to glucose over insulin resistance in impaired glucose tolerance

Aims/hypothesis. Impaired glucose tolerance (IGT) is an insulin-resistant state and a risk factor for Type 2 diabetes. The relative roles of insulin resistance and insulin deficiency in IGT have been disputed. Methods. In 40 IGT subjects and 63 sex-, age-, and weight-matched controls with normal glucose tolerance (NGT), we measured (i) indices of insulin sensitivity of fasting glucose production (by tracer glucose) and glucose disposal (M value on a 240 pmol·min^-1·m^-2 insulin clamp) and (ii) indices of beta-cell function (glucose sensitivity, rate sensitivity, and potentiation) derived from model analysis (Am J Physiol 283:E1159-E1166, 2002) of the insulin secretory response (by C-peptide deconvolution) to oral glucose. Results. In comparison with NGT, IGT were modestly insulin resistant (M=29±2 vs 35±2 μmol·min^-1·kg _FFM^-1, p=0.01); insulin sensitivity of glucose production also was reduced, in approximate proportion to M. Despite higher baseline insulin secretion rates, IGT was characterized by a 50% reduction in glucose sensitivity [53 (36) vs 102 (123) pmol·min^-1·m ^-2·mM^-1, median (interquartile range), p=0.001] and impaired potentiation [1.6 (0.8) vs 2.0 (1.5) units, p<0.04] of insulin release, whereas rate sensitivity [1.15 (1.15) vs 1.38 (1.28) nmol·m ^-2·mM^-1] was not significantly reduced. Glucose sensitivity made the single largest contribution (∼50%) to the observed variability of glucose tolerance. Conclusion/interpretation. In IGT the defect in glucose sensitivity of insulin release quantitatively predominates over insulin resistance in the genesis of the reduced tolerance to oral glucose.