Impaired glucose tolerance is characterized by multiple abnormalities in the regulation of intermediary metabolism

The responses of circulating intermediary metabolites to a low-dose sequential insulin infusion (basal, 0.005, 0.01, and 0.05 U kg-1 h-1) were assessed in eight non-obese men with Impaired Glucose Tolerance (IGT), and in eight healthy control subjects with normal glucose tolerance matched for age, gender, and body mass index. Fasting hyperinsulinaemia was observed in the subjects with IGT (7.4 +/- 1.0 vs 2.9 +/- 0.3 mU I-1, p less than 0.001). While there was no significant difference (p greater than 0.1) in fasting venous glucose levels between the groups, fasting concentrations of lactate (p less than 0.02), alanine (p less than 0.01), and glycerol (p less than 0.05) were significantly elevated in the subjects with IGT. During the incremental insulin infusion, overall concentrations of glucose (p less than 0.05), lactate (p less than 0.05), alanine (p less than 0.05), glycerol (p less than 0.05), immunoreactive insulin (p less than 0.001), and C-peptide (p less than 0.01) were significantly higher in the subjects with IGT. Linear dose-response relationships (p less than 0.005) for circulating immunoreactive insulin (log) vs metabolite concentrations were demonstrated by analysis of variance for glucose, non-esterified fatty acids (NEFA), glycerol, and total ketone bodies. For glucose, glycerol, and NEFA, group dose-response regression lines for the subjects with IGT were displaced significantly to the right (p less than 0.001 for each) of those for the normal control subjects, implying insulin insensitivity. In addition to the recognized defect in glucose homeostasis, these results indicate impaired regulation of multiple aspects of intermediary metabolism including lipolysis in IGT.