Insulin secretion and insulin sensitivity on the oral glucose tolerance test (OGTT) in middle-aged Japanese

The aim of this study was to assess the changes in insulin secretion and insulin sensitivity in relation to fasting and 2-hour plasma glucose (PG) levels and to assess the independent contributions of their impairments to non-diabetic hyperglycemia. A total of 2157 Japanese workers (mean age 52.6±7.3 years and mean BMI 23.9±3.2 kg/m ²) underwent an oral glucose tolerance test (OGTT). Of these subjects, 1125 had normal glucose tolerance (NGT), 525 subjects had isolated impaired fasting glucose (IFG), 159 subjects had isolated impaired glucose tolerance (IGT), 263 subjects had combined IFG and IGT, and 85 subjects had newly diagnosed type 2 diabetes. Insulinogenic index and Matsuda insulin sensitivity index (ISI) were signifcantly attenuated in subjects with normal but slightly elevated fasting PG, or in subjects with normal but slightly elevated 2-hour PG. Whereas, InsAUC ₁₂₀/GluAUC ₁₂₀ was not signifcantly decreased in those subjects, and signifcant decrease of it was observed exclusively in subjects with abnormal fasting PG (≥ 106 mg/dL) or abnormal 2-hour PG (≥ 221 mg/dL). Using multiple regression analyses, both Matsuda ISI and insulinogenic index were independently correlated with PG concentrations in subjects with IFG and/or IGT, while Matsuda ISI alone was independently correlated with fasting PG concentrations in normoglycemic subjects. In conclusion, both insulinogenic index and Matsuda ISI were signifcantly attenuated in subjects with normal but slightly elevated PG. Lowering of Matsuda ISI was likely to be a strong contributor to 'elevation of fasting PG within the normal range' in this population.