Insulin and C-peptide response in healthy persons and individuals with impaired glucose metabolism during oral glucose tolerance test

Serial measurements of blood glucose concentration before and after giving a specific amount of glucose orally provide a standard method to evaluate glucose metabolism. Although the reference ranges for glucose concentration in various disease states that are based on impaired glucose homeostasis have been established, the reference values are not clearly defined for insulin and C-peptide concentrations. The aim of this work was to study the insulin and C-peptide response during OGTT. Healthy individuals exhibited the following profile of insulin and C-peptide levels: 15 ± 4.9 mU/L and 0.5 ± 0.19 nmol/L (0 h), 116 ± 52.8 mU/L and 2.3 ± 0.79 nmol/L (1 h) and 59 ± 26.7 mU/L and 2.0 ± 0.67 nmol/L (2 h). Persons with impaired glucose tolerance had higher C-peptide levels at 0 h, 0.6 ± 0.17 nmol/L, and significantly higher insulin and C-peptide concentrations after 1 h, 209 ± 63.8 mU/L and 3.5 ± 1.00, nmol/L and 2 h, 188 ± 48.8 mU/L and 3.6 ± 0.92 nmol/L. Diabetic patients had higher basal levels of C-peptide, 0.8 ± 0.23 nmol/L, insulin and C-peptide increased after 1 h similarly as in healthy people, but further continued to increase significantly, 181 ± 137.6 mU/L and 3.7 ± 1.49 nmol/L. Subjects that exhibited low blood glucose levels demonstrated lower insulin concentrations at all time intervals, 11 ± 2.5 mU/L (0 h), 63 ± 31.1 mU/L (1 h) and 44 ± 22.9 mU/L (2 h), but the concentration of C-peptide leveled with that of the healthy ones. The results of this work may be useful in establishing reference ranges for insulin and C-peptide concentrations for defined time intervals during OGTT, in health and disease.