Shape of glucose, insulin, C-peptide curves during a 3-h oral glucose tolerance test: any relationship with the degree of glucose tolerance?

We aimed to analyze the shape of the glucose, insulin, and C-peptide curves during a 3-h oral glucose tolerance test (OGTT). Another aim was defining an index of shape taking into account the whole OGTT pattern. Five-hundred ninety-two OGTT curves were analyzed, mainly from women with former gestational diabetes, with glycemic concentrations characterized by normal glucose tolerance (n = 411), impaired glucose metabolism (n = 134), and Type 2 diabetes (n = 47). Glucose curves were classified according to their shape (monophasic, biphasic, triphasic, and 4/5-phases), and the metabolic condition of the subjects, divided according to the glucose shape stratification, was analyzed. Indices of shape based on the discrete second-order derivative of the curve patterns were also defined. We found that the majority of the glucose curves were monophasic (n = 262). Complex shapes were less frequent but not rare (n = 37 for the 4/5-phases shape, i.e., three peaks). There was a tendency toward the amelioration of the metabolic condition for increasing complexity of the shape, as indicated by lower glucose concentrations, improved insulin sensitivity and beta-cell function. The shape index computed on C-peptide, WHOSHCP (WHole-Ogtt-SHape-index-C-peptide), showed a progressive increase [monophasic: 0.93 +/- 0.04 (dimensionless); 4/5-phases: 1.35 +/- 0.14], and it showed properties typical of beta-cell function indices. We also found that the type of glucose shape is often associated to similar insulin and C-peptide shape. In conclusion, OGTT curves can be characterized by high variability, and complex OGTT shape is associated with better glucose tolerance. WHOSHCP (WHole-Ogtt-SHapeindex) may be a powerful index of beta-cell function much simpler than model-based indices.