In order to gain further insights into the role contributed by incretins in the pathophysiology of pre-DM and T2DM, our study aimed to analyze (1) the fasting GLP-1 levels and GLP-1 responses to oral glucose challenge test (OGTT) in various glucose tolerance states (NGT, pre-DM, and T2DM) and (2) the association between GLP-1 levels and insulin resistance (IR) and insulin sensitivity (IS) states among the three major ethnic groups. This evidence concerns the gene GCG and type 2 diabetes mellitus.