 In this study, the investigators hypothesized that significant proportion of patients with coronary artery disease CAD has reduced capacity of glucagon-like peptide-1 GLP-1 secretion, which is detectable as blunted response of plasma active GLP-1 level to oral glucose loading and that reduced GLP-1 secretory function is associated with increased severity of coronary artery stenosis but not with classic risk factors for CAD. To test this hypothesis, the investigators will analyze correlation between GLP-1 secretory capacity and severity of coronary artery stenosis determined by Gensini Score GS, an established score system for coronary artery stenoses. Additionally, the investigators will analyze relationship between level of total GLP-1 and severity of coronary artery stenosis to determine how it is different from the active GLP-1 - coronary stenosis relationship. 