Background and aims: the incretin hormones GIP and GLP-1 exert a fundamental role in glucose homeostasis after a meal, but it’s still debated if modifications in their secretion can affect the metabolic state, as well as their relative contribution to the dynamics of incretin-induced insulin secretion. In this study we aimed to explore the relative contribution of GIP and GLP-1 secretion and GIP/GLP-1 secretion ratio (GIP/GLP-1 SR) on glucose metabolism and beta-cell function in a cohort of non-diabetic and newly diagnosed diabetic subjects. Methods: 41 subjects with no previous diagnosis of T2DM and not on antidiabetic treatments were divided into 3 groups according to glucose tolerance calculated during an oral glucose tolerance test (OGTT): NGT (n=12), IGT (n=12), DM (n=17). All subjects underwent a 4h mixed meal test (MMT) and glucose, insulin, c-peptide, GIP and GLP-1 levels were assessed every 30 min. Further, we calculated the GIP/GLP-1 SR for each timepoint normalized for basal levels and the AUC of the ratio of GIP and GLP-1 secretion. All subjects also underwent a euglycemic hyperinsulinemic clamp to measure whole-body peripheral glucose utilization (M-value), an index of insulin sensitivity. Parameters of beta-cell function were obtained by mathematical modeling by c-peptide deconvolution. Results: during MMT we observed higher glucose levels in DM subjects compared to NGT and IGT (p<0.01), while insulin and c-peptide secretion showed a non-significant decreasing trend in DM subjects compared to the other 2 groups. We also found a trend of decreasing GIP secretion in IGT and DM subjects compared to NGT, even if not statistically significant, whereas no differences in GLP-1 secretion were detected among the 3 groups. Interestingly, IGT and DM groups showed a reduced GIP/GLP-1 SR compared to NGT. Further, we found that the AUC of GIP/GLP-1 SR was positively correlated with the M-value (r=0.45, p=0.01) Conclusions: our data show that, rather than an altered secretion of the incretin hormones per se, insulin resistance leads to an imbalance in the dynamics of incretin secretion overall after a meal. In particular, the GIP/GLP-1 SR appears to be an early marker of impaired glucose homeostasis in the time course towards type 2 diabetes. Further studies are needed to investigate whether the altered GIP/GLP-1 secretion is the cause or the effect of altered beta-cell incretin sensitivity and the potential metabolic effects of unbalanced incretin secretion.