Thus, aims of our study are: 1) to determine serum apelin levels in a large cohort of Italian subjects with T2D, T1D and in non-diabetic controls; 2) to identify putative metabolic determinants of modified apelin concentrations, in order to explore possible mechanism of apelin control; 3) to explore changes in apelin levels in response to sharp modifications of glucose/insulin metabolism in T2D obese subjects before and 3 days after bariatric surgery. Here, APLN is linked to type 2 diabetes mellitus.