Diabesity is a term often used to indicate the association of type-2 diabetes mellitus (T2DM) with obesity; the prevalence of both conditions is rapidly increasing worldwide and has reached epidemic proportions. Insulin resistance represents the major determinant of T2DM, which becomes manifest once relative β-cell failure ensues and insulin secretion is no longer sufficient to compensate for insulin resistance. In recent years, gastrointestinal surgery has emerged as the most effective option for the treatment of obesity and diabetes, with level-1 evidence of diabetes remission. Restrictive gastric operations such as gastric banding can improve insulin resistance in proportion to weight loss, while gastrointestinal bypass procedures, such as roux-en-y gastric-bypass (RYGB) and biliopancreatic diversion (BPD), can improve glucose homeostasis even before a significant weight loss is reached, suggesting weight-independent mechanisms of action. Studies comparing RYGB to BPD show that RYGB primarily enhances insulin secretion and reduces hepatic glucose output, whereas BPD rapidly improves glycemia primarily through the normalization of insulin sensitivity. Given the fact that BPD involves a significantly longer bypass of the proximal intestine than RYGB, these data suggest that the exclusion of a greater length of small bowel from the transit of food may cause broader and more profound influence on insulin sensitivity.