Prospective Randomized Controlled Trial on the Effect of Gastric Bypass and Biliopancreatic Diversion on Type 2 Diabetes Mellitus in Patients With BMI > 35 vs. Medical Therapy

ß-cell dysfunction and insulin resistance are the main pathophysiological defects responsible for the development of hyperglycaemia [1]. Both these defects predict incident diabetes in high-risk subjects [2]. Insulin resistance per se is not sufficient to cause hyperglycaemia; mild degrees of ß-cell dysfunction, on the other hand, may not result in diabetic hyperglycaemia in insulin sensitive individuals. It is only when impaired ß-cell function occurs in the background of insulin resistance that plasma glucose levels begin to rise (as is the case of individuals with impaired glucose tolerance [3]). The occurrence of postprandial, or day-long, hyperglycaemia further compromises both ß-cell function and insulin action, a phenomenon called glucose toxicity [4-6]. As a consequence, the vast majority of patients with established type 2 diabetes present, in addition to marked insulin resistance, a clear defect in ß-cell function, which is generally proportional to the severity of the hyperglycaemia [7]. Of note is that the extent of ß-cell dysfunction in type 2 patients may be misjudged when ß-cell function is inferred from simple measurements of fasting or postprandial plasma insulin concentrations. In fact, insulin secretion increases (in non-linear manner [8]) in insulin resistant individuals, a compensatory response aimed at maintaining glucose tolerance. As a consequence, the absolute insulin hypersecretion (particularly in the fasting state) commonly found in patients with IGT or diabetes masks the underlying defect in the ability of the ß-cell to cope with nutrient stimulation.

It is generally held that ß-cell function is irreversibly lost already at the time the disease manifests itself and thereafter continues to decline linearly with time. Several studies, however, have documented the possibility that ß-cell function may be restored, at least partially, in type 2 diabetes [9-13]. Of major relevance to the issue of ß-cell recovery in diabetes are the following findings: (a) bariatric surgery in morbidly obese patients with type 2 diabetes can restore euglycaemia, the acute insulin response to glucose [14-17] and insulin sensitivity [18,19]; (b) recent studies have reported that diabetic subjects return to euglycaemia and normal insulin levels within days after surgery, long before a significant weight loss has occurred [20]; and (c) whereas RYGB improves insulin sensitivity in proportion to weight loss, BPD improves insulin action out of proportion to weight loss, i.e., it normalises it at a time when patients are still markedly obese [21]. Because RYGB is a predominantly restrictive procedure involving the foregut whereas BPD is a predominantly malabsorptive procedure involving the distal GI tract, these findings suggest that the control of both insulin action and ß-cell function is influenced by signals originating from the GI tract.

Some studies have investigated the hormonal changes that follow bariatric surgery. In most cases, however, clinical testing was performed after significant weight reduction, thereby making it difficult to establish whether any observed hormonal effect was the cause or the consequence of weight loss and diabetes resolution. Recently, it has been reported that RYGB induces rapid normalisation of blood glucose and insulin levels in concomitance with significant changes of the levels of hormones involved in the regulation of glucose metabolism (ACTH, leptin and GIP) in the early postoperative period [22]. It has been proposed that the incretins could be one of the key mediators of the anti-diabetic effects of certain types of bariatric surgery. Previous data have shown that the significant weight loss observed after various bariatric procedures was accompanied by improvement of diabetes control and increased GLP-1 levels. However, most studies were cross sectional [23,24], reported fasting [25] rather than post-prandial GLP-1 levels, and compared various types of surgery such as jejuno-ileal bypass (JIB) [26,27] or bilio-pancreatic diversion (BPD) [27], often leading to inconclusive results. Data on fasting GIP levels after bariatric surgery are inconsistent, reporting either a decrease [25,28,29] or an increase [23,24]. GLP-1 levels increase after a meal in patients after RY-GBP [30] or with oral glucose after BPD [30]. Meal-stimulated GIP levels have been reported to increase after JIB [23], or to decrease after GBP, JIB or BPD surgery [26,29,31,32]. None of these studies, however, measured GLP-1 and GIP simultaneously, reported the incretin levels and effect on insulin secretion (with the exception of the last quoted one, which reported both GIP and insulin response to meal markedly reduced after BPD), or was done in diabetic patients.

Some authors have suggested that an enhanced release of GLP-1, triggered by the earlier presentation of undigested food to lower segments of the bowel, might be involved in the glycaemic improvement consequent to bypass procedures for obesity surgery.

Collectively, these observations clearly suggest that there is a large margin for ß-cell recovery of function in type 2 diabetes and that different segments of the gut participate differentially in such recovery.