Introduction: This article reviews the possible mechanisms behind the beneficial effects of bariatric surgery on glycemic control as well as obesity and obesity-related morbidity and mortality.
Methods: The authors reviewed two of their own meta-analyses of 621 studies published before April 2006 plus significant studies on this topic published more recently.
Results: In the two meta-analyses, complete resolution of type 2 diabetes occurred in 78.1% of patients and diabetes improvement or resolution occurred in 86.6% of patients. Data from the literature review suggested that bariatric surgery reduces contact between food and the foregut; decreases insulin levels, glycosylated hemoglobin A1C and fasting blood glucose levels; and reduces mortality and the risk for comorbidities of severe obesity (eg, asthma, gastroesophageal reflux, hypertension, stress incontinence). The beneficial effects on glycemic control were found within days of surgery (before significant weight loss occurred) suggesting that bariatric surgery changes the signaling mechanism of the gut with pancreatic islet cells, muscles, fat, the liver, and other organs.
Conclusion: While the various factors involved in resolution of type 2 diabetes following bariatric surgery is still poorly understood, the beneficial effects of surgery on glycemic control occur before weight loss suggesting other mechanisms of action.
Commentary by Louis Kuritzky MD
There is no longer any room for doubt that bariatric surgery provides remission of type 2 diabetes (T2DM) in as many as two-thirds of patients, and that such remission is durable (mean time to relapse = 8.3 years).1 In the meantime, not only are glucose levels improved, but—a claim that no other intervention for diabetes can make—even MORTALITY is improved! Wilson and Pories performed two meta-analyses of bariatric surgery for T2DM and came away with the conclusion that T2DM resolves or improves in most patients following surgery, and that although responses are most marked in those with the greatest weight loss, the dramatic reversal of T2DM occurs with such rapidity that it cannot be readily ascribed to weight loss alone.2
In a 16-year follow up of more than 600 T2DM patients who underwent gastric bypass, resolution of T2DM was seen in 83%; resolution of impaired glucose tolerance was seen in 99%!3 A critical ingredient in success of bariatric surgery may reside in the impact upon glucagon-like peptide (GLP-1). It is currently recognized that T2DM patients have deficient production of GLP-1. GLP-1 agonists (exenatide, liraglutide) have found a valuable niche in treatment due to their efficacy, tolerability, and associated weight loss—usually a welcome “side effect.” Indeed, whether due to weight loss or some other direct GLP-1 effect, cardiovascular risk factors improve in persons treated with GLP-1 agonists.
Bariatric surgery also has been associated with remarkable changes in GLP-1. For instance, in a trial performed 1 month postoperatively, GLP-1 levels rose five to six times higher than preoperative levels.4 In a control group matched for weight loss through diet, similarly positive GLP-1 changes were NOT seen, hence the conclusion that GLP-1 changes must be specifically due to the surgical intervention rather than the weight loss itself.
Improved GLP-1 levels may be a critical ingredient in the metabolic successes seen after bariatric surgery.