Our understanding of obesity and weight loss surgery mechanisms of action has rapidly evolved over the past few years. From the naïve assumptions of restriction and malabsorption to the discovery of complex gut centered neuro-hormonal signaling pathways, bariatric surgery has swiftly evolved into the field of metabolic surgery. The dramatic metabolic responses secondary to surgical manipulation of the gastro-intestinal system were difficult to ignore. The failure of purely restrictive procedures, like adjustable gastric banding, to improve blood sugar and insure durable weight loss is now solid evidence. Unfortunately, the demise of such procedures has taken many years. Many patients were lured into a safe and simple procedure only to end up with disappointment, failure and frustration. Massive esophageal dilations, hiatal hernias, aspiration pneumonias, esophageal dysmotility, band erosion and slippage are still common occurrences among many gastric band patients in Houston.
The lessons we have learned from adjustable gastric banding are numerous. First, an obese patient cannot be forced to loose weight. Second, creating a restriction or obstruction or narrowing at he gastro-esophageal junction does not lead to weight loss but to esophageal motility loss. Third, applying pressure over the esophageal fat pad area does not control appetite. Fourth, vomiting and starvation do not lead to permanent weight loss.
Beyond mechanical restriction, we have noticed a rapid blood sugar improvement following gastric bypass and gastric sleeve. Type 2 Diabetes resolution is shortly becoming a primary indication for metabolic surgery. There are now 11 randomized controlled studies favoring surgery over medical treatment of type 2 diabetes. Metabolic surgery is no longer limited to weight reduction. Rather, gastric sleeve or bypass surgery will soon be acknowledged as first line treatment for type 2 diabetic patients including those with BMI lower than 35.
The definition of type 2 Diabetes is changing from insulin resistance due to fat accumulation in muscle and adipose tissue and pancreatic failure to increase insulin secretion, to a gastro-intestinal disorder. Otherwise, how can one explain the improvement in blood sugar following gastric bypass or gastric sleeve surgery before any significant weight loss? It is safe to assume that diabetes starts in the gut. At this early stage, blood sugar is still within normal but GLP-1 secretion is diminished. Metabolic surgery restores GLP-1 secretion independent of weight loss. In fact, postoperative increase in GLP-1 levels contributes to weight loss. Why and how do GLP-1 and other neuro-hormone secretion change after metabolic surgery remain to be elucidated. It is interesting to note that only the post-prandial GLP-1 level changes. Fasting levels remain the same before and after surgery. Hence, we conclude that metabolic surgery changes the interaction between the gut and ingested food. The nature of this interaction is not well understood. One aspect of this interaction relates to gastric emptying. The stomach is a complex endocrine organ. It controls intestinal motility and function by various neuronal and hormonal pathways. It analyzes the content of ingested food and sends a number of signals to the brain, liver, pancreas and intestine. Squirts of ingested food are delivered to the duodenum in a coordinated fashion to stimulate GLP-1 among other neuro-hormones to signal the pancreas, liver and other organs to prepare for the influx of nutrients across the intestinal wall. Gastric emptying is a complex process to say the least. Both gastric bypass and gastric sleeve alter gastric emptying. Both procedures either bypass or eliminate the gastric fundus that plays an important role in gastric emptying. Interestingly, the gastric fundus secretes both Ghrelin and Leptin two important metabolic hormones. Understanding the mechanism of action of metabolic procedure boils down to figuring out the interaction between gastric fundus and ingested food. If the interaction results in the appropriate neuro-hormonal secretion, the body is able to handle absorbed nutrients like glucose. If the interaction is disrupted, several downstream issues arise leading disrupted insulin secretion and function. The end result is weight gain and impaired glycemic control leading to overt type 2 Diabetes.
From this perspective, type 2 Diabetes becomes a gastro-intestinal disorder. Metabolic surgery effectively targets the root cause of type 2 Diabetes. Future investigations will unravel these mechanisms and allow us to develop less invasive surgeries like gastric fundus invagination and possibly more effective medications for treatment of diabetes and obesity.