Laparoscopic surgery for obesity:
Theories of weight reduction and reversal of diabetes - The success of gastric bypass surgery to produce weight loss are amazing but how does it produce its effects?
The short answer is that nobody is yet sure exactly how the effects are produced, but several theories are being pursued:
Reduction in capacity of stomach - simpistically put it means less eaten at each sitting adds up to fewer calories over the day. This is a physical effect and does not explain the reversal of diabetes which can occur within days of the surgery and certainly long before significant weight loss occurs.
Loss of absorbtive length of the gut - malabsorption will certainly lead to weight loss in the presence of normal calorie intake. If the duodenum and first part of the jejunum are included in the food pathway, reversal of diabetes doe not appear to be an early event and only occurs when the total weight loss is advanced.This appears to reduce the sensation of hunger by manipulating levels of GLP-1 and PYY in the person.
Hormonal changes - The theory behind the success of this procedure seems to be the changes in the levels of hormones produced by the elimination of the duodenum from the pathway taken by food. Indeed, simply bypassing the duodenum without any change in stomach size or intoduction of malabsorption has been shown to reverse Type 2 diabetes in a very short time without consequent weight reduction.
Ghrelin - This hormone is produced by the stomach and its levels rise with starvation. Reduction of its levels is associated with lowering of hunger signals in the brain. Its levels fall after eating to signal cessation of feeding. Manipulating these levels cause the person to feel less hungry and satiated with less food.
GLP-1 - Glucagon like peptide is a hormone produced by the small bowel. It has an extremely short life in the body and many effects:
It stimulates insulin secretion by the pancreas.
It increases beta cell mass and function in the pancreas.
It increases satiety.
PYY - Peptide YY is produced by cells in the terminal ileum and colon. Artificially increasing the levels of PYY in obese subjects has been demonstrated to reduce appetite and weight.
In short it would appear that the effects of bypass surgery depend wholly, or to a great degree, on the manipulation of these three hormones in the obese patient.
The reversal of diabetes seems particularly associated with changes in GLP-1 levels.
Physicians may treat diabetes, but surgeons can cure it!