Understanding Weight Loss Surgery
Bariatric, or weight loss surgery, works by disrupting some of the biological mechanisms that keep us fat. These operations traditionally limit the size of the stomach (being ‘restrictive’) or bypass a length of the intestine (being ‘malabsorptive’) - or a mix of the two. This by no means explains all the weight loss mechanisms associated with these procedures and there is a lot of active research being undertaken to increase our understanding in this area.
The sleeve gastrectomy reduces the size of the stomach which means patients are only able to tolerate a small portion in one meal sitting. Some of the weight loss associated with sleeve gastrectomy has been attributed to removing the part of the stomach that produces the hunger hormone Ghrelin, and an increased transit of the meal into the small intestine. These mechanisms lead to changes in the many hormones involved in hunger and satiety and have complex interactions with other hormones in the system.
Single-anastomosis Gastric Bypass
The single-anastomosis gastric bypass Bypass and Roux-en-Y gastric bypass (RYGB) reduce the stomach size and make a direct join between the new small stomach pouch and intestine, excluding the upper intestine from the gastrointestinal circuit. This means that nutrients cannot be absorbed by the first part of the small intestine. Bypass operations also cause an increase in the release of the hormone GLP-1 in the lower intestine, which increases insulin secretion after meals, decreases gastric-emptying times and acts on the brain to reduce hunger. Once again emerging research indicates there are many more complex hormonal mechanisms at work.
The most obvious signs of this are the metabolic effects of weight loss surgery within days to weeks post-surgery - even before substantial weight loss has been achieved. These early effects have been noted to improve blood sugar control, resolvere obstructive sleep apnoea and fatty liver.
Research is examining how weight loss surgery leads to a change in food preferences, increases energy expenditure, alters bile acid circulation, changes the gut microbiome and effects signaling of the vagus nerve.