Gastrointestinal weight-loss surgery, also known as Gastric Bypass surgery refers to surgical processes in which the stomach is divided and reduced (usually by more than 90%), and then the small intestine is rearranged to connect to the new, smaller, stomach 'pouch'.

It's normally viewed as a 'last-resort' method to combat severe obesity. It's very effective at weight reduction. and has been found to reduce obesity-related mortality by around 40%.

Variants of the procedure are now performed in hundreds of thousands of patients, worldwide, each year.

Since its development in the last decades of the 20th century, it was initially assumed that its effectiveness was due to a simply 'mechanical' effect which prevented excess eating and lowered digestion rates. This is now known to be an oversimplified viewpoint.

The way in which gastrointestinal weight-loss surgery leads to such significant weight-loss is currently regarded as an unsolved medical problem.

Gastrointestinal weight-loss surgery (GIWLS) is currently the most effective treatment for severe obesity, with Roux-en-Y gastric bypass (RYGB) among the best of the available surgical options. Despite its widespread clinical use, the mechanisms by which RYGB induces its profound weight loss remain largely unknown. This procedure effects weight loss by altering the physiology of weight regulation and eating behavior rather than by simple mechanical restriction and/or malabsorption as previously thought.

Source : Obesity Journal (Silver Spring). 2009 Oct; 17(10): 1839–1847.

Also see : Appetite Regulationplugin-autotooltip__plain plugin-autotooltip_bigAppetite Regulation

Very extensive research, over many decades, has identified a number of biochemical routes that can influence appetite in humans.

For an overview, see: Mechanisms of Appetite Regulation Journal of Pediatric Gastroenterology and Nutrition 51():p S123-S124,

In particular, the so called 'hunger hormones'