Roux-en-Y Gastric bypass surgery is an operation in which we create a small pouch of the stomach to reduce the amount of food intake and we bypass an initial segment of the small intestine (duodenum which is the first part of the small intestine with some part of the second part of the small intestine termed as jejunum and connect the lower segment of the small intestine directly to the small stomach pouch. By this arrangement there is delay in mixing of ingested food and the digestive enzymes.
The net effect of this re-arrangement is a restricted intake of food and due to the bypass, some malabsorption- both of these leading to weight loss. However it is not only the restriction and malabsorption that leads to weight loss and correction of the co-morbidities in obese people. The intestines in our body are a source of large number of hormones, which are secreted in increased amounts when exposed to undigested food. These hormones modify the metabolism in our body such that diabetes is either resolved or controlled. Similarly blood pressure and a number of other problems are corrected due to this hormonal mechanisms. Some of these hormones also increase our resting metabolic rate such that we consume more energy even at rest.