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The gastric bypass reduces the size of the stomach by well over 90%. A normal stomach can stretch, sometimes to over 1000 ml, while the pouch of the gastric bypass may be 15 ml in size. The Gastric Bypass pouch is usually formed from the part of the stomach which is least susceptible to stretching. That, and its small original size, prevents any significant long-term change in pouch volume. What does change, over time, is the size of the connection between stomach and bowel, and the ability of the small bowel to hold a greater volume of food. Over time, the functional capacity of the pouch increases; by that time, weight loss has occurred, and the increased capacity serves to allow maintenance of a lower body weight.

When the patient ingests just a small amount of food, the first response is a stretching of the wall of the stomach pouch, stimulating nerves which tell the brain that the stomach is full. The patient feels a sensation of fullness, as if they had just eaten a large meal — but with just a thumbful of food. Most people do not stop eating simply in response to a feeling of fullness, but the patient rapidly learns that subsequent bites must be eaten very slowly and carefully, to avoid increasing discomfort, or even vomiting.

Normally when we eat, food passes out of the stomach into the duodenum after about 30 minutes. When it reaches the lower end of the duodenum, a new hormonal message is generated, telling the brain that enough food has been eaten. The person with a normal GI tract experiences this hormone release as a sense of satisfaction or “satiety” — a feeling of indifference toward eating any more. Recently, a hormone called ghrelin has been discovered, which may contribute to this effect.

To gain the maximum benefit from this physiology, it is important that the patient eat only at mealtimes, 2 to 3 small meals daily, and avoid snacks and grazing between meals, which can effectively “bypass the bypass”. This requires a change in eating behavior, and alteration of long-acquired habits for finding food. In almost every case where weight gain occurs late after surgery, capacity for a meal has not greatly increased. The cause of regaining weight is eating between meals, usually high-caloric snack foods. There is no known operation which can completely counteract the adverse effects of destructive eating behavior.