During the 1950s an operation, designed solely for the purpose of weight loss, was developed at the University of Minnesota. This operation, known as the Jejunoileal bypass, was designed to bypass much of the small intestine and thus prevent the absorption of calories. Unfortunately, although good weight loss was observed, the majority of patients suffered sever complications and the majority of these early operations had to be reversed.

In the 1960s Dr Mason and Dr Ito together developed what has become know as the Roux-en-Y gastric bypass operation after observing weight loss in patients who were suffering from ulcers and for whom treatment involved the partial removal of the stomach.

Today Roux-en-Y gastric bypass surgery is the most widely performed weight loss operation in the United States and, although other forms of surgery are rapidly gaining in popularity, the fact that so many surgeons and familiar with, and skilled in, Roux-en-Y gastric bypass surgery means that it remains the top choice for many patients. In 2005 approximately 140,000 Roux-en-Y gastric bypass surgeries were performed in the United States.

The Roux-en-Y is a form of combination surgery which is designed to both physically reduce the amount of food that can be eaten and then to reduce the number of calories which the body can absorb from food as it passes through the body.

In essence gastric bypass surgery starts with the creation of a small pouch at the top of the stomach which restricts the amount of food that can be eaten. Then the gastrointestinal tract is reconstructed to enable food to pass out of both the newly created pouch and the remaining bulk of the stomach. The manner in which reconstruction is affected varies and gives rise to two main forms of Roux-en-Y gastric bypass surgery.

The most commonly used technique is known as a Proximal Roux-en-Y. In this form of the operation the small bowel is divided about 18 inches below the main stomach outlet and about 30 to 60 inches of small intestine is used to connect the new stomach pouch to the small intestine. In this form of gastric bypass surgery much of the small intestine remains intact and, while this still allows for reasonable absorption of calories, it lessens the risk of nutritional problems resulting from a low uptake of various essential vitamins and minerals.

A less common, but still widely used, form of gastric bypass surgery is the Distal Roux-en-Y. This is essentially the same as the proximal form of the operation except that connection from the small pouch is moved further down the gastrointestinal tract effectively bypassing a greater length of the small intestine. The benefit here is that the body is not able to absorb as many calories from food as it passes through the intestine which leads to greater, or faster, weight loss. The trade-off however is that there is also a reduction in the adsorption of essential vitamins and minerals which, although manageable post-operatively, can lead to additional complications.