There are two basic types of weight loss surgery: restrictive surgeries and malabsorptive/restrictive surgeries. They help with weight loss in different ways.
Restrictive surgeries work by shrinking the size of the stomach and slowing down digestion.
A normal stomach can hold about 3 pints of food. After surgery, the stomach may at first hold as little as an ounce, although later that could stretch to 2 or 3 ounces. The smaller the stomach, the less you can eat. The less you eat, the more weight you lose.
Malabsorptive/restrictive surgeries change how you take in food. They give you a smaller stomach and also remove or bypass part of your digestive tract, which makes it harder for your body to absorb calories.
Doctors don't do purely malabsorptive surgeries -- also called intestinal bypasses -- anymore because of the side effects.
How it works: The surgeon uses an inflatable band to squeeze the stomach into two sections: a smaller upper pouch and a larger lower section. The two sections are still connected by a very small channel, which slows down the emptying of the upper pouch. Most people can only eat a 1/2 to 1 cup of food before feeling too full or sick. The food also needs to be soft or well-chewed.