The American Society for Metabolic & Bariatric Surgery  describes three approaches that weight loss surgery takes to achieve change: 

• Restrictive procedures that decrease food intake such as the lap band or the sleeve gastrectomy. 
• Malabsorptive procedures that alter digestion, thus causing the food to be poorly digested and incompletely absorbed so that it is eliminated in the stool. These procedures are not being used as a single operation and are combined with a restrictive operation for better results and less complications. 
• Combined procedures that use both approaches to achieve weight loss such as the gastric bypass.

Combined Restrictive & Malabsorptive Procedure – Gastric Bypass Roux-en-Y

In recent years, better clinical understanding of procedures combining restrictive and malabsorptive approaches has increased the choices of effective weight loss surgery for thousands of patients. By adding malabsorption, food is delayed in mixing with bile and pancreatic juices that aid in the absorption of nutrients. The result is an early sense of fullness, combined with a sense of satisfaction that reduces the desire to eat.

According to the American Society for Metabolic & Bariatric Surgery and the National Institutes of Health, Roux-en-Y gastric bypass is the most frequently performed weight loss surgery in the United States. In this procedure, stapling creates a small (15 to 20cc) stomach pouch. The remainder of the stomach is not removed, but is completely stapled shut and divided from the stomach pouch. The outlet from this newly formed pouch empties directly into the lower portion of the jejunum, thus bypassing calorie absorption. This is done by dividing the small intestine just beyond the duodenum for the purpose of bringing it up and constructing a connection with the newly formed stomach pouch. The other end is connected into the side of the Roux limb of the intestine creating the "Y" shape that gives the technique its name. The length of either segment of the intestine can be increased to produce lower or higher levels of malabsorption.

Advantages 

• The average excess weight loss after the Roux-en-Y procedure is generally higher in a compliant patient than with purely restrictive procedures.
• One year after surgery, weight loss can average 77% of excess body weight. 
• Studies show that after 10 to 14 years, 50-60% of excess body weight loss has been maintained by some patients. 
• A study done in 2000 of 500 patients showed that 96% of certain associated health conditions studied (back pain, sleep apnea, high blood pressure, diabetes and depression) were improved or resolved.

Risks

• Because the duodenum is bypassed, poor absorption of iron and calcium can result in the lowering of total body iron and a predisposition to iron deficiency anemia. This is a particular concern for patients who experience chronic blood loss during excessive menstrual flow or bleeding hemorrhoids. Women, already at risk for osteoporosis that can occur after menopause, should be aware of the potential for heightened bone calcium loss. 
• Bypassing the duodenum has caused metabolic bone disease in some patients, resulting in bone pain, loss of height, humped back and fractures of the ribs and hip bones. All of the deficiencies mentioned above, however, can be managed through proper diet and vitamin supplements. 
• A chronic anemia due to Vitamin B12 deficiency may occur. The problem can usually be managed with Vitamin B12 pills or injections. 
• A condition known as "dumping syndrome" can occur as the result of rapid emptying of stomach contents into the small intestine. This is sometimes triggered when too much sugar or large amounts of food are consumed. While generally not considered to be a serious risk to your health, the results can be extremely unpleasant and can include nausea, weakness, sweating, faintness and, on occasion, diarrhea after eating. Some patients are unable to eat any form of sweets after surgery. 
• In some cases, the effectiveness of the procedure may be reduced if the stomach pouch is stretched and/or if it is initially left larger than 15-30cc. 
• The bypassed portion of the stomach, duodenum and segments of the small intestine cannot be easily visualized using X-ray or endoscopy if problems such as ulcers, bleeding or malignancy should occur. 
• Higher chance for gallstones

Laparoscopic Adjustable Gastric Banding

A Laparoscopic Adjustable Gastric Band procedure is a purely restrictive surgical procedure in which a band is placed around the upper most part of the stomach. This band divides the stomach into two portions, one small and one larger portion. Because food is regulated, most patients feel full faster. Food digestion occurs through the normal digestive process.

Advantages

• Restricts the amount of food that can be consumed at a meal. 
• Food consumed passes through the digestive tract in the usual order allowing it to be fully absorbed into the body. 
• In multiple studies involving over 3,000 patients, excess weight loss ranged from 28-87%, with a minimum of 2 year postoperative follow-up. 
• Band can be adjusted to increase or decrease restriction. 
• Surgery can be reversed.

Risks

• Gastric perforation or tearing in the stomach wall may require additional operation. 
• Access port leakage or twisting may require additional operation. 
• May not provide the necessary feeling of satisfaction that one has had enough to eat. 
• Nausea and vomiting. 
• Outlet obstruction. 
• Pouch dilatation. 
• Band migration/slippage. 

Laparoscopic Sleeve Gastrectomy

A Laparoscopic Sleeve Gastrectomy is also a restrictive surgical procedure like the gastric band. Instead of placing a device around the stomach, 60 to 80 percent of the stomach is removed. The part of the stomach that remains is closed with permanent staples. This creates a smaller stomach just like the band. Most people feel full after just a few bites and may feel less hungry the rest of the time. The portion of the stomach that is removed helps make a chemical hormone that makes people feel hungry. With that portion removed, there is less of the hormone in the body so you will feel less hungry throughout the day.

Advantages

• Restricts the amount of food that can be consumed at a meal. 
• Less hunger throughout the day
• Food consumed passes through the digestive tract in the usual order allowing it to be fully absorbed into the body.
• Weight loss can be achieved within 12-18 months.

Risks

• Not a reversible procedure.
• If you eat too much, the effectiveness of the procedure may be reduced if the stomach is stretched.
• The new stomach can become too narrow. Rarely, another surgery may be needed to widen the sleeve.