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Gastric bypass surgery is the most commonly performed type of weight loss surgery (bariatric surgery) in the United States. This surgery alters the patient’s digestive anatomy to restrict the amount of food they are physically able to eat at one time. This promotes weight loss by causing the patient to feel full more quickly. The procedure also involves a bypass of part of the stomach and part of the small intestine that results in fewer calories being absorbed by the body.

Physicians generally encourage patients to lose weight through dietary modifications and lifestyle changes, including regular exercise. There are also a few medications that promote short-term weight loss. Bariatric surgery is considered only when these interventions prove insufficient to bring a severely obese patient’s weight to a healthy level. This type of surgery typically is performed on patients with a body mass index (BMI) of 40 or more. BMI is a measure of a person’s height and weight, and a BMI of 40 represents a man who is about 100 pounds overweight, or a woman who is about 80 pounds overweight. A normal BMI is somewhere between 18.5 and 25.

The digestive process begins as soon as a person swallows food. Once the food enters the digestive tract, digestive juices and enzymes digest food and help with the absorption of calories and nutrients. The food moves down the esophagus and into the stomach, where acid continues to digest the food. In the average person, the stomach can hold about 32 ounces (946 milliliters) of food at one time.

As this food is digested, it moves to the first part of the small intestine (known as the duodenum), where bile and pancreatic juices facilitate digestion and absorption. Most of the remaining calories and nutrients are absorbed in the final two segments of the small intestine (jejunum and ileum). Any remaining food moves into the large intestine before it is eliminated from the body.

Bariatric surgery techniques promote weight loss in one of two ways: either by restricting food intake or by interrupting the digestive process. Some procedures (e.g., gastric bypass) combine stomach restriction with partial bypass of the small intestine. Gastric bypass is often the preferred form of weight loss surgery because it is considered to be safer and to have fewer complications than other alternatives.

In this procedure, also known as Roux-en-Y gastric bypass (RGB), the surgeon first creates a small stomach pouch that serves to restrict food intake. A Y-shaped section of the small intestine is attached to the pouch. This allows food to bypass the lower stomach, the duodenum and the first portion of the jejunum, reducing the amount of calories and nutrients the body absorbs.

Gastric bypass can be performed as either traditional open surgery or as less invasive laparoscopy. In the latter technique, a small, tubular instrument called a laparoscope is inserted through a short incision in the abdomen. The laparoscope has a camera at the end that allows the physician to view the interior of the body. Surgical instruments are then passed through other short incisions to complete the surgery.

Patients who undergo the laparoscopic technique usually require a shorter hospital stay than those who undergo open surgery. In addition, laparoscopy usually requires a shorter recovery period, and presents a lesser risk of wound-related complications. However, people who weigh more than 350 pounds (158 kilograms) or have a history of abdominal surgery may not be good candidates for laparoscopy.

Only certain people are considered to be good candidates for gastric bypass surgery, such as those with a BMI of 40 or greater. Physicians may also permit gastric bypass for patients with a BMI of 35 to 39.9 who also have weight-related health problems such as diabetes, heart disease, severe sleep apnea (cessation of breathing during sleep) or high blood pressure (hypertension).

A team of health professionals (e.g., physicians, surgeons, dietitians, psychologists) will work together to help determine if a patient is a good candidate for bariatric surgery. Factors to be considered include potential benefits to a patient’s health, potential risks involved in the surgery and whether or not the patient can comply with the lifestyle changes necessary to ensure that the weight stays off following surgery.

The evaluation will include a thorough physical examination. The patient's medical history will be obtained, including a current list of medications and supplements. Blood tests and imaging tests (e.g., EKG, chest x-ray, upper gastrointestinal x-ray, gallbladder ultrasound) may also be performed.

Patients with certain preexisting conditions (e.g., alcoholism, hepatic cirrhosis with impaired liver function) may not be good candidates. Individuals with correctable hormonal causes of obesity may also not qualify.

A psychologist may help determine if a patient is able and willing to make the dietary and lifestyle adjustments necessary to ensure that the weight stays off, and to reduce the risk of complications. Patients with certain serious psychiatric conditions may be ruled out.