Gastric bypass procedures (GBP) are any of a group of similar operations used to treat morbid obesity—the severe accumulation of excess weight as fatty tissue—and the health problems (comorbidities) it causes. Bariatric surgery is the term encompassing all of the surgical treatments for morbid obesity, not just gastric bypasses, which make up only one class of such operations.

A gastric bypass first divides the stomach into a small upper pouch and a much larger, lower "remnant" pouch and then re-arranges the small intestine to allow both pouches to stay connected to it. Surgeons have developed several different ways to reconnect the intestine, thus leading to several different GBP names. Any GBP leads to a marked reduction in the functional volume of the stomach, accompanied by an altered physiological and psychological response to food. The resulting weight loss, typically dramatic, markedly reduces comorbidities. The long-term mortality rate of gastric bypass patients has been shown to be reduced by up to 40%; however, complications are common and surgery-related death occurs within one month in 2% of patients.

Comorbid conditions

Life-threatening health problems arise from obesity as a consequence of its mechanical or metabolic effects. These comorbidities may in turn lead to severe deterioration of health, shortened life expectancy, and lower quality of life.

Major comorbidities include:

• Atherosclerotic cardiovascular disease. Obesity is not only associated with the occurrence of hypercholesterolemia and hypertriglyceridemia, but it is also a factor in the occurrence of atherosclerosis, the deposition of fats within the walls of the blood vessels. This leads to conditions such as coronary artery disease, congestive heart failure, and "hardening of the arteries." This group of conditions is a leading cause of death in the United States.
• Diabetes mellitus type 2 occurs mostly in middle and old age, but it is up to 40 times more likely in those who are severely overweight. It is associated with ASCVD, kidney failure, blindness, nerve damage, and amputations of the extremities, and is also a leading overall cause of death in the United States. Dysmetabolic Syndrome X, a pre-diabetic condition often associated with obesity, is accompanied by elevated levels of insulin in the blood and a high incidence of early development of coronary heart disease.
• Essential hypertension or "high blood pressure", is much more common in obese individuals. It can lead to early development of ASCVD, as well as to kidney disease. Weight loss is considered to be an important feature of treatment.
• Obstructive sleep apnea (OSA) Persons with this condition tend to suffer from airway obstruction when asleep, as the muscles relax and the weight and bulk of tissues collapses the throat passages. An observer notices loud snoring, frequent periods when breathing ceases (apneas), and episodes of restlessness and partial awakening. The afflicted patient is often unaware of the nature of the problem, but may notice frequent awakening at night, dry mouth, a sense of having slept poorly, daytime drowsiness and fatigue, or inappropriate sleeping (such as at work, in meetings, or while driving). This condition has a significant associated mortality.
• Gastroesophageal reflux disease (GERD) is characterized by regurgitation (reflux) of acid and gastric contents into the esophagus, and sometimes into the back of the throat. Gastric acid and bile are very corrosive to the lining membrane of the esophagus, and cause it to become inflamed (esophagitis) and sometimes scarred (esophageal stricture). Reflux which occurs while sleeping can lead to sudden coughing and choking at night, a burning sensation in the throat (pyrosis), and inhalation of acid and stomach contents into the lungs, with the risk of hoarseness, bronchitis, pneumonia, lung abscess and lung scarring. GERD is often associated with development of asthma, and causation of asthmatic attacks, and may also be aggravated by OSA.
• Gallbladder disease is much more likely in obese individuals, being associated with formation of gallstones, usually composed of crystallized cholesterol, within the gallbladder. Although readily treatable by removal of the gallbladder (cholecystectomy), it may lead to life-threatening problems such as obstruction of the ducts from the liver, jaundice, and inflammation of the pancreas (gallstone pancreatitis).
• Liver disease is present in some degree in 90% of persons who undergo bariatric surgery, usually a manifestation of the metabolic effects of obesity on the liver. This may take the form of large fat globules within the liver cells (steatosis), chronic inflammation of the liver (steatohepatitis), and in a few instances, cirrhosis of the liver. The latter condition may lead to liver failure and the need for a liver transplant.
• Venous thromboembolic disease affects the legs, and causes swelling, thickening and discoloration of the skin, and ulceration of the skin. This condition begins with damage to the veins of the legs, associated with formation of blood clots (thrombophlebitis), often associated with an injury, a pregnancy (even use of birth-control pills or hormones), or a surgical operation. When a newly formed blood clot breaks loose, and floats through the veins to the heart and lungs, it is called a Pulmonary embolus, which may sometimes be fatal within minutes. More commonly, the blood clot remains in place locally, and heals by becoming a scar, which permanently damages the vein. Once damaged, the veins cannot fully function to return blood to the heart, and increased venous pressure in the legs causes swelling, impaired circulation in the skin, and sometimes skin breakdown. Obesity is a major risk factor in development of VTE, and may also aggravate the increased venous pressure in the legs.
• Degenerative disc disease is a progressive "wearing-out" of the cartilaginous disks between the vertebral bones of the spine. It occurs more often and earlier in life in obese persons, due to the markedly increased mechanical stress on the disks from the extra weight. Its most common sign is chronic low back pain, which may be disabling. This condition is also associated with sciatica, lumbar spondylosis, and spinal stenosis.
• Degenerative disease of the weight bearing joints, or osteoarthritis, affecting the hips, knees, ankles and feet, occurs earlier in life, and in greater degree, in obese individuals, due to the mechanical stresses of excess weight. Joint pain, loss of mobility, and joint replacement surgery are much more likely in obese persons.