Surgery to produce weight loss is a serious undertaking, and every prospective patient should clearly understand that the operation involves risks and complications can occur.

However, it is important to realise that the most serious risks of gastric bypass surgery are similar to any type of surgery said the Lahey Clinic's Dr. Shah.

The risks and complications of gastric bypass surgery include:

The surgery creates an open wound or wounds, which can bleed, fail to heal, become infected, or develop a hernia.

Bleeding may also occur if a blood vessel that has been cut and tied off during surgery reopens. Should a severe haemorrhage occur, transfusions and or re-operation may be required.

Fluid build-up in the abdomen may cause an abscess, or a collection of pus, that must be drained.

The new stomach outlet, connecting the stomach to the intestine, or bowel may become obstructed due to scarring, causing vomiting. This situation must be surgically corrected so that food can exit the stomach.

The bowel connection may also leak if a complete seal is not achieved, which may require re-operation.

An increased blood clotting tendency during and after surgery combined with immobility may trigger the formation of a blood clot in the deep veins of the legs or pelvis. Should this clot travel through the bloodstream to the lungs, it can obstruct one of the narrow pulmonary arteries - causing what is known as a pulmonary embolism. If a large enough portion of the lung is affected, pulmonary embolism can be fatal.

Adhesions, or scars, from abdominal surgery can obstruct the bowel, either soon after surgery or years later. If the adhesions do not break up on their own, surgery may be required to correct the situation and restore bowel function.

Abdominal hernias are the most common complications requiring follow-up surgery. Less common complications include breakdown of the staple line, stretched stomach outlets, and intra-abdominal infection.

Gallstones may develop, but may be prevented with supplemental bile salts taken for the first six months after surgery.

Nutritional deficiencies and metabolic disorders such as anaemia, osteoporosis, and metabolic bone disease may develop and may be avoided if vitamin and mineral intakes are maintained or supplemented.

Patients may develop a food intolerance to items such as red meat, sugar, and milk.

Consuming sugar and some fats on an empty stomach may trigger the dumping syndrome, an unpleasant physiological reaction to concentrated calories hitting the intestine.

It is characterised by the movement of body fluids into the intestine, in response to the high calorie load. The effect of this fluid movement is to transiently decrease the circulating blood volume.

As a result, patients may experience weakness, nausea, vomiting, sweating, elevated heart rate, and diarrhoea. The dumping syndrome can be prevented by eating protein first or avoiding sugar altogether.

Women of childbearing age should avoid pregnancy until their weight becomes stable because rapid weight loss and nutritional deficiencies can harm a developing foetus.

The risk of death following the operation is small, but does exist, and is approximately 0.5 percent to one percent.

Historically, weight reduction operations have had a bad reputation, largely because the risks involved with two early procedures clearly outweighed any benefits.

The procedures have now been largely abandoned, according to Dr. Shah.

The jejuno-ileal bypass, which did not involve the stomach and as such was not a gastric bypass, was common in the 1970s and is responsible for much of the scepticism surrounding weight-loss surgery.

"That's because many patients who underwent this procedure developed kidney or liver disease, and some even died," said Dr. Shah. "For many patients, the risks of the operation clearly outweighed the benefits.

"As a result came the development of newer, safer, and superior operations and the procedure is no longer performed in the United States."

In addition the loop gastric bypass or Mini Gastric Bypass is also generally considered obsolete due to concerns about severe post-operative complications, such as leakage, ulcers, oesophageal cancer, and extreme patient discomfort.

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