Gastric bypass surgery: how it works, who can have it and the risks

The NHS describes weight loss surgery, also called bariatric surgery, as a treatment for people who are severely obese. It works by making the stomach smaller so that you feel full sooner and eat less. The main types are gastric bypass, gastric band, gastric balloon and sleeve gastrectomy.

In the standard gastric bypass, called Roux-en-Y, your stomach is divided in two with a stomach staple. This makes a small pouch at the top of the stomach, and the new pouch is attached to the lower part of your small intestine. After surgery, food passes from the pouch straight into the lower small intestine.

The NHS also describes biliopancreatic diversion and duodenal bypass surgery, which are similar operations in which the stomach pouch is connected further along the small intestine. They involve having part of the stomach removed and can cause more side effects than a gastric bypass, so they are less commonly used.

The NHS says gastric bypass is performed by a surgeon using keyhole surgery while you are under general anaesthetic, meaning you are asleep. The operation usually takes around 2 hours. You usually go home 1 to 3 days after weight loss surgery, and recovery time is usually 4 to 6 weeks.

Compared with other options, the bypass changes both the size of the stomach and the route food takes through the digestive tract. The NHS table of procedures is a useful way to see how the main choices differ.

The NHS says weight loss surgery may be available if you have a body mass index (BMI) of 40 or more, or a BMI of 35 to 40 and a condition that might improve if you lost weight. Examples given are high blood pressure, diabetes, arthritis, breathing problems and asthma. You must also agree to make healthy lifestyle changes and to attend regular check-ups after surgery.

You will usually have tried to lose weight through a healthy diet, exercise and medicine before surgery is considered. For people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, the NHS says surgery may be considered with a BMI below 40.

Availability varies across areas of the UK. The NHS also notes you can pay for surgery privately, but that this can be expensive.

The NHS calls weight loss surgery a common and safe procedure but says that, as with any operation, there are risks that your doctor will explain. The complications it lists are summarised here.

• Blood clots, often starting in the lower leg (deep vein thrombosis) and sometimes travelling to the lungs (pulmonary embolism). Anticoagulant medicines can be given.

• A stomach leak: in the days or weeks after a gastric bypass or sleeve gastrectomy there is a small chance food could leak out of the stomach, causing a serious infection that needs antibiotics.

• Wound infection, which keeping the wound clean helps to prevent.

• A blocked gut, which can follow scarring and reduced blood flow. Cutting food into small chunks, chewing thoroughly and not drinking during meals reduces the risk.

• Gallstones, which can form when you lose weight quickly.

• Excess skin, which may be removed with surgery that is usually considered cosmetic and is not always available on the NHS.

The NHS says to call 999 or go to A&E if you have difficulty breathing with pain and swelling in a leg, chest pain with pain and swelling in a leg, severe tummy pain, bleeding non-stop from the bottom, a lot of blood (for example the toilet water turns red or you see large clots), or you are vomiting blood or your vomit looks like coffee grounds or soil.

It advises asking for an urgent GP appointment or contacting NHS 111 if, after weight loss surgery, you have redness, tenderness, swelling or pain in a leg that is not getting better, a high temperature or feeling hot, cold or shivery, a fast heartbeat, tummy pain, fast breathing, difficulty swallowing or regular vomiting. These could be a sign of a blood clot or a problem with the stomach. Pain around a wound, red, hot and swollen skin or pus from a wound should be seen by a GP.

On the NHS, gastric bypass is offered to people who meet the criteria above through specialist weight management services, with the operation and follow-up free at the point of use, although local availability varies. Private surgery is an option for people who pay themselves.

Whichever route you choose, it is worth asking who will provide long-term follow-up and what happens if a complication develops after you have left hospital. Because the bypass permanently changes how the digestive tract works, ongoing check-ups matter as much as the operation itself, and the NHS makes them a condition of being offered surgery.