Gastric sleeve vs gastric bypass: how the operations differ and what to weigh up

The NHS describes weight loss surgery, also called bariatric surgery, as a treatment for people who are severely obese. It makes the stomach smaller so that it feels fuller sooner and less food is eaten. The main types are gastric bypass, gastric band, gastric balloon and sleeve gastrectomy. Of these, the sleeve and the bypass are the two most often compared, because both are permanent changes to the digestive tract that lead to substantial weight loss in many people.

The choice is not simply one operation being better than the other. It depends on your weight and health conditions, your eating pattern, any existing reflux, what your local service offers and the surgeon's experience, and it should be discussed with a specialist bariatric team.

Gastric sleeve (sleeve gastrectomy). The NHS says surgery starts with about five small cuts in the tummy, which is then filled with air so the surgeon can remove part of the stomach. Around 80% of the stomach is removed to make it much smaller and sleeve shaped. The operation usually takes 1 to 3 hours. Because part of the stomach is physically removed, it is not an operation that can simply be undone.

Gastric bypass (Roux-en-Y). The NHS says the stomach is divided in two using a stomach staple, creating a small pouch at the top of the stomach. The new pouch is attached to the lower part of the small intestine, so after surgery food passes from the pouch to the lower small intestine. The operation usually takes around 2 hours. The NHS also describes biliopancreatic diversion and duodenal bypass as similar but less common procedures that can cause more side effects than a gastric bypass.

The NHS says weight loss surgery may be available if you have a BMI of 40 or more, or a BMI between 35 and 40 and a condition that might improve if you lost weight, such as high blood pressure, diabetes, arthritis, breathing problems or asthma, and you agree to make healthy lifestyle changes and attend regular check-ups. Elsewhere it adds that you will usually need to have tried to lose weight through a healthy diet, exercise and medicine.

It also says that if you have a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, surgery may be considered if your BMI is below 40. Availability varies across areas of the UK, and you can also pay for surgery privately, but this can be expensive.

The NHS describes weight loss surgery as a common and safe procedure, with risks as with any operation. The complications it lists for both a gastric bypass and a sleeve gastrectomy include blood clots, a stomach leak in the days or weeks after surgery, wound infection, a blocked gut, gallstones and excess skin. It lists a slipped band as a risk specific to gastric band surgery.

The NHS pages used for this guide do not give head-to-head figures for weight loss, diabetes remission or complication rates between the sleeve and the bypass, and studies reported in the research literature vary by population and follow-up, so this guide does not quote them. Questions about expected weight loss, the effect on type 2 diabetes or reflux, and the chance of needing a further operation are best put to your own bariatric team, who can relate them to your health.

Recovery is broadly similar after both operations. The NHS says you can usually leave hospital 1 to 3 days after surgery and return to normal activities after 4 to 6 weeks. A typical diet plan moves from water and fluids for the first 2 weeks, to runny food in weeks 3 and 4, soft food in weeks 5 to 8 and a gradual return to a healthy balanced diet from week 8.

Having a smaller stomach means you must eat slowly, chew carefully, eat small amounts and avoid foods that could block the stomach, such as soft white bread, and you will need to take vitamin and mineral supplements. The NHS says you will be asked to attend regular follow-up appointments for the rest of your life, usually in a weight loss surgery clinic for at least the first 2 years and then possibly with a GP once a year, including blood tests to check vitamin and mineral levels. It advises avoiding pregnancy for 12 to 18 months after surgery.

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, non-stop bleeding from the bottom, a lot of blood, or vomiting blood or poo, or vomit that looks like coffee grounds. It says to ask for an urgent GP appointment or call NHS 111 for redness or pain in a leg that is not getting better, a high temperature, a fast heartbeat, tummy pain, fast breathing, difficulty swallowing or regular vomiting. These could signal a blood clot or a problem with the stomach.

It helps to arrive at a bariatric clinic with specific questions rather than a preference for one operation.

• Which operation does the team recommend for me, and why, given my weight, diabetes, reflux and other conditions?

• What are the realistic risks of each in my case, and how often does this unit see them?

• What will my eating look like at 6 weeks, 6 months and 2 years?

• Which supplements will I need and for how long, and who will arrange the blood tests?

• What follow-up does this service provide, and what happens if I move area?

• If the result is not what we hoped, what could be done next?