Understanding what physically happens to your stomach during a sleeve gastrectomy helps make the procedure feel less abstract and the post-operative rules feel less arbitrary. When you know why certain things happen, the instructions make more sense and are easier to follow.
Before surgery: the pre-op liver reduction diet
Before surgery even begins, the stomach and surrounding area need to be prepared. In the weeks before the procedure, you will be placed on a high-protein, low-carbohydrate diet, sometimes involving meal replacement shakes. The purpose of this is to reduce the size of the liver.
The liver sits directly over the stomach. In people with significant excess weight, the liver is often enlarged and fatty, a condition called hepatic steatosis or NAFLD. James had been living with NAFLD for over ten years before surgery and was approaching stage 3 liver failure. During surgery, the surgeon needs to retract the liver to access the stomach. A smaller, less fatty liver is safer to retract and reduces the risk of liver damage during the procedure. This is not optional. Arriving for surgery with an enlarged liver because the pre-op diet was not followed can result in the surgery being abandoned.
The anaesthetic and positioning
Once you are in theatre and under general anaesthesia, you are positioned in a specific way to give the surgical team the best access to the upper abdomen. The operating table may be tilted and your legs positioned to create a working space. A breathing tube is inserted to manage your airway under general anaesthesia.
Creating the working space
The abdomen is inflated with carbon dioxide gas, a process called insufflation. This creates a working space inside the abdominal cavity, pushing the abdominal wall away from the organs so the surgeon can see clearly and manoeuvre instruments. The bloating and shoulder pain that many people experience in the days after surgery is related to this gas, which takes time to be absorbed by the body.
The calibration tube
Before the stomach is cut, a calibration tube, sometimes called a bougie, is passed down through the mouth and oesophagus into the stomach. This acts as an internal guide, sitting along the inner curve of the stomach. The surgeon uses it as a reference to ensure the remaining sleeve is formed to a consistent and appropriate size. Without this guide, there would be no reliable way to standardise how narrow the sleeve is or ensure the staple line runs in the correct position.
Dividing and removing the stomach
The surgeon then uses a surgical stapling device to cut and seal the stomach simultaneously. The stapler cuts through the stomach wall and applies a line of staples on both sides of the cut, sealing the tissue as it divides it. Multiple firings of the stapler are required to divide the stomach along the full length of the cut line, from the lower antrum up toward the gastro-oesophageal junction.
The removed portion, roughly 75 to 80 percent of the original stomach including the fundus and most of the greater curvature, is then extracted through one of the port incisions. The fundus is where the majority of ghrelin, the hunger hormone, is produced. Its removal is why appetite often decreases noticeably after surgery.
Testing the staple line
Once the stomach is divided and the removed portion extracted, the surgeon typically tests the integrity of the staple line. This can involve injecting methylene blue dye or air through the tube and checking for leaks. A staple line leak is a serious complication, and checking before closing is an important safety step.
What remains and how it behaves
What remains is a narrow, tube-shaped stomach running along the inner curve of the original stomach. It holds a small fraction of the original volume. In the early weeks, the stomach is swollen and healing, which is why even soft foods need to be consumed in very small amounts and swallowed slowly. As healing progresses over several months, the stomach does relax slightly and capacity increases modestly, though it never returns to its pre-surgical size.
The valve at the top of the stomach, the lower oesophageal sphincter, and the valve at the bottom, the pylorus, both remain intact. This means the stomach continues to function as a stomach. It receives food from the oesophagus, begins mechanical and chemical digestion, and empties into the small intestine through the normal pathway. The digestive process is not rerouted.
Sources
NHS: Weight loss surgery. NICE: Obesity: identification, assessment and management (NG238). BOMSS: Patient information for sleeve gastrectomy.
About this content
This blog is written by James and Kirsten, a couple from the UK who had gastric sleeve surgery together in March 2024.
We started this blog because we couldn't find any sources of content that details before surgery, the surgery and then life post surgery - so we decided to write one ourselves.
Everything on this site is based on our own experience and the research we have done along the way. It is not medical advice. Gastric sleeve surgery is a serious procedure and every patient's journey is different. Please always consult your own bariatric team or GP before making any decisions about your health or treatment.
Some posts on this site may contain featured or sponsored content, or affiliate links. Where this is the case, it will always be clearly stated at the top of the article. Our opinions are always our own.
Publish Date: 19 May 2026 | Last Reviewed: 7 June 2026 | Next Planned Review: 7 December 2027