WLS stands for weight loss surgery, a group of procedures that alter your stomach, your digestive tract, or both to help you lose a significant amount of body weight. Also called bariatric surgery, these operations work by limiting how much food your stomach can hold, reducing the calories your body absorbs, or changing the hormonal signals that drive hunger. The three most common types are gastric sleeve, gastric bypass, and duodenal switch.
Gastric Sleeve
Gastric sleeve surgery, formally called sleeve gastrectomy, is the most widely performed bariatric procedure today. A surgeon removes about 80% of your stomach, leaving behind a narrow tube roughly the size and shape of a banana. The smaller stomach holds far less food, so you feel full after eating a fraction of what you used to.
But portion control is only part of how it works. The removed portion of the stomach is the main source of ghrelin, the hormone that triggers hunger. With most of that tissue gone, your appetite and cravings drop noticeably. This hormonal shift is a major reason people keep weight off after the procedure rather than gradually eating more over time. The sleeve is permanent because the removed stomach tissue cannot be reattached.
Gastric Bypass
Gastric bypass, known clinically as Roux-en-Y gastric bypass, is a two-part operation. First, the surgeon staples off a small pouch at the top of your stomach, separating it from the rest. Then a section of your small intestine is rerouted and connected directly to that pouch. Food skips the larger portion of your stomach and the first stretch of the small intestine entirely, which means your body absorbs fewer calories and nutrients from each meal.
The bypass also changes gut hormones and bacteria in ways that reduce appetite and improve how your body handles blood sugar. Because food passes directly into the small intestine without going through the natural valve at the bottom of your stomach, it can trigger a reaction called dumping syndrome, where sugary or high-fat foods cause nausea, cramping, or diarrhea. This happens in roughly 15% of patients and, while unpleasant, tends to discourage eating the foods most likely to cause weight regain. Gastric bypass is technically reversible, though surgeons rarely do so unless medically necessary.
Duodenal Switch
The duodenal switch is the most aggressive option and produces the greatest weight loss. It combines a sleeve gastrectomy with a significant intestinal bypass. After the stomach is reduced to a sleeve, the surgeon reroutes a large portion of the small intestine so that food travels through one path while digestive juices travel through another. The two paths meet near the end of the intestine, leaving a relatively short segment where calories and fat are actually absorbed.
A newer, simplified version called SADI-S requires only one intestinal connection instead of two, which shortens operating time and lowers the risk of complications like internal hernias and leaks at connection points. The duodenal switch is typically reserved for people with the highest BMIs because of its potency and the nutritional management it demands afterward.
How Much Weight You Can Expect to Lose
Weight loss after bariatric surgery is measured as a percentage of “excess weight,” meaning the weight above what’s considered healthy for your height. Gastric bypass patients lose about 50% of their excess weight in the first year and around 70% by the end of the second year. Sleeve gastrectomy patients typically lose about 43% at one year and 63% at two years. The duodenal switch generally produces higher numbers than either, though long-term data on the newer SADI-S version is still accumulating.
These numbers represent averages. Individual results depend on how closely you follow post-surgery dietary guidelines, your physical activity level, and your starting weight. Most of the weight loss happens in the first 12 to 18 months, then gradually levels off.
Who Qualifies
Updated 2022 guidelines from the American Society of Metabolic and Bariatric Surgery expanded eligibility beyond the older, stricter thresholds. Surgery is now recommended for anyone with a BMI of 35 or higher, regardless of whether they have other health conditions like diabetes or high blood pressure. People with a BMI between 30 and 35 should be considered if they have a weight-related metabolic condition that hasn’t responded to other treatments.
For people of Asian descent, the thresholds are lower: a BMI above 27.5 qualifies. This adjustment reflects the fact that serious metabolic complications develop at lower body weights in Asian populations. Adolescents can also be candidates in cases of severe obesity, defined as a BMI at or above 120% of the 95th percentile for their age, with a major related health problem.
Risks and Complications
Bariatric surgery has become considerably safer over the past two decades, especially at high-volume centers. In a 14-year study of over 1,000 patients, the overall complication rate was about 7%, with roughly 3% of complications occurring within the first 30 days and 4% developing later. No deaths were recorded during the study period.
The most serious early risk is a leak at a staple line or connection point, where stomach contents can seep into the abdominal cavity. Leak rates range from about 0.5% at specialized centers to as high as 5% depending on the procedure and the surgeon’s experience. Strictures, where scar tissue narrows a connection point and blocks food from passing through, occur in less than 1% of bypass patients. Dumping syndrome is the most common ongoing issue after gastric bypass, affecting roughly 15% of patients on average.
Recovery and the Post-Surgery Diet
Most bariatric procedures are done laparoscopically through small incisions, and hospital stays typically last one to two days. The recovery timeline for your digestive system, however, is measured in weeks. Your diet advances through four stages designed to let your newly altered stomach heal before handling solid food.
You start with clear liquids while still in the hospital. About two days after surgery, you move to full liquids like protein shakes and thin soups, and you stay on those for roughly two weeks. Pureed foods begin around week two or three. Soft foods are introduced at about five weeks. Most people are eating regular foods, in much smaller portions, by six to eight weeks after surgery. Rushing through these stages risks stretching your new stomach pouch or causing a leak at a surgical connection.
Lifelong Nutritional Needs
Because bariatric surgery limits how much you eat and, in bypass procedures, how much your body absorbs, vitamin and mineral deficiencies are a predictable consequence. Supplementation isn’t optional or temporary. It’s a daily commitment for the rest of your life.
At minimum, you’ll need a daily multivitamin with minerals (most bypass and sleeve patients take two per day), 1,200 to 1,500 milligrams of calcium spread across the day in doses no larger than 600 milligrams, at least 3,000 IU of vitamin D, and vitamin B-12 either daily by mouth or through periodic injections. Women of childbearing age or anyone at elevated risk of anemia typically need 50 to 100 milligrams of supplemental iron daily. Calcium and iron supplements need to be taken at least two hours apart because they interfere with each other’s absorption.
Skipping these supplements leads to deficiencies that develop slowly but can cause serious problems: bone loss from low calcium, nerve damage from B-12 deficiency, and anemia from low iron. Regular blood work, usually every three to six months in the first year and annually after that, helps catch shortfalls before they cause symptoms.