This page gives general, neutral information. Whether surgery is right for you, and which type, is decided together with a specialist team. Criteria and reimbursement differ by country. Obesity Foundation does not promote surgery or weight-loss medication; our focus is good food, appropriate exercise and walking.
How does bariatric surgery work?
Bariatric surgery works mainly by changing the signals between the gut and the brain, not only by making the stomach smaller. After surgery, gut hormones such as GLP-1 change, hunger falls and people feel full sooner. Blood sugar often improves within days, even before much weight is lost.
That is why experts now speak of metabolic and bariatric surgery. Almost all operations are done by keyhole (laparoscopic) surgery.
What types of surgery are there?
The two most common operations worldwide are the sleeve gastrectomy and the Roux-en-Y gastric bypass.
| Operation | What the surgeon does | Points to know |
|---|---|---|
| Sleeve gastrectomy | Removes most of the stomach, leaving a narrow tube | Cannot be reversed. Can cause or worsen acid reflux. |
| Roux-en-Y gastric bypass | Creates a small stomach pouch connected directly to the small intestine, so food bypasses most of the stomach and the first part of the intestine | On average more weight loss. Often preferred when there is severe acid reflux. Higher risk of vitamin and iron shortage. Difficult to reverse. |
| Duodenal switch and similar operations | A sleeve plus a longer bypass of the small intestine | Most weight loss, but highest risk of nutrient shortages. Done less often. |
| Adjustable gastric band | Places an adjustable band around the top of the stomach | Less weight loss and frequent adjustments. Now rarely used. |
Other options, such as the one-anastomosis gastric bypass, and endoscopic procedures through the mouth, are available in some countries. The surgeon will discuss which operation fits your health.
Who is eligible for bariatric surgery?
According to the joint international guidelines of the American Society for Metabolic and Bariatric Surgery (ASMBS) and the International Federation for the Surgery of Obesity and Metabolic Disorders (IFSO), surgery is recommended for adults with a BMI of 35 or higher, whether or not they have other health problems.
It should be considered for adults with a BMI of 30 to 34.9 and metabolic disease, such as type 2 diabetes.
Because health risks start at a lower BMI in people of Asian descent, the same guidelines advise offering surgery to Asian people from a BMI of 27.5.
National criteria are often stricter. For example, in the UK, NICE advises assessment for surgery for adults with a BMI of 40 or higher, or 35 to 39.9 with a significant obesity-related condition. Many countries also require an assessment by a team that includes a surgeon, a dietitian and a psychologist.
Ask your doctor about the rules where you live. Our page on BMI and waist size explains these measurements.
What results can you expect?
Bariatric surgery usually leads to considerable weight loss, and results differ between people. Some weight regain over the years is common. Studies have also looked at effects beyond weight:
- Type 2 diabetes: many people need fewer or no diabetes medicines, although diabetes can return over time.
- Other conditions: high blood pressure, sleep apnoea, fatty liver and joint pain often improve.
What are the risks?
Bariatric surgery is now a safe operation in experienced centres, but like all major surgery it has risks. In a US registry of almost 250,000 operations, 0.1 percent of people died within 30 days. Possible complications include:
- leaks, bleeding, infection or blood clots soon after surgery;
- narrowing, ulcers or hernias, sometimes needing another operation;
- gallstones, as with any fast weight loss;
- acid reflux, especially after a sleeve gastrectomy;
- shortages of vitamins and minerals, such as iron, vitamin B12, folate, calcium and vitamin D.
What does lifelong follow-up involve?
Life after bariatric surgery requires lifelong care, because the body absorbs nutrients differently. This usually includes:
- Daily vitamins and minerals for life, plus regular blood tests to check for shortages.
- Eating in a new way: small meals, eating slowly, enough protein, and drinking between meals rather than with them.
- Dumping syndrome: after a gastric bypass, sugary or fatty food can pass too quickly into the intestine, causing cramps, sweating, dizziness or diarrhoea. Low blood sugar a few hours after eating can also occur.
- Care with alcohol: alcohol is absorbed faster after surgery. In a large US study, people who had a gastric bypass had more than twice the risk of developing alcohol problems over seven years compared with people who had a gastric band.
- Pregnancy planning: doctors usually advise waiting until weight is stable, often 12 to 18 months, and getting extra checks. See obesity and pregnancy.
- Mental health support: big changes in body and eating can bring up strong emotions. Psychological support can help.
If weight returns after surgery, help is available: talk to your specialist team about your options. Good food, daily walking and regular exercise stay important for life. Read more about weight regain.
Can adolescents have bariatric surgery?
Yes, international guidelines say that carefully selected adolescents with severe obesity should be considered for surgery in specialist centres. In the US Teen-LABS study, teenagers lost as much weight as adults five years after gastric bypass, and were more likely than adults to see their type 2 diabetes and high blood pressure go into remission.
They also more often needed further abdominal operations and had more often low iron and vitamin D, so close follow-up and family support are essential. Read more about obesity in children and adolescents.
Frequently asked questions
Is bariatric surgery the easy way out?
No. Surgery is a major medical treatment, considered when other treatment has not been enough. It requires careful preparation, permanent changes in eating and lifelong follow-up. Whether it suits you is a personal decision made with a specialist team, and nobody should be judged for it.
Which is better: sleeve gastrectomy or gastric bypass?
There is no single answer. A gastric bypass often leads to somewhat more weight loss and is usually preferred for people with severe acid reflux, while a sleeve is technically simpler and has a lower risk of some long-term problems. The choice depends on your health and is made with your surgeon.
Do I have to lose weight before surgery?
Rules differ between countries and hospitals. Many surgical teams ask for a short low-energy diet in the weeks before the operation, because it shrinks the liver and makes surgery safer. Ask your team what applies to you.
Can I take obesity medication after bariatric surgery?
Yes, in some situations. If weight returns or health problems come back, doctors can add obesity medication after surgery. This is decided by your specialist team.
Sources
- Eisenberg et al. – 2022 ASMBS and IFSO indications for metabolic and bariatric surgery (summary, HCPLive)
- NIDDK – Types of weight-loss surgery
- University of Gothenburg – Higher average life expectancy after obesity surgery (SOS study, NEJM 2020)
- NIH – Early weight-loss surgery may improve type 2 diabetes, blood pressure outcomes (Teen-LABS, 2019)
- King's College London – Gastrointestinal surgery as a cure for type 2 diabetes: 10-year trial results, The Lancet (2021)
- BMJ – Referral criteria and assessment for bariatric surgery: summary of updated NICE guidance (2023)