Short answer
A one-anastomosis gastric bypass (OAGB), often called a mini gastric bypass, is a weight-loss operation in which the stomach is made into a long, narrow pouch joined to the small intestine at a single connection, rather than the two connections of a standard Roux-en-Y bypass. In a randomised trial its weight loss at five years was not inferior to the standard bypass. The main trade-offs are more reflux, including bile reflux, and a higher risk of nutritional problems, which depend partly on how much intestine is bypassed.
Key points
- “Anastomosis” means a surgical join; OAGB has one join instead of two.
- Five-year weight loss in the YOMEGA trial was similar to a standard Roux-en-Y bypass.
- Reflux symptoms were reported in 41% after OAGB against 18% after Roux-en-Y bypass in that trial.
- Lifelong vitamin and mineral supplements and monitoring are needed.
- Long-term evidence is still thinner than for the sleeve or standard bypass.
How the operation works
In OAGB, the surgeon divides the stomach to make a long, narrow pouch that holds less food, then attaches it to a loop of the small intestine further down, so food skips the first part of the intestine. There is just one connection point, which makes it faster and less complex than a traditional bypass, in which the stomach and small intestine are rerouted using two connections.1
Bile reflux, in which bile from the intestine flows back into the stomach pouch and gullet, is a specific concern with this design.13 The length of intestine bypassed (the biliopancreatic limb) varies between surgeons, averaging about 200 cm with no consensus; poor absorption and malnutrition have been reported when it exceeds 250 cm.3
| Feature | One-anastomosis bypass (OAGB) | Roux-en-Y bypass |
|---|---|---|
| Surgical joins | One1 | Two1 |
| Stomach pouch | Long and narrow1 | Small pouch at the top of the stomach |
| Excess BMI loss at 5 years (YOMEGA) | 75.6%2 | 71.4%2 |
| Clinical reflux (YOMEGA, 5 years) | 41%2 | 18%2 |
| Nutritional complications (YOMEGA, 2 years) | 21.4%3 | None reported3 |
What the main trial shows
The YOMEGA trial in France randomly assigned 253 adults to OAGB or Roux-en-Y bypass. At five years, excess BMI loss (the share of BMI above a healthy level that was lost) was 75.6% after OAGB and 71.4% after Roux-en-Y bypass, meeting the trial’s test for non-inferiority. Type 2 diabetes remission and overall nutritional status were similar between the groups at five years.2
The most common problem was clinical gastro-oesophageal reflux disease (GERD, stomach contents flowing back into the gullet): 41% after OAGB against 18% after Roux-en-Y bypass. Ten of 127 people (8%) who had OAGB were converted to a Roux-en-Y bypass. The authors said the high reflux rate needs further investigation.2 Only 68% of participants were followed up at five years, which weakens the certainty of these figures.2
Evidence that OAGB gives weight loss comparable to Roux-en-Y bypass over five years is Moderate, resting mainly on this single randomised trial. A 2020 review concluded that prospective studies with more than five years of follow-up are still needed to settle concerns about bile reflux and nutrition.3
Risks to understand
Cleveland Clinic lists a higher risk of bile reflux, ulcers where the stomach and intestine join, poor absorption of nutrients with a lifelong need for vitamins, dumping syndrome (symptoms when food moves too quickly into the intestine) and general surgical risks such as bleeding and infection.1 It also describes a lower risk of bowel obstruction, leaks and hernias than with a traditional bypass.1
In published series, reported rates vary widely: the 2020 review gives figures of about 0.6% to 4% for ulcers at the join, 0.1% to 1.9% for leaks and about 1% for malnutrition, with iron-deficiency anaemia in 5% to 10%.3 Because these come from studies of different designs and limb lengths, the true incidence is best described as Uncertain incidence.
Bile reflux is particularly relevant for anyone with existing reflux. The International Federation for the Surgery of Obesity and Metabolic Disorders (IFSO) has issued and updated a position statement on the role of OAGB, reflecting continuing debate about where it fits.4 For how reflux differs between other operations, see can a gastric sleeve cause reflux?
When to seek medical advice
After weight-loss surgery, the NHS advises calling 999 or going to A&E for severe tummy pain, vomiting blood or vomit that looks like coffee grounds, heavy bleeding from the bottom, or leg pain and swelling with breathlessness or chest pain. Get urgent advice from a GP or NHS 111 for a high temperature, fast heartbeat, difficulty swallowing or regular vomiting.5
Common questions
Is a mini gastric bypass a smaller operation?
Not in effect; “mini” refers to having one surgical join instead of two, but it is still a full bypass with a long-term impact on digestion.1
Will I need vitamins for life?
Yes; Cleveland Clinic describes a lifelong need for vitamins and lifelong monitoring for deficiencies.1 See why vitamin supplements are needed for life.
Can OAGB be converted to another operation?
Yes; in the YOMEGA trial, 8% of people who had OAGB were converted to a Roux-en-Y bypass within five years.2 See when revision surgery is considered.
How long is recovery?
Cleveland Clinic advises avoiding strenuous activity for up to six weeks and returning to a typical diet over up to 12 weeks.1
Bottom line
OAGB is a simpler-to-perform bypass with five-year weight loss similar to the standard Roux-en-Y operation in a randomised trial. Its main drawbacks are more reflux, including bile reflux, and nutritional risks that depend on technique, so existing reflux and commitment to lifelong follow-up weigh heavily in whether it is appropriate.
Related reading
- Gastric bypass guide
- Gastric sleeve vs gastric bypass: how do they compare?
- What is dumping syndrome?
- Why are vitamin supplements needed for life after bariatric surgery?
- When is revision bariatric surgery considered?
Sources
- Cleveland Clinic. Mini gastric bypass. Cleveland Clinic. https://my.clevelandclinic.org/health/procedures/mini-gastric-bypass. Accessed 5 November 2026.
- Robert M, et al. Efficacy and safety of one anastomosis gastric bypass versus Roux-en-Y gastric bypass at 5 years (YOMEGA): a prospective, open-label, non-inferiority, randomised extension study. Lancet Diabetes and Endocrinology; 2024. PMID 38452784. https://read.qxmd.com/read/38452784/efficacy-and-safety-of-one-anastomosis-gastric-bypass-versus-roux-en-y-gastric-bypass-at-5-years-yomega-a-prospective-open-label-non-inferiority-randomised-extension-study
- Aleman R, Lo Menzo E, Szomstein S, Rosenthal RJ. Efficiency and risks of one-anastomosis gastric bypass. Annals of Translational Medicine; 2020;8(Suppl 1):S7. doi:10.21037/atm.2020.02.03. https://atm.amegroups.org/article/view/37738/html
- De Luca M, Piatto G, Merola G, et al. IFSO update position statement on one anastomosis gastric bypass (OAGB). Obesity Surgery; 2021. doi:10.1007/s11695-021-05413-x. https://link.springer.com/article/10.1007/s11695-021-05413-x
- NHS. Weight loss surgery: complications. NHS website. https://www.nhs.uk/tests-and-treatments/weight-loss-surgery/complications/. Accessed 5 November 2026.
Article history
- — First published.