Key points

The bypass reduces the stomach to a small pouch joined directly to the intestine: food no longer passes through the rest of the stomach or the first part of the intestine.
It leads to significant and lasting weight loss in many people, but results vary from one person to another.
It has its own specific risks and requires vitamins and follow-up for life.

How the operation works

The surgeon creates a small pouch at the top of the stomach and joins it directly to the small intestine. The rest of the stomach is left in place, but food no longer passes through it. Further down, a second join brings the digestive juices back into contact with the food. The operation is done by laparoscopy (keyhole surgery), through small incisions.

Concept and medical supervision: Dr Pierre Fournier

Subtitles

All steps as text

Before surgery (0–4 s). This is the route food takes before gastric bypass.
The small pouch (4–11 s). A small pouch is separated from the larger stomach, which stays in place.
Dividing the jejunum (11–15 s). The jejunum is divided.
Bringing up the limb (15–24 s). One limb is brought up to the pouch: the gastrojejunal anastomosis.
Second connection (24–31 s). The other limb is connected further down: the jejunojejunal anastomosis.
Both routes (31–40 s). Food follows the new route. Digestive juices join it at the second connection.

Simplified diagram, not to scale.

What you can expect

The bypass does more than reduce the size of meals: it changes the hormonal signals that regulate hunger, fullness and blood sugar¹.

It leads to significant and lasting weight loss in many people². Type 2 diabetes often improves markedly, sometimes to the point of remission, but this effect can wane over the years³.

Results vary from one person to another, and weight regain remains possible, sometimes years later. See Weight regain.

For healthcare professionals, the SOPHIA research calculator (University of Lille) estimates a weight trajectory after surgery based on large cohorts of people who have had an operation⁴. Its result is an estimate, to be discussed in consultation. SOPHIA calculator (external website, in English).

Risks and constraints specific to the bypass

The risks common to all obesity surgery are set out on the page Benefits, risks and constraints. The bypass also has its own particular features:

Internal hernia: the intestine can become trapped in a space created by the new arrangement of the bowel. It shows up as abdominal pain, sometimes coming in bouts, and may require emergency surgery⁵.
Ulcer at the join (anastomosis), made more likely by smoking and anti-inflammatory medicines⁶.
Feeling unwell after meals (dizziness, palpitations, drowsiness, low blood sugar), linked to sugars passing through quickly; adjusting your diet is the first treatment⁵.
Deficiencies in vitamins and minerals, and tests that are harder to carry out: the bypassed part of the stomach can no longer be examined by a standard gastroscopy⁵.

No technique is best for everyone: see Choosing an operation.

Hospital stay and follow-up

The hospital stay usually lasts two to three nights. The team supports you as you return to eating and to your activities, adapting this to your situation.

Vitamin and mineral supplements, regular blood tests and follow-up are needed for life, even if you feel well^{1, 2}. Life and follow-up after surgery

Frequently asked questions

Will I be able to eat normally?

Yes, after a period of adjustment, but in smaller amounts and eating slowly. Fast-acting sugars and alcohol are less well tolerated. Dietary support is part of the follow-up.

What next?

If this page sounds like your situation, you can book a surgical assessment consultation. Its purpose is to review your situation and the possible options; any operation is then decided collectively by the multidisciplinary team, with you.

Your request is received by the Centre’s appointments team, who will organise the next steps with you. You do not need to check for yourself whether you meet the criteria.

Book a surgical assessment consultation

Prepare for your first consultation

Medical review of the French version: Dr Pierre Fournier

Scientific sources and references

Sources accessed on 12 October 2026. Reimbursement rules and information on medicines change over time.

Swiss Society for the Study of Morbid Obesity and Metabolic Disorders (SMOB). Directives pour le traitement chirurgical de l’obésité, in force from 1 July 2023 (in French). View source
Gerber PA, Collet TH, Favre L, et al. Swiss obesity clinical practice guidance. Swiss Med Wkly. 2026;156:5415. View source
Adams TD, Davidson LE, Litwin SE, et al. Weight and metabolic outcomes 12 years after gastric bypass. N Engl J Med. 2017;377(12):1143-1155. View source
Saux P, Bauvin P, Raverdy V, et al. Development and validation of an interpretable machine learning-based calculator for predicting 5-year weight trajectories after bariatric surgery: a multinational retrospective cohort SOPHIA study. Lancet Digit Health. 2023;5(10):e692-e702. View source
IFSO-EC, version adopted by the SMOB. La prise en charge en urgence du patient opéré de chirurgie métabolique et bariatrique (poster for emergency departments), 2026. View source
American Society for Metabolic and Bariatric Surgery. Literature review on risk factors, screening recommendations, and prophylaxis for marginal ulcers after metabolic and bariatric surgery. 2024. View source

Author: Prepared for the Centre from the sources cited
References: See “Scientific sources and references” on this page

Gastric bypass