What is it?

A gastric bypass is a stomach operation in which a small upper stomach or pouch of 20 to 30 cc is created, which will make you feel full sooner. This pouch is connected with a small intestinal loop through which the food is transported. Digestive juices (from the remnant stomach, pancreas and gall) only enter after 150 cm to start digestion (i.e. malabsorptive effect).

The gastric bypass has both a restrictive effect (restricted intake of nutrients) and a malabsorptive effect (less absorption of nutrients).

Its advantage is that both ‘volume eaters’ and so-called ‘sweet eaters’ are candidates for this surgery (due to the reduced absorption of sugars). Reflux symptoms may decrease or disappear, and patients do not develop (foul-smelling) diarrhoea since the largest part of the small intestine takes care of digestion.

The gastric bypass is the golden standard and therefore the most commonly performed weight loss surgery. The procedure is also fully reversible.


The main weight loss takes place in the first year after surgery with 15% of the original weight after three months, 20 to 25% after six months and 30 to 30% after 12 months. A (small) weight gain of 5% after the second post-operative year is quite normal.

Side effects

  • Staple-line leaks, bleeding, abscess and thromboembolism are the most important complications that may occur during or shortly after the procedure. In the long term, stomach ulcers, gall stones and internal hernia or obstruction are the most common complications.
  • Dumping syndrome: this is one of the most important side effects after a gastric bypass. This causes nausea, heart palpitations, sweating, dizziness, and more briefly after eating sugary or fatty foods or eating too much or too quickly. The symptoms usually disappear by themselves (and often improve by lying down). In the first instance, treatment consist of dietary adjustments and if this is insufficiently effective, medication may be started (diabetes medication or somatostatin).
  • Vitamin and/or mineral deficiencies (iron, vitamin B12, folic acid, etcetera) can be avoided by taking a daily multivitamin.

  • Hair loss and/or brittle nails may occur from the third month after surgery due to substantial, rapid weight loss; this will recover by itself nine months after surgery.

  • Finally, gastroscopy or ERCP through the classic route (through the mouth) will be difficult since the pouch is closed off from the remnant stomach.

Centres and specialist areas

Obesity Centre

Latest publication date: 05/02/2021