What it means
The surgeon staples off a small pouch at the top of the stomach, then attaches part of the small intestine to it. Food skips most of the stomach and the first section of the small intestine. Digestive juices from the bypassed section rejoin further down.
This limits how much you can eat at once and how many calories are absorbed. It also changes gut hormones that affect hunger, fullness and blood sugar, which is why type 2 diabetes often improves soon after surgery.
Because fewer nutrients are absorbed, people need lifelong vitamin and mineral supplements and regular blood tests. Possible problems include dumping syndrome, ulcers, low blood sugar after meals and nutrient shortfalls.
The NIDDK notes that gastric bypass is difficult to reverse, though a surgeon may do so if medically necessary. Long-term follow-up with a bariatric team is part of the treatment, not an optional extra.
Why it matters
Some people compare bypass with GLP-1 medicines, and some use both at different points in care.