Jejuno-ileal (JI) bypass is an obsolete bariatric surgery procedure. The term "jejuno-ileal" refers to two of the three parts of the small intestine. The jejunum is the very long middle portion of the small intestine, and the ileum is the shorter part at the end that empties into the large intestine.
JI bypass was performed by dividing the proximal jejunum and then plugging the top end of the jejunum into the ileum, much further down the small intestine. After the procedure, food would move from the stomach to the upper small intestine, then directly into the ileum, bypassing all of the absorptive areas of the jejunum. Thus, patients would eat a normal meal but would absorb only a small fraction of the calories it contained. This procedure is no longer performed for two reasons. First, patients with no jejunum cannot effectively absorb some vitamins, especially vitamin B complex and vitamin C. This leads to severe malnutrition and liver failure.
The second problem also arose from the bypassed jejunum, which emptied normally downstream into the ileum but had no 'upstream' connection and hence no inflow of food or liquids from the stomach. The flow of liquid through the intestine is necessary to cleanse it; with no liquid flow, bacteria from the large intestine are free to grow and colonize the small intestine, all the way up to the blind end of the jejunum. This leads to sepsis by bacterial translocation, so effectively in fact that JI bypasses are used in the lab to create sepsis in animal experimental models.