Reduced absorptive surface, rapid transit and loss of specialised ileal function
Expansion
Normal small bowel length is about 300 to 600 cm. Symptoms appear when less than about 200 cm remains, and dependence on parenteral nutrition is likely below 100 cm without a colon.
Determinants of outcome
- Length remaining
- Which segment: losing the ileum is worse than losing the jejunum, since the ileum absorbs B12 and bile salts and adapts better
- Ileocaecal valve preserved: slows transit and limits bacterial overgrowth
- Colon in continuity: salvages energy as short-chain fatty acids, worth up to 1000 kcal a day, and absorbs fluid
Early problems: massive fluid and electrolyte loss, and gastric acid hypersecretion, because inhibitory gut hormones are lost.
Later problems: malabsorption, bile salt diarrhoea, gallstones, oxalate renal stones (unabsorbed fat binds calcium, so oxalate is absorbed freely), bacterial overgrowth and D-lactic acidosis.
Intestinal adaptation over 1 to 2 years increases villus height and absorptive capacity, aided by enteral feeding, which is why enteral nutrition is maintained even when parenteral support is needed.