Bile acid diarrhea
In patients with ileal resection or disease, the part of the small intestine with high-affinity bile acid transporters has been removed or is dysfunctional. Thus excessive bile acid finds its way into the colon. If the bile acid concentration in colonic contents reaches a critical level of approximately 3 to 5 mmol/L, salt and water absorption by the colonic mucosa is inhibited and diarrhea results. Patients who have had extensive small bowel resections (more than 100 cm) often have so much fluid entering the colon that this critical bile acid level is not reached, even though bile acid malabsorption may be extensive.
In addition to this classic form of diarrhea caused by bile acid malabsorption, some investigators have speculated that bile acid malabsorption causes chronic diarrhea in some patients with an intact ileum. Although tests of bile acid absorption frequently are abnormal in patients with idiopathic diarrhea, treatment with bile acid–sequestering resins, such as cholestyramine, is not often as effective in this group of patients as in those who have had surgical resection of the ileum.