Recycling rate of bile acids in the enterohepatic recirculation as a major determinant of whole body ⁷⁵SeHCAT retention

Measurement of the whole body retention of orally administered ⁷⁵SeHCAT is used to investigate patients with unexplained diarrhoea. Retention values of <15 % at 7 days post-administration are taken to indicate bile acid malabsorption (BAM). Whilst idiopathic BAM is frequently diagnosed with ⁷⁵SeHCAT, functional and morphological studies of the terminal ileum rarely show any abnormality, so the disorder may be more appropriately termed bile acid diarrhoea (BAD). In addition to malabsorption, excess bile acid may reach the colon, where the events leading to diarrhoea take place, as a result firstly of increased bile acid synthesis and secondly of an increased recycling rate of bile acids. Increased recycling has been largely ignored as a cause of BAD, but, as shown in this study, can readily result in excess bile acids reaching the colon even when ileal absorption efficiency is normal (i.e. 95–97 %). There needs to be a re-evaluation of the causes of BAD in patients without a history of previous intestinal resection or evidence of ileal pathology, such as Crohn’s disease.