The rate of biliary bile acid excretion is a product of bile acid pool size and enterohepatic cycling frequency. Cycling frequency, in turn, is determined by the time necessary for bile acid to travel from the ampulla of Vater to its small bowel absorptive site, from its absorptive site to the liver, and from the liver to the duodenum. In 10 subjects receiving constant intraduodenal infusion of an isocaloric formula containing the nonabsorbable markers polyethylene glycol and β-sitosterol, the authors altered small intestinal treinsit time and measured the effect upon biliary bile acid excretion rate. Sorbitol (15% formula) shortened transit time by 38% and increased hourly bile acid excretion rate by 31%. Atropine (0.04 mg i.m. q 2 hr) increased transit time by 38% and decreased hourly bile acid excretion rate by 38%. Biliary excretion rates of cholesterol and of bilirubin, substances excreted in bile but not subject to enterohepatic circulation, did not change with changes in transit time. This suggests that sorbitol and atropine were not acting upon the biliary limb of the enterohepatic circulation but rather on the intestinal limb. The authors conclude that small bowel transit time is a major determinant of enterohepatic cycling frequency.
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