Distribution of biliary cholesterol between mixed micelles and nonmicelles in relation to fasting and feeding in humans.

To further investigate the nonmicelle mode of cholesterol transport in human bile, we examined its levels in relation to fasting and feeding. T-tube bile samples were collected (for 30 min) every 4 h over a 24-h period. All patients (3 with cholesterol gallstones; 1 with pigment stone) had their T tube clamped for a minimum of 4 days before the study to allow the bile acid pool to replete. Biliary lipid concentrations in all patients increased with feeding and decreased with fasting. However, because of a greater decrease in bile acid concentration relative to cholesterol concentration during the fasting period, fasting bile was consistently more saturated than bile obtained while feeding. Associated with the increase in lithogenic index with fasting was a decrease in the micelle solubilized cholesterol and an increase in nonmicelle solubilized cholesterol. At low bile acid concentrations (fasting) most biliary cholesterol is therefore transported as a nonmicelle complex, whereas at high bile acid concentrations (feeding) most of the cholesterol is transported in the mixed micelle. No nucleation of the biliary samples was observed in any of the patients over a 10-day period of observation. Thus the biliary nonmicelle complex (presumably vesicles) has a major cholesterol transport function especially at low bile acid concentrations.