The intercalation of phospholipids, principally lecithin, between bile salt molecules improves the efficiency with which micelles solubilize cholesterol. Free bile salt molecules exist in equilibrium with mixed micelles in a water solution. The combined molar concentration of bile salt and phospholipid is about 11 times that of cholesterol. Cholesterol gallstone formation is potentiated by hepatic secretion of a bile containing excess cholesterol relative to the concentration of bile salt. This occurs most often because of increased biliary concentration of cholesterol but may also result from decreased bile acid secretion in certain disease states.
Excess cholesterol is solubilized in micelles and in vesicles composed of phospholipid bilayers.
Cholesterol crystal formation seems to occur at the surface of these vesicles. Supersaturated bile, however, is not the sole factor in gallstone formation. Bile proteins and lipoproteins also have the capacity to promote the nucleation of microcrystals and the growth of gallstones. Biliary stasis may also play a role. Gallbladder sludge and gallstones often form during pregnancy, parenteral nutrition, and rapid weight loss.