Inhibition of cholesterol absorption in rats by plant sterols

The extent and site(s) of inhibition of cholesterol absorption by plant sterols, sitosterol and fucosterol, were studied in rats. The intragastric administration of a single emulsified lipid meal containing 25 mg [3H]cholesterol and 25 mg of either sitosterol or fucosterol inhibited the lymphatic absorption of cholesterol by 57% and 41%, respectively, in 24 hr. Less than 2% of each plant sterol was absorbed in the 24-hr period. In contrast, neither plant sterol (50 microM) inhibited cholesterol absorption when co-administered with equimolar amounts of cholesterol in phospholipid-bile salt micelles nor was either absorbed from the micellar solution. A series of in vitro studies was conducted to identify the site(s) of plant sterol inhibition of cholesterol absorption and to account for the difference in inhibitory effectiveness of sitosterol and fucosterol. A comparison of the micellar solubility of each sterol alone and in equimolar binary mixtures (to 2.0 mM) revealed that the solubility of individual sterols decreased in the following order: cholesterol, fucosterol, sitosterol, and that in binary mixtures cholesterol solubility was decreased by sitosterol and, to a lesser extent, by fucosterol relative to its solubility alone. A comparison between micellar-solubilized cholesterol and either sitosterol or fucosterol for binding to isolated brush border membranes, intestinal mucin, or for esterification by either cholesterol esterase or acyl coenzyme A:cholesterol acyltransferase revealed moderate to no competition. The data suggest that plant sterols displace cholesterol from bile salt (taurocholate) micelles and that sitosterol is more effective than fucosterol in this capacity.