CHOLESTEROL 7-ALPHA-HYDROXYLASE ACTIVITY IS INCREASED BY DIETARY MODIFICATION WITH PSYLLIUM HYDROCOLLOID, PECTIN, CHOLESTEROL AND CHOLESTYRAMINE IN RATS

Sources of dietary fiber known to alter cholesterol metabolism and/or bile acid pool size were fed to rats, and activity of the rate-limitin g step in bile acid synthesis, cholesterol 7 alpha-hydroxylase, was me asured. In the first experiment, semipurified diets containing 5% cell ulose, psyllium hydrocolloid, pectin or oat bran as dietary fiber sour ces or 2% cholestyramine were fed to groups of 10 male Wistar rats for 4 wk. In the second experiment, groups of six rats were fed diets con taining 5% cellulose, rice bran, oat bran or psyllium with and without 0.25% cholesterol. In the first experiment, the activity of cholester ol 7 alpha-hydroxylase (pmol . min(-1). mg protein(-1)) was highest in the cholestyramine-treated group (95.6 +/- 3.6), followed by groups f ed psyllium (35.5 +/- 3.5) or pectin (36.0 +/- 4.5), which exhibited m ore than twice the enzyme activity of groups fed cellulose (16.9 +/- 1 .9) or oat bran (12.3 +/- 2.0). In the second experiment, feeding chol esterol resulted in significantly higher enzyme activity when cellulos e (65%), oat bran (118%) and rice bran (60%) were fed, but no differen ce in activity was observed when cholesterol was added to the psyllium -containing diet. Higher activity of cholesterol 7 alpha-hydroxylase w hen pectin or psyllium rather than cellulose was fed may explain the a lmost twofold higher bile acid pool sizes previously reported in respo nse to feeding either of these fibers. These data support the hypothes is that the hypocholesterolemic effect of soluble fibers is modulated through increased synthesis and therefore pool size of bile acids.