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-limiting step in bile acid synthesis, cholesterol 7 alpha-hydroxylase, was measured. In the first experiment, semipurified diets containing 5% cellulose, psyllium hydrocolloid, pectin or oat bran as dietary fiber sources 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 containing 5% cellulose, rice bran, oat bran or psyllium with and without 0.25% cholesterol. In the first experiment, the activity of cholesterol 7 alpha-hydroxylase (pmol.min-1.mg protein-1) was highest in the cholestyramine-treated group (95.6 +/- 3.6), followed by groups fed psyllium (35.5 +/- 3.5) or pectin (36.0 +/- 4.5), which exhibited more 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 cholesterol resulted in significantly higher enzyme activity when cellulose (65%), oat bran (118%) and rice bran (60%) were fed, but no difference in activity was observed when cholesterol was added to the psyllium-containing diet. Higher activity of cholesterol 7 alpha-hydroxylase when pectin or psyllium rather than cellulose was fed may explain the almost twofold higher bile acid pool sizes previously reported in response to feeding either of these fibers. These data support the hypothesis that the hypocholesterolemic effect of soluble fibers is modulated through increased synthesis and therefore pool size of bile acids.