Fecal steroid excretion of rats fed rice bran oil and oryzanol
Rice bran oil (RBO) has been considered healthy cooking oil with potent effects to reduce the risks of atherosclerosis and coronary heart disease. The relatively high level of unsaponifiable components contributes to the healthy benefits of RBO. This study investigated the possibility that the cholesterol-lowering effects of oryzanol are through increased fecal steroid excretion. The objective was to compare fecal cholesterol and bile acid excretion as well as bioavailability among different forms of oryzanol. The correlation between bioactivity and bioavailability of oryzanol was investigated as well. Forty-seven female breeder rats were used as animal models. Oryzanol was added to an AIN-93 M maintenance diet at 2.8g/kg as the treatments. Treatments were either oryzanol in RBO, dissolved oil form of oryzanol in corn oil, or crystalline form of oryzanol added directly to the diet. Control was corn oil. Fecal samples were collected at week 6 and 10. Fecal cholesterol, oryzanol, and bile acids were analyzed with GC-FID. Results showed no significant differences between the two collection periods (P>0.05). Oryzanol in RBO produced the greatest fecal cholesterol excretion among the diet groups (P<0.05). Both fecal cholesterol and total bile acid excretion were significantly higher in rats fed RBO oryzanol than the average of the other two treatment groups (P<0.001). It may indicate that the hypocholesterolemic activity of RBO with oryzanol was more effective than oryzanol in corn oil, through synergistic effects of the components in the unsaponifiable contents. The possible mechanism for RBO and oryzanol to lower cholesterol levels appeared to be through inhibiting cholesterol absorption and bile acid reabsorption to enhance fecal cholesterol and bile acid excretion. The effect of treatment compared to control was greater for bile acid excretion than for cholesterol excretion, which may suggest the primary mechanism of the hypocholesterolemic effect of oryzanol may be through the interference of bile acid reabsorption in the enterohepatic circulation. The correlation among different forms of oryzanol to the bioactivity was unclear, although the results showed fecal recovery of oryzanol was positively related to fecal steroid excretion. It might indicate the absorbability of oryzanol was negatively related to fecal steroid excretion.