Cholesterol supplementation attenuates the hypocholesterolemic effect of rice bran oil in rats.

ABSTRACT

Rice bran oil (RBO), when blended with safflower oil (SFO) at the ratio of 7 to 3, has been shown to lower serum cholesterol in humans consuming cholesterol. The mechanism as to how this oil blend exerts its effect is not yet clear. This study examined the effect of cholesterol supplementation on the cholesterol-lowering ability of different RBO/SFO blends. Male Sprague Dawley rats (4 wk old) were fed purified diets containing 10% fat with or without the addition of 0.5% cholesterol for 3 wk. The fat was either SFO or RBO alone, or the mixture of these two oils at the ratio of 7 3 (7S/3R), 55 (5S/5R), or 37 (3S/7R). Without cholesterol supplementation, there were no significant differences in the serum and liver total cholesterol levels among different dietary fats. However, the HDL cholesterol level of rats fed the RBO-containing diets (especially in rats fed the 3S/7R diet) was higher than that of rats fed the diet containing SFO alone. This resulted in an increase in the ratio of HDL/total cholesterol-a desirable outcome. Supplementation of the diets with 0.5% cholesterol significantly increased the cholesterol level in both the serum and the liver. Increasing the proportion of RBO in the diet further raised the total cholesterol level in the serum whereas it reduced liver cholesterol. Then, the specific effect of the 3S/7R mixture on the ratio of HDL/total cholesterol disappeared. These findings suggest that cholesterol supplemented at the level of 0.5% in this study masked the cholesterol-lowering effect of RBO. Smaller percentages of polyunsaturated fatty acid (i.e., 182n-6) in the RBO-containing diets than in the SFO diet might have reduced their ability to dispose the circulating serum cholesterol into the liver.