Dietary Protein Sources Modify the Cholesterol-Lowering Effect of Saponins in Chicks.

Abstract

Effects of casein (CAS) and casein hydrolysate (CHY) as the dietary protein sources on serum and liver cholesterol concentrations in chicks were compared in the first experiment. Next, effects of CAS, CHY and soybean protein isolate (SPI) on the cholesterol-lowering action of Quillaja or tea saponin were investigated. Seven-day-old male chicks were fed experimental diets added with cholesterol for 10 days. In experiment 1, chicks were given diets containing 20 and 30% crude protein. Increasing protein content in CAS and CHY diets suppressed the elevation of serum total cholesterol and liver cholesterol, but CHY was less effective than CAS in lowering LDL-cholesterol concentrations. When beef tallow was added to diets containing 20% crude protein in experiment 2, chicks fed CAS diet had lower serum total cholesterol concentrations than those fed CHY diet. CHY was water-soluble, and approximately half of total nitrogen was amino-acid nitrogen. These results indicate that the hypocholesterolemic effect of dietary CAS relative to CHY is related to the structure of protein. Serum total cholesterol and liver cholesterol concentrations were much higher in chicks fed CHY diet than those fed SPI diet. However, by the cholesterol-lowering action of saponins, there were no differences in these concentrations between both groups. The two saponins also lowered serum total cholesterol and liver cholesterol concentrations in chicks fed CAS diet, but their concentrations were still higher than those of chicks given another protein diets. Thus, in addition to the interference between saponins and steroids in the intestine, it seems likely that dietary proteins affect the cholesterol-lowering action of saponins.