Serum and liver cholesterol levels of rats and mice fed soy-bean protein or casein.

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Abstract
Rats and mice were fed soybean protein or casein diets for 10 and 50 wk, respectively, during which terms their serum cholesterol levels were analyzed periodically. Rats fed high-cholesterol diets containing soy protein or the amino acid mixture simulating soy protein produced lower levels of serum cholesterol throughout the experiments, as compared with those on the corresponding casein-type diets. Feeding soy protein resulted in a significant decrease in serum apo[apolipoprotein]A-I and apoB, but the relative concentration of high-density lipoprotein-cholesterol was kept at the higher level. The concentration of liver cholesterol was also lower in rats fed the plant protein. In mice fed a cholesterol-free diet, the cholesterol-lowering effect of soy protein was noticeable at an early stage of the feeding periods, by 20 wk. The extent of lipid peroxidation in rats and mice determined as TBA[thiobarbituric acid]-reactive substances in serum was the same when protein diets were given, while it was significantly higher when an amino acid mixture of the soy protein type was fed to rats. Soy protein apparently exhibits its hypocholesterolemic effect even when a diet rich in cholesterol is fed. The cholesterol-lowering effect of soy proteins is apparently a phenomenon common to rodents.