The effect of semipurified diets containing different proportions of either casein or soybean protein on the concentration of cholesterol in whole serum, serum lipoproteins and liver in male and female rats.

Male and female lean Zucker strain rats were fed cholesterol-enriched semipurified diets containing 2 levels (20% and 50%, w/w) of either casein or soybean protein for a period of 14 weeks. In the female rats, the feeding of casein diets resulted in significantly higher levels of serum cholesterol than when diets containing soybean protein were fed. In addition, the hypercholesterolemic effect of dietary casein could be enhanced by increasing the proportion of this protein in the diet. Modulations in the proportion of dietary soybean protein did not significantly affect the serum cholesterol levels. In the male rats, however, no such differential effects were observed, indicating a difference between male and female rats in susceptibility to the induction of changes in serum cholesterol levels by dietary means. Upon feeding casein diets, both the male and female rats exhibited a shift of cholesterol from the high density lipoproteins to the lipoproteins with a lower density. This effect was more pronounced in the female than in the male rats. Liver cholesterol concentrations were markedly affected by modulations both in the type and proportion of dietary protein in both sexes. The concentration of cholesterol in the liver of the rats was highest in those fed the 50% casein diet and progressively lower in the animals on diets containing 20% casein, 20% soybean protein and 50% soybean protein.