The effect of semipurified diets containing either casein or soybean protein on the concentration of serum cholesterol and the lipoprotein composition in rabbits

This thesis deals with the effect of dietary casein and soybean protein on the concentration of serum cholesterol and the lipoprotein composition in rabbits. Special attention has been paid to the time course of the changes produced by the protein in the diet.After a short introduction, a review of the studies on the effect of dietary protein on serum cholesterol is presented. From these studies, it was concluded that the effect of dietary protein on the concentration of serum cholesterol is only manifested in hypercholesterolaemic subjects and in experimental animals fed a diet high in cholesterol. An exception to this generalisation is the rabbit, an animal highly susceptible to the induction of hypercholesterolaemia and atherosclerosis. In the rabbit, dietary protein is also able to influence serum cholesterol levels when cholesterol-free diets are used.In the next chapter a method is presented for the separation of serum lipoproteins by density gradient ultracentrifugation. By staining the serum lipoproteins prior to ultracentrifugation the various lipoprotein classes can be easily localized in the gradient after the separation. By means of this technique it was observed that the density profile of the serum lipoproteins of rabbits and other experimental animals differs to that in man.Subsequently the time course of the changes in serum cholesterol concentration and lipoprotein composition were studied, when rabbits were transferred from a commercial diet to a semipurified diet containing either casein or soybean protein. It was observed that after only one day of feeding a semipurified diet containing casein the serum cholesterol levels had more than doubled. In the rabbits fed soybean protein the serum cholesterol level increased only slightly. The ingestion of semipurified diets resulted in a steep increase in the ratio of cholesterol to protein in all the serum lipoprotein fractions. This suggests that lipoprotein particles relatively rich in cholesterol wer