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 were formed. Furthermore, marked variations in the density profile of the serum lipoproteins were observed between individual rabbits fed semipurified diets.In chapter 4 the effect of higher proportions of casein in the diet on the enhancement of the hypercholesterolaemia produced by this protein was examined. A low casein diet (10%) resulted in lower serum cholesterol levels than did a high casein diet (40%), whereas a diet containing 20% casein produced intermediate concentrations of cholesterol in the serum. In the animals with the highest levels of total serum cholesterol (the 40% casein group) most of the cholesterol was transported in the very low density lipoproteins. With moderate hypercholesterolaemia (the 20% casein group), the low density lipoproteins were the main carrier of cholesterol. Elevations in lipoprotein cholesterol were associated with an increased ratio of cholesterol to protein in all of the groups.In the final experiment, the time course of the regression and progression of hypercholesterolaemia was studied when rabbits were transferred from a semipurified diet containing casein to a semipurified diet with soybean protein and vice versa. In this study high protein diets (40%) were used. When casein in the diet was replaced by soybean protein, a rapid decrease in serum cholesterol occurred. This decrease in serum cholesterol was initially reflected in a decrease in the amount of cholesterol in the very low density lipoproteins followed by a subsequent drop in the cholesterol in the low density lipoproteins. Conversely, the replacement of soybean protein by casein resulted in an steep elevation in the serum cholesterol levels, which was mainly caused by an increase in the cholesterol in the low density lipoproteins.These studies show that in rabbits very rapid and pronounced changes in serum cholesterol concentrations and lipoprotein composition can be produced by changing the type and amount of dietary protein. These findings underline the suitability of the rabbit as a model for studies of hypercholesterolaemia.