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

A longitudinal cross-over study of serum cholesterol and lipoproteins in rabbits fed on semipurified diets containing either casein or soybean protein

1. Two groups, each of six rabbits, were fed on semi-purified diets containing either 400 g casein or 400 g soya-bean protein/kg for 20 d and then the diets of the two groups were crossed-over. 2. Just before the cross-over, the serum cholesterol concentration (mean ± SE) was 3068 ± 592 and 800 ± 143 mg/l for the groups fed on casein and soya-bean protein respectively. 3. Changes in the serum cholesterol concentration were observed 1 d after crossing-over the diets. By 10 d, the cholesterol levels in the two groups had also crossed-over. 4. The changes in serum cholesterol level after the cross-over were reflected in the very-low-density lipoproteins (VLDL) and low-density lipoproteins (LDL). 5. Lipoprotein protein concentrations in the LDL changed in the same way as cholesterol. In the VLDL however, the protein concentration decreased in both groups after the change in diet. 6. The cholesterol:protein values for the LDL and VLDL markedly increased in the rabbits changed from the soya-bean-protein diet to the casein diet, reaching a maximum 2 d after the cross-over. In the animals switched from casein to soya-bean protein, the values progressively declined. 7. The source of dietary protein exerts a rapid effect on the composition of both the VLDL and LDL which is proposed to be attributed to changes in the number and size of lipoprotein particles

Density profile and cholesterol concentration of serum lipoproteins in experimental animals and human subjects on hypercholesterolaemic diets

1. 1. The density profile of Sudan black stained serum lipoproteins was studied in human subjects and various animal species on diets supplemented with cholesterol. 2. 2. In the animals studied (rabbits, calves, mice, chickens, rats and guinea-pigs), the feeding of cholesterol resulted in an elevation of serum cholesterol levels together with marked changes in the density profile and the cholesterol concentration of the serum lipoproteins. Large differences between animal species in their response to dietary cholesterol were found. 3. 3. In a human subject, an increased concentration of serum cholesterol due to the consumption of a diet supplemented with six egg yolks per day was reflected in an elevated level of LDL cholesterol, while changes in the density profile of stained serum lipoproteins were not observed. 4. 4. In subjects with familial type III and type IV hyperlipoproteinaemia, marked differences in the density profile of lipoproteins were found when compared with that of normolipoproteinaemic subjects. 5. 5. The density profile of stained lipoproteins in the type III patients was remarkably similar to that in cholesterol-fed chickens and lean Zucker rats

Density profile and cholesterol concentration of serum lipoproteins in experimental animals and human subjects on hypercholesterolaemic diets

1. 1. The density profile of Sudan black stained serum lipoproteins was studied in human subjects and various animal species on diets supplemented with cholesterol. 2. 2. In the animals studied (rabbits, calves, mice, chickens, rats and guinea-pigs), the feeding of cholesterol resulted in an elevation of serum cholesterol levels together with marked changes in the density profile and the cholesterol concentration of the serum lipoproteins. Large differences between animal species in their response to dietary cholesterol were found. 3. 3. In a human subject, an increased concentration of serum cholesterol due to the consumption of a diet supplemented with six egg yolks per day was reflected in an elevated level of LDL cholesterol, while changes in the density profile of stained serum lipoproteins were not observed. 4. 4. In subjects with familial type III and type IV hyperlipoproteinaemia, marked differences in the density profile of lipoproteins were found when compared with that of normolipoproteinaemic subjects. 5. 5. The density profile of stained lipoproteins in the type III patients was remarkably similar to that in cholesterol-fed chickens and lean Zucker rats