Relations between dietary salt type, acidosis and hyperlipemia in rabbits on casein containing semipurified diets

Casein-induced hypercholesterolaemia in rabbits is calcium-dependent

Young male rabbits were fed semi-purified diets containing either casein or soy protein, both at a normal (0.84%, w/w) and a high (1.44%, w/w) level of dietary calcium. At the normal calcium level, casein, as compared with soy protein, increased the concentration in serum of total and free cholesterol and the ratio of free cholesterol to phospholipid. Also, casein increased the intestinal phosphate absorption and decreased the faecal fat excretion. The hypercholesterolaemic response of the rabbits on the casein diet was significantly correlated with both phosphate absorption (r = +0.744) and fat excretion (r = -0.701). The increased amount of dietary calcium inhibited the casein-specific effects on both the intestinal and the serum lipid parameters. In contrast, calcium did not change these parameters in rabbits fed the soy protein diet. These results support the hypothesis that the degree of phosphorylation of casein is involved in the mechanism of the casein-induced hypercholesterolaemia by means of its effect on the enterohepatic cycle of bile acids.

The importance of the non-protein components of the diet in the plasma cholesterol response of rabbits to casein

To characterize the hypercholesterolaemic effect of casein further, four groups of young male rabbits in two separate experiments were placed on cholesterol-free semi-purified diets for 12 weeks. The diets were similar in composition, with either casein or soya-bean-protein isolate providing the protein source (250 g/kg). In two of these diets the salt mix was reduced by 45% (normally 40 g/kg) and replaced by potassium bicarbonate. Growth was unaffected by these alterations in dietary salts except for one group given the soya-bean-reduced-salts diet. The mean concentrations of plasma cholesterol were significantly higher in all casein-fed groups as compared with their soya-bean-fed counterparts but the response was much greater in those given the casein-reduced-salts diet. Contrary to expectations, analysis of the diets showed the zinc and copper concentrations of the casein diets to be less than those of the soya-bean diets. This was due to the greater concentrations of Cu (threefold) and Zn (twofold) in the soya-bean-protein isolate compared with casein. The mean concentration of Zn in fur was significantly decreased in casein-fed rabbits and these animals also excreted less Zn but more Cu in their urine than those given the casein-reduced-salts diet. The rabbits given the casein diet with the least salt mix showed the greatest degree of hypercholesterolaemia, suggesting an interaction between trace elements and the casein effect.

Regression of casein and cholesterol-induced hypercholesterolaemia in rabbits

Hypercholesterolaemia was induced in rabbits by feeding semipurified diets containing soy protein plus cholesterol (0.8 g/kg) or casein for four weeks. Subsequently for a period of six weeks, some of the rabbits were transferred to diets containing soy protein while others continued to receive the hypercholesterolaemic diets to which was added a mixture of amino acids (g/kg feed; glycine, 3.9; arginine, 6.9 and alanine, 1.6). Such additions increased the concentration of these amino acids in the casein diet to that in the hypocholesterolaemic soy protein diet. The cholesterol levels in the serum of the rabbits transferred to the soy protein diets declined rapidly, becoming significantly different from animals remaining on the hypercholesterolaemic diets after only three days. Serum cholesterol levels comparable to those in rabbits fed soy protein throughout the entire experiment were reached after about two weeks. The addition of the amino acids tended to reduce the concentration of cholesterol in the serum of the rabbits made hypercholesterolaemic by feeding the diets containing either casein or cholesterol. However, the effect reached significance only with the diet containing casein to which amino acids were added and then only at one time point. Six weeks after the cholesterol-fed animals were transferred to the cholesterol-free soy protein diet or to the diet containing the additional amino acids, apo E disappeared from the IDL1 (1.006 less than d less than 1.012 g/ml) and IDL2 (1.012 less than d less than 1.019 g/ml) fractions, but not from the VLDL fraction. Both with the cholesterol-free soy protein diet and the cholesterol diet fortified with amino acids, cholesterol in the VLDL fraction was reduced to values seen in animals fed the soy protein diet throughout the entire experimental period. The amount of cholesterol in the IDL and LDL fractions was decreased only in the soy protein group. Replacement of casein by soy protein, or the addition of amino acids to the casein diet did not induce the disappearance of apo E from the IDL or VLDL fractions. Only the soy protein diet lowered the amount of cholesterol in the VLDL and LDL fractions appreciably. It is concluded that the amount of apo E present in both IDL and VLDL does not invariably correlate with the level of serum cholesterol in rabbits. This study also indicates that the hypercholesterolaemic nature of casein resides only partially in the fact that it contains relatively low proportions of glycine, alanine and arginine compared with soy protein.

Failure of dietary-casein-induced acidosis to explain the hypercholesterolemia of casein-fed rabbits

The partial replacement of casein by a mixture of gelatin, fish protein and soy protein in cholesterol-free semipurified diets of rabbits reduced the hypercholesterolemic response. The partial replacement of casein by the protein mixture also increased the feed intake and alleviated or reversed the weight loss observed from the casein diet. The data indicate that casein alone is not an ideal protein source for rabbits probably because of the imbalance of the amino acid composition. When KCl in the semipurified diets was replaced by KHCO3, a higher feed intake and a better growth were obtained, irrespective of the protein source in the diet. In addition, the feeding of semipurified diets containing KCl resulted in acidosis, which could be prevented by its replacement with KHCO3. A semipurified diet containing casein and KCl produced a more severe acidosis and higher serum cholesterol levels than the diet containing the protein mixture. Nevertheless, the correction of the acidosis by the replacement of KCl in the diet by KHCO3 did not lead to an abrogation of the casein-induced hypercholesterolemia.

The effect of replacement of dietary soybean protein by casein on the fecal excretion of neutral steroids in rabbits

Der Einfluß des Ersatzes von Sojaprotein durch Kasein auf die Ausscheidung von Neutralfetten beim Kaninchen

Der Ersatz von Sojaprotein in einer halbsynthetischen Diät durch Kasein führte bei Kaninchen zu einem ständigen Anstieg von Cholesterin im Serum und zwar innerhalb von 15 Tagen um 75%. Gleichzeitig waren die Ausscheidung von Cholesterin und Coprostanol bei Kaseinfütterung signifikant erniedrigt und zwar bereits nach zwei Tagen um etwa 40%. Dieser Prozeß setzte sich in etwa 3 Wochen fort. Es wird vermutet, duß die herabgesetzte Ausscheidung von Neutralfetten mit dem Kot der Kausalfaktor ist für die Entwicklung einer Hypercholesterinämie des Kaninchens bei Kaseinfütterung.

The effect of age on the development of hypercholesterolemia in rabbits fed semipurified diets containing casein

Young and adult male rabbits were alternately fed semipurified diets and a commercial diet over a period of 57 weeks. The semipurified diets, containing either casein or soy protein, and the commercial diet were fed either ad libitum or on a restricted basis. When a restricted feeding regime was applied, both in the young and adult rabbits, significantly higher levels of serum cholesterol were observed in the animals fed casein compared with their counterparts fed soy protein. However, during the first period of feeding the semipurified diets, the hypercholesterolemic response of the casein diet was significantly greater in the young than in the adult rabbits. During the second and third period, no significant differences in cholesterolemic response were observed between young and adult rabbits fed casein diets. Further, the cholesterolemic response to semipurified diets containing casein was progressively lower during the second and third period. Similar results were found when the rabbits were fed ad libitum. However, in the adult rabbits no significant differences were observed between the rabbits fed casein and soy protein. Thus, the results of this study show that adult rabbits are less susceptible to the induction of hypercholesterolemia by feeding casein diets than are young ones.