The distribution of cholesterol between lipoprotein fractions of serum from rabbits fed semipurified diets containing casein and either coconut oil or corn oil

Feeds of animal origin in rabbit nutrition – a review

Rabbits are classified as obligate herbivores. However, under natural conditions, some members of the family Leporidae incorporate animal products into their diets. Therefore, it seems biologically justified to supplement the diets of farmed rabbits with feeds of animal origin as sources of protein, fat and minerals. The aim of this review was to describe, from a historical perspective, the use of various feeds of animal origin in rabbit nutrition. The applicability of by-products from mammal, poultry, fish and invertebrate processing for rabbit feeding was evaluated, including the future prospects for their use. A review of the available literature revealed that various animal-based feeds can be valuable protein sources in rabbit diets, but their inclusion levels should not exceed 5-10%. Studies investigating their efficacy have been conducted since the 1970s. In some regions of the world, the use of animal-derived protein in livestock feeds was prohibited due to the risk of spreading bovine spongiform encephalopathy (BSE). However, the interest in animal by-products as protein sources in livestock diets is likely to increase since the above ban has been lifted.

The hypercholesterolemic effect of dietary coconut fat versus corn oil in hypo- or hyperresponsive rabbits is not exerted through influencing cholesterol absorption

In two inbred strains of rabbits with high or low response of plasma cholesterol to dietary saturated versus polyunsaturated fatty acids, the efficiency of intestinal cholesterol absorption was measured. The feeding of a cholesterol-free purified diet containing saturated fatty acids in the form of coconut fat, when compared with a diet containing corn oil as polyunsaturated fatty acids, did not influence the efficiency of cholesterol absorption in the two rabbit strains. Irrespective of the dietary fat source, the hyperresponsive rabbits absorbed cholesterol more efficiently. It is concluded that the hypercholesterolemic effect of dietary coconut fat versus corn oil is not exerted by influencing cholesterol absorption.

Dietary animal proteins and cholesterol metabolism in rabbits

The effect in rabbits of giving isonitrogenous purified diets containing casein, ovalbumin, fish protein, milk-whey protein and soya-bean protein were compared. The diets were balanced for cholesterol and for the amount and type of fat. When incorporated into low-cholesterol diets (0.08 g cholesterol/kg), casein, ovalbumin and soya-bean protein produced similar levels of serum cholesterol. With a high background of dietary cholesterol (1.5 g/kg), serum cholesterol concentrations increased with soya-bean protein, whey protein, casein and fish protein, in that order. Thus, the hypercholesterolaemic effect of casein in carefully balanced diets was only seen against a high-cholesterol background. The development of hypercholesterolaemia produced by giving fish protein was different from that produced by casein. First, less cholesterol accumulated in the very-low-density-lipoprotein fractions and more in the lipoproteins of higher density with fish protein than with casein. Second, fish protein, unlike casein, did not increase liver cholesterol. Third, transfer of rabbits from a diet containing soya-bean protein to one containing casein resulted in an immediate marked depression in neutral steroid and bile acid excretion in faeces. However, when rabbits were fed on the diet with fish protein after the diet with soya-bean protein, there was no significant depression in neutral steroid output and the depression in bile acid output was delayed. The present study suggests that different animal proteins cause hypercholesterolaemia by different mechanisms.

Effects of cholesterol-free, semipurified diets containing different levels of casein or soy protein on distribution of cholesterol and protein among serum lipoproteins of rabbits

Rabbits fed cholesterol-free, low-fat, semipurified diets have more cholesterol and protein in serum low density lipoprotein (LDL) relative to high density lipoprotein (HDL) than rabbits fed Chow diet. This difference was accentuated by a casein semipurified diet but was also observed with a soy protein diet even though the latter did not produce an elevation of serum cholesterol. To investigate the reason for these differences, the formulation of the semipurified diets was altered by reducing the level of protein from 27 to 16%, increasing the fat from 1 to 4% and the fiber from 5 to 13%, to correspond more closely to the proportions in Chow. With this formulation, the soy protein diet gave a lipoprotein pattern similar to that of Chow, whereas the casein diet produced a moderately elevated serum cholesterol level with more cholesterol in LDL than in HDL. When the protein in the newly formulated diets was increased back to 27%, the lipoprotein patterns reverted to those obtained with the original formula. In this case, soy protein-fed rabbits had moderately elevated serum cholesterol whereas casein-fed animals showed hypercholesterolemia. These results indicate that the altered lipoprotein pattern observed previously in rabbits fed semipurified diets is related to the high level of protein in those diets.

Evidence for an inverse relation between plasma triglyceride and aortic cholesterol in the coconut oil/cholesterol-fed rabbit

Rabbits fed a commercial chow diet containing 0.5% cholesterol and 14% coconut oil developed more severe hyperlipidemia and atherosclerosis than rabbits fed the same diet containing olive oil in place of coconut oil. Average plasma cholesterol was twice as high in the coconut oil/cholesterol-fed rabbits than in olive oil/cholesterol-fed rabbits. Final plasma triglycerides, although highly variable, were approx. 20-fold higher than basal plasma triglyceride in coconut oil/cholesterol-fed rabbits; plasma triglyceride in olive oil/cholesterol-fed rabbits remained unchanged throughout the study period. In coconut oil/cholesterol-fed rabbits, a direct relationship between plasma triglyceride and aortic cholesterol was not found. Plasma cholesterol and aortic cholesterol were also not correlated at a statistically significant level (r = 0.26, P greater than 0.25). However, when both plasma cholesterol and triglyceride were simultaneously introduced as predictors of aortic cholesterol, the correlation between these plasma lipids and aortic cholesterol became highly significant (r = 0.64, P less than 0.02). Aortic cholesterol increased in proportion to plasma cholesterol concentrations but appeared to be inversely related to plasma triglyceride levels.

[Effect of milk constituents on lipid metabolism]

The cholesterol lowering effect of various milk constituents was examined in six trials with growing pigs. Each trial is described. By means of main component analysis 44 factors were tested in order to see whether they influenced the lipid metabolism. We observed that there was no specific component in the milk which showed a cholesterol lowering effect. Milk and milk constituents change the nutrient composition of the food and can, e.g. by means of the changed amino acid pattern, indirectly lower the lipid content in the blood.

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.

The effect of dietary saturated fat versus polyunsaturated fat on serum cholesterol and phospholipid concentrations in rabbits with partial ileal bypass

Rabbits were fed semipurified diets containing either corn oil or coconut fat. Dietary coconut fat significantly elevated serum total cholesterol and phospholipid concentrations. Rabbits with partial ileal bypass (PIB) had significantly lower serum cholesterol and phospholipid values, irrespective of whether the diet contained corn oil or coconut fat. The effect on serum lipids of the type of fat was similar in control and PIB animals. Since PIB rabbits are known to excrete extremely high amounts of steroids with the feces, we suggest that our data point to a lack of interaction of the type of dietary fat with the fecal excretion of steroids.

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.

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.