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.