Further studies on the hypocholesterolaemic effect of soya-bean protein in rats.

1. The hypocholesterolaemic effect of soya-bean protein compared with that of casein, when fed at 200 g/kg diet for 3 weeks, was studied in rats. 2. Using a cholesterol-free low-fat (10 g maize oil/kg) diet, the hypocholesterolaemic effect was demonstrated not only with soya-bean protein but also with an amino acid mixture simulating this protein. The addition of lysine to these diets did not modify their intrinsic effects on serum cholesterol. 3. By feeding the vegetable protein or its amino acid mixture, both high-density-lipoprotein-and very-low-density-lipoprotein plus low-density-lipoprotein-cholesterol decreased to a similar extent, and the concentration of serum apolipoprotein A--I decreased and that of apolipoprotein B increased. 4. Although rats given soya-bean protein excreted more neutral and acidic steroids, the difference disappeared when the amino acid mixture was fed, indicating that the hypocholesterolaemic effect of the vegetable protein is not ascribed only to the increased elimination of steroids. 5. It is likely that the difference in the amino acid composition between animal and vegetable proteins is in fact responsible for the different response of serum cholesterol.