Given evidence of a hypocholesterolemic effect in rats, soy protein compared with casein showed the following effects on cholesterol dynamics: a) lower cholesterol absorption and greater fecal steroid excretion, b) more rapid turnover of serum cholesterol, c) a marked size reduction of the rapidly exchangeable cholesterol pool (pool A) through a significant increase in the removal rate in that compartment without influencing the production rate, and d) greater hepatic sterogenesis both in vitro and in vivo. In contrast, the amino acid mixture equivalent to soy protein compared with casein-type mixture showed: a) no effects on cholesterol absorption and fecal steroid excretion, b) no effects on the turnover of serum cholesterol, c) the reduction of pool A size as a result of decreased production, and d) the reduction of hepatic steroidogenesis in vitro, but not in vivo. These results indicate that decreased intestinal absorption of cholesterol, and increased fecal steroid excretion are primarily responsible for the antihypercholesterolemic effect of soy protein compared with casein. The fecal loss of steroids far outweighs the activation of hepatic steroidogenesis. In the case of the soy protein-type amino acid mixture, the depression of hepatic cholesterol synthesis seems accountable for reducing serum cholesterol levels.

1. The effect of variation in arginine: lysine on the relative cholesterolaemic effects of dietary soya-bean protein and casein was studied. Male rats received semi-purified diets containing soya-bean protein isolate or casein supplemented respectively with varing amounts of lysine or arginine for 40 d and blood samples were taken after a 5 h fast.2. Neither the addition of arginine to casein nor lysine to soya-bean protein modified the intrinsic effect of these proteins on serum cholesterol.3. Serum triglyceride levels tended to rise with increasing amounts of lysine supplementation. The opposite trend was obtained with arginine supplementation.4. Casein caused an increase in the concentration of serum insulin, but not glucagon. The glucagon level was increased proportionately with increasing amounts of arginine, while the addition of lysine showed no effect. The effects of added amino acids on serum insulin were inconclusive.5. There was a parallel increase in serum apo E and glucagon in response to arginine supplementation, while lysine supplementation increased serum apo E. 6. Thus, arginine:lysine was more effective in regulating serum triglyceride than serum cholesterol. Insulin was associated with different effects of these proteins on serum lipids.

Male rats were fed for 4 weeks cholesterol-free diets containing proteins from different sources, milk, fish, egg yolk, soybean, rice and peanut. The antihypercholesterolemic effect of vegetable proteins compared to animal proteins was certified. There was a negative correlation (γ = -0.74) between the serum cholesterol level and the arginine/lysine ratio of dietary protein, suggesting a role of this ratio in determining the serum cholesterol level. In general, the fasting level of circulating insulin was lower whereas that of glucagon was higher on feeding vegetable proteins, thus resulting in a fall of the insulin/glucagon ratio. Changes in the hormonal status may be relevant to the protein effect.