Studies on the Mechanism of Antihypercholesterolemic Action of Soy Protein and Soy Protein-Type Amino Acid Mixtures in Relation to the Casein Counterparts in Rats

Abstract

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.

Comparison of the Mechanisms Proposed to Explain the Hypocholesterolemic Effect of Soybean Protein versus Casein in Experimental Animals

“…Digestion and absorption of intact proteins differs from that of single amino acids. Nagata et al (11) have shown that the feeding of amino acid mixtures simulating soybean protein or casein produced serum cholesterol concentrations in rats that were identical to those seen after feeding the intact proteins. However, the amino acid mixtures and intact proteins had different effects on other aspects of cholesterol metabolism, such as fecal steroid excretion (11).…”

“…There is direct evidence that soybean protein impairs cholesterol absorption (10)(11)(12). There is only indirect evidence that soybean protein inhibits the absorption of bile acids: as mentioned above , in animals fed soybean protein, bile acid excretion in feces is increased.…”

Effects of Dietary Soybean Peptides on Hepatic Production of Ketone Bodies and Secretion of Triglyceride by Perfused Rat Liver

“…29) Sugano et al have indicated that the effect of soybean protein on cholesterol metabolism might not be fully elucidated by the amino acid profile, and that the sequence of amino acid linkage and peptides formed in the process of protein digestion might be more important than amino acid composition. 19) Since the amino acid profile is comparable among the SPI, LD3, and HD1 used in the present study, the hypolipidemic activities, that is, the triglyceride-and cholesterol-lowering activities, of soybean peptides are estimated to be caused by lowmolecular peptide and high-molecular peptide per se respectively, but not the differences in amino acid composition.…”

Effects of Simultaneous Intake of Soybean Protein and Diacylglycerol on Lipid Profiles and Body Fat Accumulation in Rats

“…7) These effects were attributed to a reduction in cholesterol absorption and increases in fecal steroid excretions in rabbits and rats. 8,9) It is also assumed that SPI has cholesterol-lowering effects by stimulating cholesterol 7 -hydroxylase, the key enzyme converting cholesterol to bile acid. 10) On the other hand, DAG, which consists mainly of sn-1, 3 species, has been shown to suppress body fat accumulation in mice much more than dietary TG with a similar fatty acid composition, 11) and also to prevent increases in postprandial serum TG concentrations in healthy humans.…”