The cholesterol-lowering effect of dietary soybean protein has been experimentally established in both animal and human subjects (1-3). The various mechanisms underlying the hypocholesterolemic action of soybean protein include the suppression of intestinal cholesterol absorption, promotion of fecal steroid excretion, and increases in the rate of low-density lipoprotein (LDL) receptor-mediated cholesterol removal (3-11). Soybean peptides containing high-molecular-weight (undigested) peptide fractions, obtained through the digestion of soybean protein isolate (SPI), are believed to play an important role in the hypocholesterolemic effect of soybean proteins (6-9). High-molecular-weight soybean peptides are responsible for bile acid-binding, which results in the stimulation of fecal steroid excretion (6-9).Soybean protein also has a hypotriglyceridemic effect that is mediated by the stimulation of fatty acid b-oxidation, suppression of fatty acid and triglyceride synthesis, and suppression of hepatic triglyceride secretion (3,(12)(13)(14)(15)(16). We have previously investigated the effect of certain soybean peptides, obtained through the digestion of SPI, on triglyceride metabolism in rats (17). Our results showed that the low-molecular-weight (digested) peptide fraction of dietary soybean (LD) caused a reduction in triglyceride secretion from perfused rat liver (17). Post-perfused liver triglyceride levels were also lowered by LD feeding (17). These reductions were inversely related to the stimulation of fatty acid b-oxidation in the perfused rat liver (17). Furthermore, the responses induced by LD appeared to be greater than those induced by intact SPI (17). Therefore, LD may modulate triglyceride metabolism and contains the component(s) responsible for the hypotriglyceridemic activity of soybean protein. Soybean pentapeptides (18) and oligopeptides (19) have been recently implicated as the components responsible for the triglyceride-lowering effect of soybeans. However, the particular peptides in soybean protein responsible for the triglyceride-lowering effect have not been identified. In this study, we investigated the effect of dietary LD on lipid parameters, particularly serum and liver triglyceride levels, in rats. MATERIALS AND METHODSPreparation of soybean peptides. Soybean protein isolate (SPI, Fujipro R, Fuji, Osaka, Japan) was added to 10 volumes of water, followed by artificial digestion with a commercial protease (protein AY10; Amano Enzyme, Summary We investigated the effects of dietary soybean peptides, particularly lowmolecular-weight peptides, on serum and hepatic concentrations of lipids in rats. Soybean protein isolate (SPI) was digested with protease to produce low-molecular-weight peptides (LD) or a mixture of high-and low-molecular-weight peptides (HLD). Rats were fed diets containing 20% casein, SPI, LD or HLD as a nitrogen source, with or without 0.5% cholesterol, for 2 wk. Next, rats were fed cholesterol-free diets containing 0%, 5%, 10%, or 20% LD at the expense of casein for 2 wk. Seru...
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