Effects of isoflavones containing soy protein isolate compared with fish protein on serum lipids and susceptibility of low density lipoprotein and liver lipids to in vitro oxidation in hamsters

The effects of dietary soy protein isolate (SPI), ethanol-extracted SPI (E- SPI) low in isoflavones, and fish protein (FP) on the concentration of bloo d lipids and the susceptibility of low density lipoprotein (LDL) to copper- induced oxidation were compared in male golden Syrian hamsters fed a modera te hypercholesterolemic semi-purified diet for 10 weeks. SPI, E-SPI, and FP were incorporated into the isonitrogenous experimental diets as protein so urces. The SPI group exhibited significantly lower serum total cholesterol concentration compared with the E-SPI group (P < 0.05) and the FP group (P < 0.01). Both the SPI and E-SPI groups showed lower LDL cholesterol (P < 0. 001 and P < 0.05, respectively) and less LDL apolipoprotein B (P < 0.01) co mpared with the FP group. The distribution pattern of serum lipoprotein cho lesterol fractions of the SPI and E-SPI groups were similar to each other, but different from that of the FP group. The lysine/arginine ratio of the t hree diets was significantly correlated with serum total cholesterol concen tration (r = 0.462, P = 0.023). The resistance of LDL to copper-induced oxi dation was greater in the SPI group than in the E-SPI and FP groups as asse ssed by the lower concentrations of thiobarbituric acid-reactive substances (TBARS) and the longer lag time required for the formation of conjugated d ienes (P < 0.01). Livers of hamsters fed the FP diet had a higher amount of TEARS than those of hamsters fed SPI (P < 0.01) and E-SPI (P < 0.05) diets . The SPI diet showed sparing effects on alpha-tocopherol contents in both serum and liver. It seems likely that soy isoflavones protect the circulati ng and membrane lipids by sparing alpha-tocopherol and endogenous antioxida nts. (C) Elsevier Science Inc. 1999. All rights reserved.