Soy Isoflavones Modify Liver Free Radical Scavenger Systems and Liver Parameters in Sprague–Dawley Rats

Soyfoods contain estrogenic isoflavones—namely, genistein (G) and daidzein (D)—that, like estrogens, display physiological effects in humans and animals. Previously we and others have demonstrated antioxidant and cardioprotective effects of orally ingested soy diets and soy isoflavones. The overall objective of this study was to test the effects of injected soy isoflavones, G and D, on liver lipids, liver free radical scavenger systems, and parameters of cardiovascular risk. Forty male rats were injected with G, D, estradiol (E), or a vehicle control (V) for 6 weeks. At the end of the study, body weight, food intake, feed efficiency ratio (FER), plasma glucose and cholesterol, abdominal fat pad weight, reproductive organ weight, liver weight, liver lipids, and liver free radical scavenger systems were compared. Food intake was significantly (P < .04) higher in the D-, G-, and E-treated animals compared with V-treated animals. FER was lower (P < .001) in D-, G-, and E-treated animals compared with the V- and the E-treated animals. Body weight, testis weight, and prostate weight were markedly (P < .001) lower in the E-treated animals compared with D-, G-, and V-treated animals. Intraabdominal fat pad weights were also significantly (P < .001) lower in the E group, although this effect was lost when corrected for body weight. Liver weights were considerably lower in the D-, G-, and E-treated animals versus the V group (P < .001). Total plasma cholesterol was reduced (P < .05) in D- and E-treated animals versus the V group. Liver lipids appeared to be unchanged by the isoflavones and slightly elevated by E treatment (P < .02). Liver catalase levels were numerically higher in the D- and E-treated animals compared with the V group (P < .1). Similarly, Cu/Zn superoxide dismutase (SOD) activity was significantly elevated in the D and E groups (P < .01), while G treatment (P < .03) elevated SOD to a lesser degree, versus the V group. These results suggest that subcutaneous injections of the naturally occurring soy isoflavone D and, to a lesser extent, G exert cardioprotective effects and stimulate antioxidant systems, while minimizing the undesirable effects elicited by E treatment.