The effect of dietary soybean isoflavones on the rate and efficiency of growth and carcass muscle content in pigs and rats
The in vitro biological properties of soy isoflavones suggest that they may negatively impact muscle growth in vivo. Three experiments were conducted to determine the impact of dietary soybean isoflavones on body and muscle growth in pigs and rats. In experiment one, pigs were self fed a basal diet containing 0 or 1585 ppm supplemental isoflavones from 6 to 30 kg body weight. The isoflavones source consisted of an extract of soybeans that contained the isoflavones genistein, daidzein, and glycitein in relative proportions and physical forms similar to that present in soybeans. Isoflavones increased (P < .08) daily body weight gain (579 vs 595) and carcass muscle content (12.48 vs 12.87 kg). Increased muscle growth was observed in predominantly red-fibered muscles. In experiment two, pigs were fed diets that contained supplemental genistein concentrations of 0, 200, 400, 600 and 800 ppm from 5 to 28 kg body weight. Serum genistein increased linearly (P < .01) from .55 to 3.44 muM as dietary genistein levels increased independent of stage of growth. Feed intake and body growth rate were decreased by dietary genistein during initial stages of growth, but were increased at later stages of growth in response to increasing dietary genistein. The combined weights of four red and four white muscles at 27 kg body weight, were not altered by dietary genistein concentration. In experiment three, gravid rats were fed a basal diet supplemented with 0, 431, 862 or 1724 ppm isoflavones supplied by a soy extract similar to experiment one. Growth of male offspring was not altered by maternal dietary isoflavones but growth and efficiency of feed utilization of female offspring increased (P < .04) with increasing maternal dietary isoflavone concentration. Hind limb muscle weights of male, but no female offspring, increased (P < .03) with increasing maternal isoflavone concentrations. Based on these data, dietary soy isoflavones increase muscle growth when fed either pre- or postnatally. Further, the inhibitory effects of genistein on muscle growth, which are observed in vitro at concentrations as low as 1.0 muM, are not observed in vivo with serum concentrations as high as 3.4 muM.