Effects of dietary soy isoflavones on feed intake, growth performance and digestibility in juvenile Japanese flounder (Paralichthys olivaceus)

An 8-week feeding trial was conducted to investigate the effects of dietary soy isoflavones on feeding intake, growth performance, and digestion of juvenile Japanese flounder (Paralichthys olivaceus). Four isonitrogenous (49% crude protein) and isoenergetic (20.1 MJ kg−1) diets were formulated to contain four graded levels of soy isoflavones, namely, 0, 1, 4 and 8 g soy isoflavones in 1 kg of diet. Each diet was randomly fed to triplicate tanks of fish (Initial average weight: 2.58 g ± 0.01 g), and each tank was stocked with 35 fish. No significant difference was observed among diets with levels of 0, 1 and 4 g kg−1 soy isoflavones in feed intake, weight gain, feed efficiency ratio (FER), proximate composition of fish whole body and apparent digestibility coefficients (ADC) of nutrients and energy (P>0.05). However, high dietary soy isoflavones level (8 g kg−1) significantly depressed weight gain, FER, whole-body crude lipid content of fish and ADC of nutrients (P<0.05). These results indicate that high level of dietary soy isoflavones (above 4 g kg−1) significantly depresses growth responses and FER of Japanese flounder. However, as the content of soy isoflavones in soybean meal is around 1 to 3 g kg−1, the adverse effects might be neglected when soybean products are used as a fish feed ingredient.