Effects of different soybean proteins on lipid digestion and growth of yellowtail Seriola quinqueradiata

Two experiments were conducted to determine whether the undigested high molecular fraction (HMF) of soybean protein reduces growth of yellowtail through impairment of lipid digestion and absorption. Soybean meal (SBM), soy protein isolate (SPI), digested SPI (DSPI), and digested and purified SPI (DPSPI) were used as experimental diets. In experiment 1 (exp. 1), yellowtail were fed 4 diets with aforementioned soybean products for 3 weeks. In experiment 2 (exp. 2), the fish were fed soybean protein or fish meal (FM) diets for 8 weeks. Growth performance of DPSPI-fed fish was better than that of fish fed other soybean proteins, and comparable to that of fish fed with FM. Plasma and tissue lipid levels of DPSPI-fed fish tended to be higher than those of fish fed other soybean proteins. Significantly lower intestinal lipid levels and higher bile acid levels were found in DPSPI-fed fish than in fish fed with SBM, SPI, and DSPI. These findings indicate that one of the factors contributing to growth depression in yellowtail fed with SBM-based diets is the negative effect of undigested HMF of soybean protein on bile acid levels and lipid digestion.