An experiment was conducted to determine whether the undigested high molecular fraction (HMF) of soybean protein restricts the 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.
Figure 3, taurine levels in both liver and muscle of fish fed all soybean proteins were lower than those of fish fed FM Taurine levels in fish fed DPSPI tended to be higher than those in fish fed the other soybean proteins However, no significant differences in taurine levels were detected between SPI- and DPSI-fed fish in liver, or between SBM- and SPI-fed fish in muscle Figure Taurine contents in liver and muscle of yellowtail fed the experimental diets Values are mean and standard deviation (n = 12) Different letters within each intestinal part denote significant differences (P < 0.05) 2.3 Discussion In the present study, growth performance in fish fed DPSPI was better than in fish fed other soybean proteins (SBM, SPI and DSPI) and was comparable to that of 119 Nguyen Phuc Hung and Peerapon Khaoian fish fed FM (Table 2) Low feed intake has been reported to be one reason for the low growth performance in fish fed plant protein-based diets [2] In the present study, the feed intake of yellowtail fed SBM, SPI and DSPI was higher than that of fish fed DPSPI and FM Therefore, low growth performance observed in fish fed SBM, SPI and DSPI was not due to a feed intake decrease It has been reported that FM is rich in taurine plant protein sources including soybean are a poor source of taurine [15], and taurine supplementation markedly improved growth performance of yellowtail being fed soy protein concentrate-based diets [13] In the present study, however, tissue taurine content did not differ among fish fed SBM, SPI, DSPI and DPSPI, and was lower than in fish fed with FM (Figure 3) This suggests that the low taurine content of soybean products is not the only factor reducing growth performance of yellowtail fed soybean protein Since DPSPI differs from other soybean proteins in the amount of undigested HMF, the presence of undigested HMF in fish fed other SBMs could be responsible for impaired growth rates In the present study, yellowtail fed SBM, SPI and DSPI showed significantly higher lipid contents in posterior intestinal digesta than did fish fed DPSPI (Table 4) On the other hand, the growth and lipid contents in muscle and liver of fish fed SBM, SPI and DSPI were lower than those in fish fed DPSPI and FM (Table 5) Since body lipid level is influenced by the efficiency of lipid digestion and absorption from the diet, the results of these studies suggest that fish fed SBM, SPI and DSPI have poor dietary lipid digestion and absorption, and the poor dietary lipid digestion and absorption is a factor responsible for the low growth observed in fish fed with these soybean proteins In the present study, total bile acid levels in the gallbladder and anterior intestine of fish fed SBM, SPI and DSPI were lower than those of fish fed DPSPI (Figures and 2) Bile acid is synthesized from cholesterol and is conjugated with taurine or glycine before being stored in the gallbladder [14] In yellowtail, conjugation is exclusive to taurine, and cholyltaurine and chenodeoxycholyltaurine are the main bile acids [5] Thus, lower bile acid levels found in the gallbladder and anterior intestine of SBM-, SPI-, and DSPI-fed fish could be the result of low body taurine levels On the other hand, bile acid levels in gallbladders of DPSPI-fed fish were comparable to those of FM-fed fish even though taurine in their body tissues was not higher than in fish fed other soybean proteins This could be due to a better bile acid reabsorption rate in DPSPI-fed fish compared with fish fed other soybean proteins The better reabsorption of total bile acid in DPSPI-fed fish could be due to an elimination of undigested HMF of soybean protein Therefore, fish fed DPSPI which did not contain the undigested HMF of soybean protein could have had better reabsorption rates of bile acids and higher gallbladder bile acid content even though taurine-conjugated bile acids supplied from biosynthesis were as low as in fish fed other soybean proteins due to low dietary taurine It has been reported that growth of yellowtail fed soy protein concentrate was improved by providing taurine supplements to make taurine 3% of the diet, which is 120 Effects of undigested soybean protein on lipid digestion and growth of yellowtail fish almost 10 times the amount found in FM-based diets [4, 13] This suggests that a soybean protein-based diet increases the need for taurine in yellowtail, possibly in order to increase synthesis of taurine-conjugated bile acids and to compensate for its excretion from the intestine In the present study, while there was no statistically different growth performance between DPSPI- and FM-fed fish, muscle and liver lipid contents in fish fed DPSPI were lower than in FM-fed fish These results suggest that, apart from the DPSPI diet being slightly lower in lipid content compared with FM diet, the quantity of bile acid reabsorbed from the intestine in yellowtail fed DPSPI was not sufficient for normal lipid digestion and absorption In addition, the bile acid level in the anterior intestinal digesta of DPSPI-fed fish was significantly lower than in that of FM-fed fish although no difference in bile acid levels in gallbladder was observed between these two groups Since the bile duct connects to the anterior part of the intestine, the results of the present study suggest that, apart from undigested HMF of soybean protein which reduces bile acid reabsorption, SBM may contain other factor(s) that interfere with secretion of bile acids into the intestine Conclusion In conclusion, undigested HMF of soybean protein reduced bile acid reabsorption and induced poor dietary lipid digestion and absorption in yellowtail fish The poor dietary lipid digestion and absorption was a factor responsible for low growth in yellowtail fed an SBM-based diet It is suggested that a supplementation of bile acids and/or the use of digested soybean protein could be an effective way to improve lipid digestion and growth of yellowtail fed SBM-based diets 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The Association of the Official Analytical Chemists, Inc., Arlington 122 ... secretion of bile acids into the intestine Conclusion In conclusion, undigested HMF of soybean protein reduced bile acid reabsorption and induced poor dietary lipid digestion and absorption in yellowtail. .. soy protein concentrate was improved by providing taurine supplements to make taurine 3% of the diet, which is 120 Effects of undigested soybean protein on lipid digestion and growth of yellowtail. .. efficiency of lipid digestion and absorption from the diet, the results of these studies suggest that fish fed SBM, SPI and DSPI have poor dietary lipid digestion and absorption, and the poor dietary lipid