Effects of dietary soybean meal on the bile physiology in rainbow trout, Oncorhynchus mykiss

To clarify the effect of dietary soybean meal (SBM) on bile synthesis and enterohepatic circulation in rainbow trout, three isonitrogenous and isolipidic diets; a fishmeal based diet (diet FM), a SBM based non-fishmeal diet (diet SBM) and a 1% bile acid (taurocholate) supplemented SBM diet (diet C-tau) were prepared and acute/chronic effects of the diets on the bile physiology was examined. In the acute experiment (< 24 h), fish fed the SBM based diets (diet SBM and C-tau) had higher expression levels of bile acid synthesis genes, cyp7a1-2, cyp8b11 and cyp8b1-2 in the liver compared to fish fed the FM diet. In the chronic experiment (10 weeks), fish fed the SBM and C-tau diets had significantly lower expression levels of cyp7a1 (-1 and -2), cyp8b1 (-1 and -2) genes compared with fish fed the FM diet. On the other hand, expression of bile acid synthesis inhibitor gene shp-2 was higher in fish fed the SBM diet compared with fish fed the FM and C-tau diets. Furthermore, fish fed the SBM based diet had reduced bile acid absorption rate. These data suggest that SBM up-regulates bile acid synthesis on a short-term basis, while long-term SBM administration down-regulates the bile acid synthesis and the recovery of bile through the enterohepatic circulation. The gallbladder weights of fish fed the C-tau diet were similar to those of fish fed the FM diet whereas the actual bile acid absorption ability was still suppressed in the C-tau group. This study contributes to the general knowledge of bile physiology in teleost fish under different nutritional conditions and to develop tailored and sustainable aquaculture feeds based on SBM.