Lipid digestibility, bile drainage and development of morphological intestinal changes in rainbow trout (Oncorhynchus mykiss) fed diets containing defatted soybean meal

The aim of this experiment was to study the effect of defatted soybean meal (SBM) on lipid digestibility, bile acid level and morphological changes in the distal intestine (DI) in rainbow trout, and to assess whether the lipid digestibility and bile acid level are affected by the morphological changes in the DI. Triplicate groups of 0.35-kg trout were fed a diet with fish meal as the only high-protein feed ingredient (FM diet) or a diet with 30% SBM (SBM diet) for 40 days. Faeces, blood, intestinal chyme and tissue from the DI were sampled during the experimental period. Lipid digestibility was not significantly affected by diet. Trout fed the FM and SBM diets had similar bile acid concentration in the distal part of the DI (DI 2), whereas trout fed the SBM diet had gradually decreasing bile acid concentrations in the pyloric region (PR) and mid intestine (MI) over the 40-day experimental period. The levels of cholesterol and triacylglycerols in plasma were slightly lower in trout fed the SBM diet than the FM diet, although this was only significant at day 40 and day 5 for cholesterol and triacylglycerols, respectively. Gradually enhanced trypsin activity was evident in the DI 2 of trout fed the SBM diet. The trout developed SBM-induced enteritis, but the progression was slower than reported for Atlantic salmon. The results show that the bile acid concentration in the intestinal chyme was gradually lowered by soybean meal, and indicates that this was not due to increased faecal excretion of bile acids or soybean meal-induced enteritis.