Effects of the soybean antinutrients saponin, isoflavonoid, and phytosterol on morphology and immune responses in the distal intestine of Atlantic salmon

Abstract

Two experiments were included in this study. The first experiment was designed to investigate the effects of the soybean antinutrients saponins, isoflavonoids, phytosterols and the mix of them on the distal intestine morphology of Atlantic salmon. The fish were fed six different diets: positive control with soybean meal (SBM), negative control with soy protein concentrate (SPC), SPC added one of the following (level given as pure compound): 0.5% saponins (SPC+Sap), 1.5% isoflavonoids (SPC+Iso), 1% phytosterols (SPC+Phy), and a diet with SPC added a mix of the three antinutrients (SPC+Mix) in the same amounts as in the diet with the single inclusions. Fish fed SBM diets showed the typical characteristic of SBM induced enteritis (SBMIE) in the distal intestine, followed by mild signs of enteritis in fish fed SPC+Sap and SPC+mix. Fish with SPC, SPC+Iso and SPC+Phy showed normal intestinal morphology. Compared to fish fed SPC, somatic index of the distal intestine (DISI) was significantly lowered in fish fed SBM and SPC+Mix. Apparent digestibility of dry matter (DM), crude protein (CP) and lipid in fish fed SBM was significantly higher than in fish fed SPC-based diets. However, capacity of brush border membrane enzyme leucine aminopeptidase (LAP, mmol/kg body weight) in distal intestine (DI) and bile salt concentration of DI content were significantly decreased in fish fed SBM diet compared to fish fed SPC. Antinutrient supplementation tended to decrease bile salt concentration in DI chyme and plasma triglycerides (TG). The analyses of gene expression in DI tissue in fish fed SBM, showed upregulation of proinflammatory cytokine IL-17A, proliferating cell nuclear antigen PCNA, apoptosis gene caspase CASP6, heat shock protein 70 HSP70, tissue modeling gene collagenase3 MMP13 and down-regulation of proinflammatory cytokine IL-8 indicated activation of immune responses, increased cell proliferation, as well as increased apoptosis, stress and cell repair during the development of SBMIE. In addition, upregulation of tight junction protein genes CLDN25B and CDH1 and down-regulation of CLDN15, antioxidant gene catalase CAT, and water channel gene aquaporin AQP8AB suggested that SBM interfered with barrier functions and fluid permeability of distal intestine. For fish fed SPC with saponin and the mixed antinutrients, the expression of genes IL-8, AQP8AB, CLDN15, CLDN25B and CAT showed similar regulation as SBM-fed fish, indicating similar dysfunctions of DI. The second experiment was a follow-up study of the first, for which only the results of the feed quality recordings are presented in this thesis. The diets were the following: a negative control with soy protein concentrate as the main protein source (SPC), the SPC supplemented with saponins (SPC+sap) or with a crude saponin extract from SBM (SPC+imp), and a fourth diet in which SPC was replaced with a processed soybean meal (SBMX). The processed soybean meal was provided by the company Evonik. The aim of the processing was to reduce the level of antinutrients. In this study, only the physical quality of these four diets were analyzed and evaluated. Although lower expansion ratio was showed in SBMX and higher fat leakage in SPC+Sap, the overall variations between the diets were small and would most likely not affect the nutritional value of the diets as eaten by the fish.