Dietary Sodium Butyrate Improves Intestinal Health of Triploid Oncorhynchus mykiss Fed a Low Fish Meal Diet

SIMPLE SUMMARY: Soybean meal is currently one of the most widely used ingredients in aquafeed due to its consistent supply, reasonable price, and well-balanced amino acid profile. However, anti-nutritional factors such as antigen proteins and soybean agglutinin in soybean meal can also cause intestinal barrier injury and inflammatory reactions in fish, harming fish growth, immunity, and intestinal health. Intestinal health is especially important for carnivorous fish such as rainbow trout (Oncorhynchus mykiss), which have a relatively small intestinal length index. In this study, it was confirmed that dietary NaB levels of 0.20% improved the growth of juvenile O. mykiss and reduced the severity of anti-nutritional factor-induced enteropathy by enhancing digestive enzymes and improving intestinal morphology. Dietary NaB enhanced the mRNA expression of TJ proteins while decreasing the mRNA expression of pro-inflammatory cytokines, NF-κB, and MLCK-MLC, likely reinforcing the intestinal mucosal barrier and protecting against pathogenic bacterial infections. In addition, the gut microbial composition was more conducive to O. mykiss health with the inclusion of 0.20% NaB in the diet. Furthermore, when challenged with Aeromonas salmonicida, dietary 0.20% NaB increased the survival rate of O. mykiss. This study provides a theoretical basis for the development of feed additives for use in low-fish meal feeds. ABSTRACT: This study aimed to determine the effects of dietary sodium butyrate (NaB) on the growth and gut health of triploid Oncorhynchus mykiss juveniles (8.86 ± 0.36 g) fed a low fish meal diet for 8 weeks, including the inflammatory response, histomorphology, and the composition and functional prediction of microbiota. Five isonitrogenous and isoenergetic practical diets (15.00% fish meal and 21.60% soybean meal) were supplemented with 0.00% (G1), 0.10% (G2), 0.20% (G3), 0.30% (G4), and 0.40% NaB (G5), respectively. After the feeding trial, the mortality for G3 challenged with Aeromonas salmonicida for 7 days was lower than that for G1 and G5. The optimal NaB requirement for triploid O. mykiss based on weight gain rate (WGR) and the specific growth rate (SGR) was estimated to be 0.22% and 0.20%, respectively. The activities of intestinal digestive enzymes increased in fish fed a NaB diet compared to G1 (p < 0.05). G1 also showed obvious signs of inflammation, but this inflammation was significantly alleviated with dietary NaB supplementation. In comparison, G3 exhibited a more complete intestinal mucosal morphology. Dietary 0.20% NaB may play an anti-inflammatory role by inhibiting the NF-κB-P65 inflammatory signaling pathway. Additionally, the relative abundance of probiotics was altered by dietary NaB. In conclusion, dietary 0.20% NaB improved the intestinal health of triploid O. mykiss fed a low fish meal diet.