Sodium butyrate supplementation in high-soybean meal diets for juvenile rice field eel (Monopterus albus): Effects on growth, immune response and intestinal health

Abstract

An eight weeks feeding trial was conducted to assess the effects of dietary sodium butyrate (NaB) supplementation in diets with graded levels of soybean meal (SBM) on growth performance, antioxidant status, immune response, intestinal digestive enzymes activities and morphology of rice field eel Monopterus albus (initial body weight 14.87 ± 0.11 g). Using the 2 × 4 experimental design, eight isonitrogenous (42% crude protein) and isolipidic (5.5% crude lipid) experimental diets were formulated to contain two SBM levels (18% and 33%, designed as SBM18 and SBM33) and four levels of NaB (0, 250, 500, 1000 mg kg−1 dry diet). The results showed that NaB supplementation significantly increased weight gain (WG), serum superoxide dismutase (SOD) activity and significantly decreased malondialdehyde (MDA) content of fish fed diets with 33% SBM (P < .05). While no significant effects were noticed in SBM18 group (P > .05). At each NaB level, WG of fish fed with 33% soybean meal diet was significantly lower than that of 18% soybean meal except for at 500 mg kg−1 level. Not NaB level but SBM level significantly influenced the feed intake (FI) (P < .05). With the increasing SBM levels, FI significantly decreased at all the NaB supplementation levels (P < .05). At each soybean level, the serum lysozyme (LSZ) activity in fish fed NaB supplement diets was significantly higher compared to the non-supplement diets. Intestinal mucosal morphology was damaged in the SBM33 group without NaB, in contrast to a normal appearance found in fish fed the NaB supplement diets in the SBM33 group. Dietary NaB significantly increased the value of villus height/crypt depth and the intestinal trypsin activity in SBM33 group. At each soybean meal level, lymphocyte number and intestinal creatine kinase (CK) activity significantly increased with the increasing NaB supplementation level. Dietary NaB significantly up-regulated the expression abundance of proliferating cell nuclear antigen (pcda), ornithine decarboxylase (odc), sucrase-isomaltase (si), tight junction protein ZO-1 (zo-1) and alkaline phosphatase (alp), the markers of epithelial proliferation and differentiation. These results suggested that dietary inclusion of 33% SBM significantly reduced growth and feed utilization, lowered the non-specific immune response and damaged intestinal epithelial morphology compared to 18% SBM. Adding dietary NaB partially reversed these detrimental effects imposed by the increased SBM inclusion.