The concentration of butyric acid in the intestine increased with the increase in the content of fermentable dietary fibre; however, the potential physiological impact of a high dose of butyric acid on fish has not been sufficiently studied. The aim of this study was to investigate the effect of two dosages of butyric acid on the growth and health of the liver and intestine of the largemouth bass (Micropterus salmoides). Sodium butyrate (SB) was added to the diet at 0 g/kg (CON), 2 g/kg (SB2), and 20 g/kg (SB20), and the juvenile largemouth bass were fed to apparent satiation for 56 days. No significant difference was observed in the specific growth rate or hepatosomatic index among the groups ( ). The concentration of β-hydroxybutyric acid in the liver, the activities of alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase, and the concentrations of triglyceride and total cholesterol in serum increased significantly in the SB20 group compared to the CON group ( ). The relative expression of fas, acc, il1b, nfkb, and tnfa in the liver of the SB20 groups was also significantly higher than that of the CON group ( ). The above indicators in the group SB2 had similar change tendencies. The expression of nfkb and il1b in the intestine of both the SB2 and SB20 groups was significantly downregulated compared with that in the CON group ( ). The size of hepatocytes was enlarged, and the intracellular lipid droplets and the degree of hepatic fibrosis were increased in the SB20 group compared to the CON group. There was no significant difference in intestinal morphology among the groups. The above results indicated that neither 2 g/kg nor 20 g/kg SB had a positive effect on the growth of largemouth bass, while a high dosage of SB induced liver fat accumulation and fibrosis.

The effects of plant protein sources (PPSs) on the health of the liver and intestine of the largemouth bass, Micropterus salmoides, were compared to verify the potential damaging effects of dietary fiber (DF). A diet containing 55% fish meal (FM) was used as the control. The test diets contained 25% soybean meal (SBM), rapeseed meal (RSM), cottonseed meal, or peanut meal, and the FM content was decreased to 30%. The protein and lipid contents of these five diets were balanced by casein and oil. Fish were raised for 8 weeks. The fish fed the diet containing PPS showed a trend of decreasing growth and apparent digestibility coefficients. The contents of total bile acid, lipid, and collagen in the liver were increased, and the mRNA expression levels of genes encoding inflammatory factors and enzymes involved in de novo fatty acid synthesis and bile acid synthesis were upregulated. Both the lipid and collagen contents in the liver were positively correlated with the DF content in the diet significantly. Morphology and histology showed reduced liver size, hepatic steatosis, and fibrosis in fish fed diets containing PPS. The lowest hepatosomatic index was observed in fish fed the SBM diet, and the most severe damage was observed in fish fed the RSM diet. No obvious histological abnormalities were observed in the hindgut. The bile acid profile in the liver could be used to distinguish the types of PPS very well by Fisher discriminant analysis. These results indicated that 25% of each of the four PPSs in the diet exceeded the tolerance range of largemouth bass and caused liver damage, which might be mediated by bile acid. DF in PPS might be an important agent contributing to liver damage.

Black soldier fly (Hermetia illucens) larvae are a promising source of insect lipids, characterized by rapid fatty acid accumulation and a high lauric acid content. This study investigated the effects of dietary black soldier fly oil (BSFO) on muscle quality in largemouth bass (Micropterus salmoides). Experimental diets were formulated to be isonitrogenous, isolipidic, and isophosphoric, with 1.0% and 2.0% BSFO partially replacing soybean oil. A control group received 2.3% soybean oil without BSFO or glycerol monolaurate (GML), while positive controls were supplemented with 0.35% and 0.7% GML. Fish (initial weight: 25.08 ± 0.12 g) were cultured in pond cages for 56 days, and three replicates were established for each treatment group. Muscle quality and nutritional traits were evaluated, including proximate composition, fatty acid profiles, texture properties, fiber diameter, hydroxyproline content, antioxidant capacity, and expression of genes related to myogenesis, atrophy, apoptosis, and mTOR signaling. Compared with the control, the 2.0% BSFO group showed a significant increase in muscle hydroxyproline content (p < 0.05), while GML supplementation led to a significant decrease (p < 0.05). In the 1.0% BSFO group, muscle saturated fatty acid (SFA) and monounsaturated fatty acid (MUFA) contents were unchanged (p > 0.05), but n-3/n-6 polyunsaturated fatty acid (PUFA) ratios and highly unsaturated fatty acid (HUFA) levels were significantly elevated (p < 0.05). The dietary supplementation of BSFO enhanced the levels of high-quality fatty acids in the muscle tissue. Antioxidant capacity was also significantly enhanced in the 1.0% BSFO group (p < 0.05) but reduced in the GML groups (p < 0.05). Texture analysis showed that BSFO significantly improved muscle hardness, elasticity, chewiness, and gumminess (p < 0.05). Gene expression analysis revealed no significant effects of BSFO on genes related to myogenesis (myod and myog) and muscle atrophy (mstn and murf1), or apoptosis-related genes (caspase8, caspase9, and caspase3) (p > 0.05); mTOR signaling pathway-related genes (s6k1 and akt1) were significantly upregulated in the 2.0% BSFO group (p < 0.05). In contrast, 0.7% GML significantly upregulated genes related to myogenesis (myod, myf5, and myog), muscle atrophy (mstn, fbxo32, and murf1), and apoptosis (caspase8, caspase9, and caspase3) (p < 0.05). In summary, dietary supplementation with 2.0% BSFO effectively enhances muscle quality in largemouth bass without negatively impacting muscle development.