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Long‐term consumption of high‐carbohydrate feed may adversely affect intestinal health of fish; however, the underlying roles remain ambiguous. This study examined the effects of varying carbohydrate levels on the intestinal flora of common carp and assessed how microbial metabolites influence intestinal health. Two hundred seventy common carps were chosen and distributed randomly into three groups that fed diets containing starch at levels of 15% (low‐carbohydrate diet [LCD]), 28% (medium‐carbohydrate diet [MCD]), and 45% (high‐carbohydrate diet [HCD]) for 60 days. A significant increase in final body weight, weight gain rate, and specific growth rate within the MCD group, while feed conversion ratio exhibited a decrease in comparison to the other groups (p < 0.05). Feeding with a HCD led to decreased activity of catalase and increased malondialdehyde content, which was consistent with reverse transcription‐quantitative real‐time polymerase chain reaction (RT‐qPCR) analysis results (p < 0.05). Specifically, the RT‐qPCR results revealed that HCD treatment significantly upregulated il1β, il6, and il8 transcript levels. Whereas, the il10 messenger RNA (mRNA) was markedly reduced in comparison to the LCD group. Furthermore, the HCD group exhibited an increased abundance of Proteobacteria, accompanied by a reduction in Fusobacteria abundance, and also revealed an upsurge in opportunistic pathogenic bacteria, such as Aeromonas and Shewanella. The correlation analysis demonstrated negative correlations of anti‐inflammatory active substances such as fucoxanthin, (S)‐reticuline, hecogenin, and uridine with Aeromonas, but positive correlations with Luteolibacter. In summary, dietary carbohydrates might mediate intestinal flora to regulate their metabolites and affect intestinal inflammatory response. [ABSTRACT FROM AUTHOR]

In the present study, we conducted a 10‐week culture experiment to investigate the effects of taurine on the growth performance, antioxidant properties, and muscle quality of the common carp fed an oxidized lipid diet. There were five experimental groups with three replicates each. Based on the fresh fish oil group (FO), equal amounts of oxidized fish oil (with a thiobarbituric acid‐reactive substances value of 49.57 ± 2.34 mg/kg) and 0 g/kg (OFO), 4 g/kg (OT4), 8 g/kg (OT8), and 12 g/kg (OT12) taurine were added to the diet, while the same composition ratio was maintained by changing the microcrystalline cellulose content. Compared to the FO group, the feed conversion ratio, weight gain rate, muscle crude lipid, and n‐3/n‐6 polyunsaturated fatty acids (PUFA) ratio were significantly lower in the group OFO (P < 0.05). In addition, compared to the FO group, OFO fish showed an increased content of malondialdehyde and protein carbonylation and decreased hardness, brightness, pH, superoxide dismutase, and catalase levels in the muscle (P < 0.05). Notably, the growth index significantly improved in the OT4 group compared to that in the OFO group (P < 0.05). In addition, dietary taurine increased the crude lipid content, n‐3/n‐6 PUFA, antioxidant capacity, hardness, springiness, brightness, pH, and collagen content in the muscle compared with OFO fish (P < 0.05). Moreover, taurine supplementation significantly reduced myofiber diameter and increased myofiber density (P < 0.05) and enhanced the expression levels of paired box 7 (pax7), myogenic factor 5 (myf5), myogenic differentiation antigen (myod), and myogenic regulatory factor 4 (mrf4) compared with that of the OFO group (P < 0.05). Considering growth performance and muscle quality, the optimal supplemental levels of taurine in the oxidized lipid diet were 4 g/kg and 6.84–7.70 g/kg, respectively. [ABSTRACT FROM AUTHOR]

Apelin plays pivotal roles in fluid homeostasis, food intake and energy metabolism in mammals. However, the regulatory role of apelin in glucose metabolism in teleosts is still unclear. In this study, we investigated the regulatory role of Pyr‐apelin‐13 in glucose metabolism of common carp (Cyprinuscarpio L.). The results showed that apelin mRNA abundance was significantly increased in the foregut and hepatopancreas after glucose injection. Insulin and glucagon also up‐regulated apelin mRNA levels in the foregut, hepatopancreas and hepatocytes. In vivo, intraperitoneal injection of Pyr‐apelin‐13 induced hypoglycaemia through the up‐regulation of mRNA expression and activity of hexokinase (HK) coupled with the down‐regulation of mRNA expression and activity of glucose‐6‐phosphatase (G6Pase) in the hepatopancreas. In vitro, upon incubation of the primary hepatocytes with Pyr‐apelin‐13 (0, 10, 100 and 1000 nM), the mRNA expression of glucokinase (GK) was increased, whereas the mRNA expression of phosphoenolpyruvate carboxykinase (PEPCK) and G6Pase was reduced. For glucose transport, the glucose transporter 2 (GLUT2) mRNA expression increased in the hepatopancreas, intestine, primary enterocytes and hepatocytes after Pyr‐apelin‐13 treatment. Collectively, our data may provide a basis for further studies on elucidation of the role and mechanism of apelin in glucose metabolism of teleosts. [ABSTRACT FROM AUTHOR]