Effects of different dietary vitamin C supplementations on growth performance, mucus immune responses and antioxidant status of loach (Misgurnus anguillicaudatus Cantor) juveniles

A 60-day feeding trial was conducted to determine the effects of different dietary vitamin C levels on growth performance, immune response and antioxidant capacity of loach juveniles. Six isonitrogenous (58.6% of crude protein), isoenergetic (17.5kJg(-1)) practical diets supplemented with 0 (VC0), 100 (VC100), 200 (VC200), 500 (VC500), 1000 (VC1000) and 5000mgkg(-1) (VC5000) of VC (35% ascorbic acid equivalent) were fed to fish (mean initial weight 0.11 +/- 0.02g) in triplicate. Results showed that fish fed VC0 diet had significantly lower body weight gain (BWG) and survival rate (SR). However, BWG and SR improved significantly in fish fed VC100 and VC200 diet respectively. Whole body ascorbic acid concentration increased with incremental dietary VC levels from 0 to 100mgkg(-1). The activity of mucus alkaline phophatase was significantly increased by the dietary VC level. Incremental levels of VC significantly reduced activities of glutathione peroxidase (GPx) and catalase. Moreover, fish fed diets containing more than 100mgkg(-1) VC significantly down-regulated the superoxide dismutase and GPx mRNA expression in liver. Meanwhile, the expressions of liver heat shock protein (HSP70) and nuclear factor-erythroid 2-related-2 (Nrf2) were affected by fish fed diets containing VC from 100 to 5000mgkg(-1). In conclusion, VC requirement of loach juveniles for optimum growth and functionally preventing lipid peroxidation was more than 200mgkg(-1) of diet. Moreover, high dose of VC supplementation did not show any detrimental effects on loach growth performance.