Dietary vitamin C affects growth, antioxidant status and serum immune parameter of juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) fed low fishmeal diets

Abstract

Ascorbic acid (Vitamin C - VC) is one of the essential micronutrients required for normal growth and physiology of fish. In this research, the optimum dietary VC requirement of juvenile hybrid grouper (Epinephelus fuscoguttatus♀ × Epinephelus lanceolatus♂) was evaluated in a comparative feeding trial and subsequent oxidative stress test. Six isoenergetic (340 kcal/kg), isoproteic (49%) and isolipidic (7%) experimental diets were supplemented with graded amounts of L-ascorbyl-2-polyphosphate (analyzed VC concentrations of 76, 86, 104, 126, 137 and 152 mg/kg dry matter) and hand-fed to fish (initial weight of 15 ± 0.1 g/fish (means ± S.D.)) in three replicate cages twice a day (08:00 h and 16:30 h) to apparent satiation for 7 weeks. Percent weight gain (WG%), together with the VC concentrations in liver and head kidney, were elevated with increasing dietary VC contents, reaching a plateau in the VC137 treatment. The optimal dietary VC requirement of hybrid grouper was estimated to be 148 mg/kg dry matter based on broken-line regression analysis of WG%. Serum immunoglobulin M (IgM) concentrations and the relative mRNA expression of hepatic insulin-like growth factor-1 (IGF-1) displayed a similar variation tendency as WG%. At the transcriptional level, the activity of Nrf2-Keap1 pathway was up-regulated by VC supplementations, with the VC104 treatment showing the highest value. In summary, the optimal dietary VC requirement for the highest value of WG% of juvenile hybrid grouper was 148 mg/kg dry matter, and appropriate dietary VC supplementation improved oxidation resistance and serum IgM concentrations of the fish.