Requirements of vitamin C (L-ascorbyl-2-monophosphate-Mg and L-ascorbyl-2-monophosphate-Na) and its effects on immune responses of grouper, Epinephelus malabaricus

An 8-week feeding trial was conducted to evaluate two vitamin C derivatives, L-ascorbyl-2-monophosphate-Mg (C2MP-Mg) and L-ascorbyl-2-monophosphate-Na (C2MP-Na), to satisfy the vitamin C requirement and to test their effects on the immune responses of juvenile grouper, Epinephelus malabaricus. C2MP-Mg and C2MP-Na were each supplemented at 20, 50, 80, 150, 250, and 400 mg kg(-1) diet in the basal diet providing of 7, 18, 31, 51, 93, 145 mg ascorbic acid (AA) equivalent of C2MP-Mg kg(-1) diet and 4, 10, 17, 31, 47, 77 mg ascorbic acid (AA) equivalent of C2MP-Na kg(-1) diet, respectively. Basal diet without AA supplementation was included as control. Each diet was fed to triplicate groups of grouper (mean initial weight 3.20 +/- 0.05 g). Fish fed diets supplemented with either C2MP-Mg or C2MP-Na had significantly (P < 0.05) greater weight gain (WG), feed efficiency and survival than those fed the unsupplemented control diet. Liver ascorbate concentrations in fish generally increased as dietary C2MP-Mg or C2MP-Na supplementation level increased. Haemolytic complement activity was higher in fish fed diets supplemented with 92 mg AA equivalent of C2MP-Mg kg(-1) or 10-17 mg AA equivalent of C2MP-Na kg(-1) than fish fed the unsupplemented control diet. Lysozyme activity was higher in fish fed greater than or equal to51 mg AA equivalent of C2MP-Mg kg(-1) or greater than or equal to47 mg AA equivalent of C2MP-Na kg(-1) than fish fed the unsupplemented control diet. Analysis by broken-line regression of WG indicated that the adequate dietary vitamin C concentration from each vitamin C derivative in growing grouper is 17.9 mg AA equivalent of 2MP-Mg kg(-1) and 8.3 mg AA equivalent of C2MP-Na kg(-1), and it also indicated that C2MP-Mg is about 46% as effective as C2MP-Na in meeting the vitamin C requirement of grouper.