Dietary folic acid requirement of grouper, Epinephelus malabaricus, and its effects on non-specific immune responses

To quantify the dietary folic acid (FA) requirement of grouper, Epinephelus malabaricus, and its effects on non-specific immune responses, FA was added to a basal diet at 0, 0.4, 0.8, 1.2, 2, 4, 6 and 10 mg FA/kg, providing actual dietary values of 0, 0.39, 0.72, 1.03, 1.89, 3.36, 5.85, and 9.52 mg FA/kg, respectively for a total of 8 experimental diets. Each diet was fed to triplicate groups of fish (initial body weight: 7.28 ± 0.03 g) in a recirculating seawater rearing system for 8 weeks. Weight gain (WG) was highest (P < 0.05) in fish fed diets with 0.72, 1.03 and ≥ 3.36 mg FA/kg, followed by 0.39 mg FA/kg, and lowest in fish fed the FA-free control diet. Fish fed diets with ≥ 0.72 mg FA/kg had higher feed efficiency (FE) but lower hepatosomatic index (HSI) than fish fed the control diet. Fish fed diets with ≥ 0.72 mg FA/kg had highest hepatic FA concentration, followed by 0.39 mg FA/kg, and lowest in fish fed the control diet. Hepatic thiobarbituric acid-reactive substance (TBARS) value was highest in fish fed the control diet, followed by 0.39 mg FA/kg diet, and lowest in fish fed diets with ≥ 0.72 mg FA/kg. Leukocyte superoxide anion (O2−) production ratio was higher in fish fed diets with ≥ 0.72 mg FA/kg than that in fish fed diets with ≤ 0.39 mg FA/kg. Fish fed diets with ≥ 0.72 mg FA/kg had higher plasma lysozyme activity than fish fed the control diet. Analyses of WG, HSI, hepatic FA concentration and hepatic TBARS value indicate that the optimal dietary FA level for juvenile grouper is about 0.8 mg FA/kg diet, this dietary level is also adequate for non-specific immune responses of the species.