Dietary Supplementation with γ-Aminobutyric Acid Improves Growth, Digestive Enzyme Activity, Non-Specific Immunity and Disease Resistance against Streptococcus iniae in Juvenile Olive Flounder, Paralichthys olivaceus

SIMPLE SUMMARY: γ-aminobutyric acid (GABA) is a very important biomolecule that is found in all lifeforms and serves innumerable essential biological functions in pathways ranging from neural transmission to metabolism and immunity. In recent years, GABA was identified as an important metabolite involved in the modulation of the gut microbiome, and even appetite, in fish. The current trial aims to assess the effects of GABA as a supplement for nutritionally important biomarkers of fish health. Our results show that approximately 229–282 mg/kg of the total dietary GABA has important benefits for juvenile olive flounder, most significantly with improved disease resistance against Streptococcus iniae. ABSTRACT: Recent research is increasingly shedding light on the important role that microbial metabolites such as γ-aminobutyric acid (GABA) play in the context of nutrition, cognition, immune function, and the modulation of the gut microbiome. Yet, very few trials were conducted to assess the effects of its supplementation on biomarkers of fish health. Therefore, an eight-week feeding trial was devised to evaluate GABA supplementation in juvenile olive flounder, (Paralichthys olivaceus). A total of 630 fish with an average weight of 4.90 ± 0.10 g (±SD) were randomly assigned to one of seven triplicate groups and fed a non-GABA supplemented diet (CON, with 92 mg/kg GABA content), a positive control with 4 g/kg oxytetracycline (OTC), and five other diets supplemented with 50, 100, 150, 200 and 250 mg/kg GABA (corresponding to a total GABA content of 154, 229, 282, 327 and 352 mg/kg, respectively). Growth, blood chemistry, nonspecific immunity, digestive enzyme activity and disease resistance were assessed. The results showed that 100 and 150 mg/kg GABA supplementation consistently yielded significant improvements (p < 0.05) in growth, intestinal amylase, serum lysozyme, and survival against infection with Streptococcus iniae. Based on polynomial analysis, the optimal supplementation level was determined to be 237 mg/kg. These results support GABA as an important functional feed additive in juvenile olive flounder.