Evidence suggests that fish are more tolerant than mammals to imbalanced dietary amino acid profiles. However, the behavioral and physiological responses of fish to individual deficiencies in dietary indispensable amino acids (IDAA) remain unclear. This study examined how stomachless fish respond to diets deficient in limiting IDAA (lysine, methionine, and threonine), using Zebrafish (Danio rerio) as a model. The response to deficient diets was assessed based on; 1) growth performance and feeding efficiency; 2) feed intake; 3) expression of appetite-regulating hormones and nutrient-sensing receptors; and 4) muscle postprandial free amino acid (FAA) levels. There were 6 treatments, each with 3 replicate tanks. A semi-purified diet was formulated for each group. The CG diet was based on casein and gelatin, while the FAA50 diet had 50 % of dietary protein supplied with crystalline amino acids. Both were formulated to contain matching, balanced amino acid profiles. The remaining diets were formulated the same as the FAA50 diet, with minor adjustments to create deficiencies in selected IDAA. The (-) Lys, (-) Met, and (-) Thr diets had lysine, methionine, and threonine withheld from the FAA mix, respectively, and the Def diet was deficient in all three. The juvenile Zebrafish were fed to satiation 3 times daily from 21 to 50 days-post-hatch. Results showed that 50 % replacement of dietary protein with crystalline amino acids significantly reduced growth of juvenile Zebrafish. There were no significant differences in growth between the FAA50 group and groups that received deficient diets. The deficiency of singular IDAA did not induce significant changes in feed intake; however, the combined deficiency in the Def diet caused a significant increase in feed intake. This increased feed intake led to decreased feeding efficiency. A significant decrease in feeding efficiency was also observed in the (-) Lys group. There was an observed upregulation of neuropeptide Y (NPY), an orexigenic hormone, in the Def group. Overall, results from this study suggest stomachless fish increase feed intake when challenged with IDAA-deficient diets, and the regulation of NPY might play a role in this response.
