In pursuing more sustainable and efficient aquaculture, this study explores the concept of functional amino acids and their benefits in fish nutrition above and beyond the traditional approaches of nutritionally dispensable or indispensable amino acids (AA) . Functional AA are defined as those that participate and regulate key metabolic pathways to improve growth and health. Moreover, functional AAs supplementation is crucial for precision formulation in a challenge-oriented nutrition strategy. This study aimed to evaluate the dose-response effects of supplementing functional AA for Asian Seabass (Lates calcarifer) on growth and health. Parameters included midgut histo-morphology , digestive enzymes activity, liver and serum antioxidant capacity, blood biochemistry, immune and growth-related genes expression. The mix of functional AA used in this study was obtained from extensive hydrolysis of poultry keratin (KFAA), commercial name Kera-Stim®50. Five ison utrient diets, a control, and four diets supplemented with different KFAA levels: 0.25% (KFAA0.25), 0.50% (KFAA0.50), 0.75% (KFAA0.75) and 1.0% (KFAA1.0) were used in the experiment. F ish were fed to apparent satiation twice a day for eight weeks. Samples were collected at the end of the feeding trial after 24 h of fasting.
Overall, f ish fed the KFAA0.75 and KFAA1.0 diets resulted in significant (P < 0.05) improvement on most parameters evaluated. A regression analysis indicated an ideal inclusion of 0.876% for feed conversion and 0.752 for protein efficiency ratio. The main highlights were the midgut histo-morphology including the mucosal fold height, width, thickness, and goblet cell, were better developed in fish fed the KFAA1.0 diet . T he expression of growth-related genes including growth hormone (GH), GH receptor (GHR), insulin-like growth factor I (IGF-I), and IGF-II were greater in fish fed KFAA0.75. Antioxidant markers and innate immune enzymes were also evaluated and increased in fish fed KFAA0.75 and KFAA1.0 citing lysozyme, myeloperoxidase, catalase, and glutathione peroxidase. Digestive enzymes activity, (amylase, protease, and lipase) were boosted by the supplementation of ≥KFAA0.5%. Unlike, whole body fatty acids profiles were greater for polyunsaturated (C18:3 n6 and C18:3n3) in fish fed KFAA0.25 . These findings provide a scientific basis for applying FAAs as a sustainable solution for a more efficient and profitable aquaculture, especially for marine carnivores such as Asian seabass.