Optimal Dietary Vitamin A Fortification Improves Growth Performance, Nutrient Metabolism, Antioxidant Capacity, Immune Response, and Histomorphology in Intensively Cultured Litopenaeus vannamei

Vitamin A (VA) functions as a vital micronutrient supporting growth, immune competence, and stress tolerance in shrimp, though its requirement under industrial high-density rearing conditions has not been established. This study assessed how graded dietary VA levels affect growth, nutrient metabolism, antioxidant capacity, immune function, and hepatopancreas structure in Litopenaeus vannamei reared at 720 shrimp/m3. Five experimental diets were formulated with graded VA supplementation at 0 (A0, control), 3000 (A1), 6000 (A2), 12,000 (A3), and 24,000 IU/kg (A4). During an 8-week feeding trial, the weight gain rate (WGR) and specific growth rate (SGR) increased and subsequently decreased with rising dietary VA, while the feed conversion ratio (FCR) showed the opposite trend; the A3 group exhibited optimal performance. In the A2 and A3 groups, trypsin, lipase, and amylase activities were enhanced, accompanied by the upregulation of protein synthesis-related genes (tor, s6k) and fatty acid oxidation-related genes (cpt-1, aco), as well as the downregulation of lipogenic genes (acc, fas) (p < 0.05), which resulted in an elevated whole-body crude protein content (p < 0.05). In the hepatopancreas, activities and transcript levels of SOD, GSH-Px, and PO increased, the MDA content decreased, and histological status improved. Immune-related genes (tlr4, myd88, nf-κB, imd, relish, lzm, crustins) were upregulated, whereas hsp70 and aif were downregulated (p < 0.05), suggesting enhanced innate immune potential and reduced stress-associated responses. Under the high-density culture system in this study, L. vannamei exhibited improved growth performance when the dietary VA level was 12,647 IU/kg. Based on weight gain results, the dietary VA concentration required for maximum shrimp growth under this stocking density was estimated to be 22,800 IU/kg. This value is model-derived rather than an established dietary requirement and is subject to uncertainty, requiring further experimental validation.