Synergistic Effects Between Dietary Zinc Form Supplementation and Dietary Protein Levels on Performance, Intestinal Functional Topography, Hemato-biochemical Indices, Immune, Oxidative Response, and Associated Gene Expression of Nile Tilapia Oreochromis niloticus

The present study investigates the effect of different dietary protein levels suboptimum level (25%) and optimum level (35%), different Zn forms bulk zinc oxide (BZnO) or nanoparticles zinc oxide (NZnO), and their interaction on performance, intestinal topography, hematology, serum biochemical, antioxidant-immune responses, and related gene expression of Nile tilapia. Six experimental diets were formulated to contain approximately 25% and 35% crude protein and supplemented with Zn forms with 0 (normal level in ingredients), 60 mg kg⁻¹ BZnO and 60 mg kg⁻¹ nanoparticles of NZnO. Nile tilapia, Oreochromis niloticus, fingerlings (7.53 ± 0. 06 g) were fed on one of tested diets in triplicates with 5% of total biomass three times a day for 84 days. Results showed that, fish fed diet containing 35% crude protein and supplemented with NZnO form recorded the highest final body weight (FBW), weight gain (WG), and specific growth rate (SGR). However, no significant (P > 0.05) differences were recorded in FBW, WG, SGR, feed conversion ratio (FCR), and protein efficiency ratio (PER) between fish fed diet containing 35% crude protein without Zn supplementation and fish fed diet containing 25% crude protein supplemented with NZnO form. Either fish fed diet containing 25% or 35% crude protein and supplemented with NZnO exhibited the highest values of villi height/width. The highest absorption surface area (ASA) was obtained in fish fed diet containing 25% or 35% crude protein and supplemented with BZnO. Hemoglobin (Hb), hematocrit (Hct), and red blood cell count (RBCs) highest values were obtained for fish fed diet containing protein level 35% supplemented with NZnO. Fish fed diet containing protein level 35% and supplemented with NZnO had the lowest value of alanine amino transferase (ALT) and aspartate amino transferase (AST). The highest globulin value was recorded for fish provided with diet containing 35% crude protein and supplemented with BZnO followed by those fed diet containing 35% crude protein and supplemented with NZnO. Fish fed diet containing protein level 25% with NZnO supplementation recorded the highest super oxide dismutase (SOD), catalase (CAT), glutathione reductase (GSH), and glutathione peroxidase (GPX₎, with decreasing malondialdehyde (MAD) values. The highest values of immunoglobulin g (IgG), immunoglobulin M (IgM), complement 4 (C4), and complement 3 (C3) were obtained for diet containing 35% crude protein and supplemented with NZnO form. Growth hormone gene (GH) was upregulated in fish fed 25% dietary protein without Zn supplementation, while it was downregulated in fish fed 25% dietary protein and supplemented with NZnO. Transcription of insulin-like growth factor-1 (IGF-I) gene recorded the highest value for fish fed 35% crude protein and supplemented with BZnO. This is although the diet of 35% crude protein + NZnO induced significant (IGF-I) gene expression compared with 25% crude protein with or without BZnO. Therefore, nano zinc is useful as a feed supplement for Nile tilapia (Oreochromis niloticus)