The current study evaluated the efficiency of three forms of dietary sodium propionate (SP) on growth performance, digestive and liver antioxidant enzymes activities, as well as skin mucus immune responses of koi carp (Cyprinus carpio koi) fingerlings. Gelatine micro and nanoparticles loaded with SP were synthetized and expressed as gelatine-SP MPs and gelatine-SP NPs, respectively. Basal diet was supplemented with gelatine microparticles, gelatine nanoparticles, powder-SP, gelatine-SP MPs and gelatine-SP NPs. Fish were distributed into 18 experimental units and fed with prepared diets for eight weeks. The results indicated that the growth performance improved in fish fed SP-supplemented diets in comparison with the controls. The highest growth performance was observed in gelatine-SP NPs followed by gelatine-SP MPs. The digestive enzymes activity including protease, trypsin, lipase and α-amylase in gelatine-encapsulated SP micro and nanoparticles dietary groups increased significantly in comparison with other dietary groups (p < .05). The use of SP-loaded micro and nanoparticles decreased the liver alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels in comparison to other groups (p < .05). Feeding fish with SP-supplemented diets led to increasing antioxidant enzymes of superoxide dismutase (SOD), glutathione (GSH) and catalase (CAT), as well as alkaline phosphatase (ALP) in the liver tissue in comparison to the controls (p < .05). The skin mucus lysozyme, alternative haemolytic complement activity (ACH50) and total immunoglobulin (Ig) levels were higher in SP-dietary groups than the controls; the highest levels were in gelatine-SP NPs dietary group (p < .05). The findings confirmed that the administration of SP loaded into micro and nanoparticles of gelatine could improve its efficiency in the diet of koi fish.

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