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Objective: The objective of this study was to evaluate effects of heat treatment and soybean oil inclusion on protein oxidation of soy protein isolate (SPI) and of oxidized protein on redox status of broilers at an early age. Methods: SPI mixed with soybean oil (SPIO) heated at 100°C for 8 h was used to evaluate protein oxidation of SPI. A total of two hundred and sixteen 1-day-old Arbor Acres chicks were divided into 3 groups with 6 replicates of 12 birds, receiving basal diet (CON), heat-oxidized SPI diet (HSPI) or mixture of SPI and 2% soybean oil diet (HSPIO) for 21 d, respectively. Results: Increased protein carbonyl, decreased protein sulfhydryl of SPI were observed as heating time increased in all treatments (p<0.05). Addition of 2% soybean oil increased protein carbonyl of SPI at 8 h heating (p<0.05). Dietary HSPI and HSPIO decreased the average daily gain of broilers as compared with the CON (p<0.05). Broilers fed HSPI and HSPIO exhibited decreased glutathione (GSH) in serum, catalase activity and total sulfhydryl in liver and increased malondialdehyde (MDA) and protein carbonyl in serum, advanced oxidation protein products (AOPPs) in liver and protein carbonyl in jejunal mucosa as compared with that of the CON (p<0.05). Additionally, broilers receiving HSPIO showed decreased glutathione peroxidase activity (GSH-Px) in serum, GSH and hydroxyl radical scavenging capacity in liver, GSH-Px activity in duodenal mucosa, GSH-Px activity and superoxide anion radical scavenging capacity in jejunal mucosa and increased AOPPs in serum, MDA and protein carbonyl in liver, MDA and AOPPs in jejunal mucosa (p<0.05). Conclusion: Protein oxidation of SPI can be induced by heat and soybean oil and oxidized protein resulted in redox imbalance in broilers at an early age. [ABSTRACT FROM AUTHOR]