Moderate Methionine Reduction Alleviates Lipopolysaccharide-Induced Stress in Broiler Chickens by Enhancing Antioxidant Pathways

Methionine (Met), an essential amino acid involved in antioxidant defense and immune regulation in all vertebrates, may play a critical role in modulating acute immune stress responses; however, whether methionine reduction or supplementation in broilers is more beneficial during acute immune challenge remains unclear. To address this gap, this study compared the effects of dietary methionine reduction and supplementation on growth performance, antioxidant status, immune responses, and methionine metabolism in broilers subjected to lipopolysaccharide (LPS) challenge. In total, 504 one-day-old male broilers were assigned to four treatment groups: control (CON, 0.55%, marked as 100%Met), lipopolysaccharide-challenged (LPS, 0.55%, marked as 100%Met), methionine-restricted with LPS challenge (MR + LPS, 0.35%, marked as 60%Met), and methionine-supplemented with LPS challenge (MS + LPS, 0.75%, marked as 140%Met) groups. The experiment lasted for 21 days. On days 17, 19, and 21, broilers in the LPS-stimulated groups received intraperitoneal injections of LPS at 1 mg/kg body weight. Methionine restriction increased the feed conversion ratio before challenge, whereas average daily gain decreased in both LPS and MS + LPS groups during the challenge. Serum alanine aminotransferase, aspartate transaminase, LPS, corticosterone, interleukin-1β, interleukin-6, tumor necrosis factor-α, and hepatic malondialdehyde levels were reduced in the MR + LPS group compared with the LPS group (p < 0.05), whereas interleukin-10, antioxidant enzyme activities, total antioxidant capacity, and hepatic expression of antioxidant- and sulfur-metabolism-related genes were increased (p < 0.05). These findings indicate that moderate methionine restriction during acute immune stress enhances antioxidant capacity, alleviates hepatic burden, and supports metabolic stability in broilers.