Dose–Response Evaluation of Dietary Curcumin Supplementation on Ileal Morphology and Antioxidant Status in Broiler Chickens

This study aimed to evaluate suitable dietary curcumin supplementation levels for improving intestinal morphology and antioxidant status in medium-growing broiler chickens fed a practical corn–soybean meal diet from 57 to 84 d of age. A total of 252 male broilers (57 d of age; 1092.86 ± 8.91 g) were randomly assigned to six dietary treatments with seven replicate cages of six birds each. Broilers were fed diets supplemented with 0, 80, 160, 240, 320, or 400 mg/kg curcumin for 28 d. Asymptotic, broken-line, and quadratic models were fitted to dose–response data to estimate suitable dietary curcumin supplementation levels. Curcumin supplementation improved ileal morphology and antioxidant status, as evidenced by increased villus height (VH), VH/crypt depth (CD), villus surface area, total antioxidant capacity, and superoxide dismutase (SOD) activity, together with decreased CD and malondialdehyde (MDA) content. Curcumin also upregulated the mRNA expression of catalase (CAT), SOD, glutathione peroxidase (GPX)1, GPX3, GPX4, and nuclear factor erythroid 2-related factor 2 (Nrf2) and increased Nrf2 protein abundance. Among the candidate models, broken-line regression provided the best fit for key ileal antioxidant indices, indicating that approximately 188 mg/kg of dietary curcumin may be sufficient to maximize ileal total antioxidant capacity under the present experimental conditions. Furthermore, Nrf2 expression exhibited trends similar to those of antioxidant-related gene expression and antioxidant enzyme activities, but showed an opposite trend to MDA content. These findings suggest a potential association between Nrf2 expression and intestinal antioxidant status.