Grapes and their by-products effects on growth performance, intestinal histomorphology, immune response and antioxidant status in broiler chickens - A meta-analysis

A comprehensive meta-analysis was conducted to quantitatively assess the impact of dietary grape and grape by-products supplementation on growth performance, intestinal histomorphology, immunological response, and antioxidant status in broiler chickens. A systematic search guided by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 across PubMed, Scopus, and Web of Science up to October 19, 2025, yielded 349 records, of which 21 controlled feeding studies satisfied the inclusion criteria. Data were analyzed using a multilevel random-effects model in R (metafor package). Heterogeneity (I² and τ²) and publication bias were evaluated using Egger's regression and the trim-and-fill method. Pooled estimations indicated that grape supplementation did not significantly influence body weight gain (mean difference (MD) 12.7 g, 95% CI: -14.5 to 39.8, P = 0.360), feed intake (MD = 30.6 g, 95% CI: -51.8 to 113, P = 0.466), or feed conversion ratio (FCR) (MD = 0.01, 95% CI: -0.01 to 0.02, P = 0.565). Significant enhancements were observed in villus height (MD = 59.5 µm, 95% CI: 1.49-117, P = 0.044), reduction in crypt depth (MD = -13.3 µm, 95% CI: -18.7 to -7.95, P < 0.001), and villus height-to-crypt depth ratio (VH:CD) (MD = 1.31, 95% CI: 0.32-2.29, P = 0.009), indicating improved intestinal histomorphology and absorptive capacity. The humoral immune response showed a positive but non-significant effect (standardized mean difference (SMD) = 1.09, 95% CI: -0.69-2.87, P = 0.228). Enzymatic antioxidants (SMD = 0.25, P = 0.697), lipid peroxidation (SMD = -0.50, P = 0.297), and non-enzymatic antioxidant capacity (SMD = 2.45, P = 0.26) were also non-significant. Substantial heterogeneity across outcomes was mainly explained by grape product form, intestinal segment, immunological outcome type, and antioxidant marker type through meta-regression. Overall, grape by-products improved intestinal histomorphology but did not significantly enhance growth performance, immunity, or oxidative balance.