Dose-dependent effects of dietary quercetin on performance, egg quality, metabolic health, and antioxidant defense in laying hens: A systematic review and meta-analysis

Background and Aim: Quercetin is a plant-derived flavonoid known for its antioxidant and metabolic regulatory properties. Many studies have assessed its effects on laying hen performance, egg quality, blood metabolites, and oxidative status; however, the results have been inconsistent, mainly due to differences in dosage, duration, hen age, and quercetin form. This meta-analysis aims to quantitatively synthesize the available evidence and examine the dose–response relationships of dietary quercetin supplementation on productive performance, egg quality traits, blood metabolites, and antioxidant defenses in laying hens. Materials and Methods: A systematic literature search of Scopus and Web of Science identified 27 eligible studies published in English. Effect sizes were calculated as mean differences (MDs) using a restricted maximum likelihood random-effects model. Subgroup and meta-regression analyses were conducted to evaluate how quercetin dose, treatment duration, initial hen age, and quercetin form (extract vs. plant powder) influenced the outcomes. Heterogeneity was assessed with the I² statistic, and publication bias was examined using funnel plots and Egger’s regression test. Results: Dietary quercetin significantly improved laying rate (LR) (MD = 2.82%), egg weight (MD = 1.21 g), Haugh unit (MD = 1.84%), shell thickness (MD = 0.014 mm), and yolk color (MD = 0.53), while reducing the feed-to-egg ratio (FER) (MD = −0.15) (p < 0.05). Quercetin supplementation also decreased serum glutamate pyruvate transaminase (SGPT), glucose, total cholesterol, and malondialdehyde levels, while increasing high-density lipoprotein and superoxide dismutase (SOD) concentrations (p < 0.05). Meta-regression revealed linear dose-dependent reductions in SGPT, glucose, and total cholesterol, whereas LR, FER, and SOD activity showed quadratic responses. Optimal responses occurred at quercetin doses of approximately 400–600 mg/kg. Treatment duration, hen age, and quercetin form further influenced several outcomes. Conclusion: Dietary quercetin effectively boosts productivity, egg quality, metabolic health, and antioxidant defense in laying hens in a dose-dependent way. Supplementing at 400–600 mg/kg seems optimal for maximizing laying performance and antioxidant levels, supporting quercetin as a promising phytogenic feed additive for sustainable poultry farming.