Effects of Dietary Zeolite Inclusion on the Performance and Egg Quality of Laying Hens: A Systematic Review and Meta-Analysis

Zeolite has been increasingly investigated as a functional mineral feed additive in poultry diets because of its adsorptive and ion-exchange properties; evidence regarding its effects in laying hens remains inconsistent. A meta-analysis search was conducted in PubMed, Semantic Scholar, and Scopus following PRISMA guidelines and resulted in 30 eligible publications (1985 and 2026). Effect sizes were expressed as the natural logarithm of the response ratio (LnRR) and synthesized using robust variance estimation (RVE) as the primary analysis, while multilevel analysis was performed as a sensitivity analysis (to account for dependent effect sizes). Heterogeneity was assessed using I2 and τ2, and publication bias was evaluated using Egger’s regression test. Subgroup analyses were conducted according to zeolite type, laying hen strain, inclusion level based on the EFSA reference threshold (≤1.25% vs. >1.25%), and experimental period. Zeolite supplementation significantly reduced feed intake by 1.65% (LnRR = −0.017; p = 0.024) and feed conversion ratio by 9.21% (LnRR = −0.097; p = 0.003), while increasing final body weight by 3.91% (LnRR = 0.038; p = 0.003). Egg weight, eggshell thickness, albumen weight, and egg shape index increased by 1.57%, 4.24%, 5.05%, and 2.09%, respectively. Sensitivity analysis generally supported the direction of the responses, although statistical significance was inconsistently retained for some outcomes, including FBW, globulin, AST, tibial ash, Ca and P plasma, indicating model-dependent evidence. Several hematological and plasma biochemical parameters were not significantly altered. Subgroup analyses indicated variation according to hen strain, zeolite type, inclusion level, and experimental period, although within-subgroup significance did not consistently indicate between-subgroup differences. Overall, zeolite supplementation was associated with improved feed efficiency and selected production and egg-quality traits, but substantial heterogeneity, dependency among effect sizes, and model-dependent findings for some outcomes warrant cautious interpretation. Further well-controlled and longer-term studies are needed to clarify the factors underlying variation in response and their biological relevance.