Functional Amino Acid Supplementation Drives Early Growth and Gut Maturation in Broilers: A Meta-Analysis.

Early post-hatch feeding strategies supplementing nutrients, particularly functional amino acids, have been proposed to enhance gastrointestinal tract (GIT) maturation and health in broilers in the post-antibiotic era. However, reported effects on performance and gut morphology remain inconsistent. Hence, this meta-analysis synthesized and clarified the efficacy pattern of supplemental FAA (Arg, Gln, Gly) evidence on growth performance, gut morphology, and lymphoid organ development. From a search spanning 2015 to September 2025, data were extracted from 23 eligible studies among 582 reports identified and pooled from five online databases. Standardized mean differences (SMDs) were calculated using Hedges' g estimator with 95% confidence intervals, and heterogeneity was explored using subgroup and meta-regression procedures. Internal validity and reliability of included studies and publication bias were also assessed. The random-effects meta-analyses revealed that the FAA increased BWG (SMD = 1.01; p = 0.0006) and reduced feed conversion ratio (SMD = -0.45; p < 0.0001). Likewise, they enhanced intestinal architecture in both the jejunum and ileum. This was characterized by increased villus height (p < 0.05), reduced crypt depth (p < 0.05), and an elevated villus-to-crypt ratio (p < 0.0001), with the ileum exhibiting the greatest morphological response. In contrast, supplementation had no significant effect on spleen weight (SMD = 0.24; p = 0.2483) or bursa weight (SMD = 0.31; p = 0.1575). These effects, however, can be influenced by dosage used, dietary crude protein level, and broiler strain. In addition to enhancing the small intestine morphology early on, longer supplementation increased feed efficiency. Specifically, L-arginine and glycine efficaciously stimulated BWG, while L-glutamine and L-arginine enhanced morphology. Overall, early dietary supplementation with arginine, glutamine, or glycine is an effective post-antibiotic nutritional strategy to alleviate early post-hatch physiological stress and support broiler growth and intestinal development. However, to optimize nutrient utilization and sustain growth performance comparable to that achieved with standard CP diets, these FAAs in practical broiler nutrition should be strategically integrated into low-CP formulations.