Modulation of intestinal stress in broilers: A subgroup and meta-regression insights of selected functional amino acids

Emmanuel Nuamah¹Utibe Mfon Okon²Jongryun Kim¹Darae Kang¹Kwanseob Shim^1*

• ¹Jeonbuk National University, Jeonju, Republic of Korea
• ²Universiti Putra Malaysia, Serdang, Malaysia

Early post-hatch feeding strategies supplementing immunonutrients, particularly functional amino acids (FAA), have been proposed to enhance gastrointestinal tract (GIT) maturation and integrity in broilers in the post-antibiotic era. However, reported effects on performance and gut health remain inconsistent. Thus, this meta-analysis aimed to synthesize and clarify the efficacy pattern of supplemental FAA evidence on growth performance, gut morphology, and lymphoid organ development. From a search span between 2015 and September 2025, data were extracted from 23 eligible studies out of 582 reports identified and pooled from five online databases. Data on arginine (n = 15), glutamine (n = 4), and glycine (n = 7), with three overlapping studies, were pooled using the Hedges' g effect size estimator of standardized mean differences (SMD) at a 95% CI, while the source of heterogeneity was accounted for with subgroup and meta-regression analyses. Additionally, the internal validity and reliability of the included studies, as well as publication bias, were assessed. The random-effects model's SMD estimates showed that supplementing the target FAA increased BWG (P = 0.0006) and reduced feed conversion ratio (P < 0.0001), with arginine and glycine having significant effects. Supplementing glutamine and arginine improved villus height and the villus-to-crypt ratio, lowered crypt depth, and had the strongest effect in the jejunum. However, it had no significant effects on the weights of the spleen (P = 0.2483) and the bursa (P = 0.1575). Supplementation with L-arginine, L-glutamine, and glycine enhances broiler growth performance and intestinal morphology, particularly during the early growth phase, although effects on FCR remain modest. L-arginine and glycine are most effective for enhancing weight gain, while L-glutamine and L-arginine support gut structural integrity. Outcome variability reflects influences of strain, dietary protein level, supplementation duration, and amino acid dose, with further research needed to address unmeasured factors such as feed processing and housing conditions. Strategic integration of these amino acids with feed enzymes, fat, or insoluble fiber, tailored to the strain and environment, may optimize digestibility and achieve performance comparable to that of standard protein formulations.