Dietary chlorogenic acid modulates jejunal microbiota, short-chain fatty acids, and hypothalamic gene expression in broilers under high stocking density.

High stocking density can impair broiler growth and is associated with alterations in intestinal microbial ecology and neuroendocrine regulation of feeding. Chlorogenic acid (CGA) has antioxidant, anti-inflammatory, and microbiota-modulating properties. This study evaluated dietary CGA in a 2 × 2 experimental arrangement defined by stocking density (normal vs. high) and CGA supplementation (0 vs. 1 g/kg). Two hundred and sixteen one-day-old male Arbor Acres (AA) broilers were assigned to ND (14 birds/m2), HD (22 birds/m2), NDCGA (14 birds/m2 + 1 g/kg CGA), or HDCGA (22 birds/m2 + 1 g/kg CGA), with six replicate pens per treatment. High stocking density was associated with poorer late-stage growth performance, age-dependent alterations in circulating cytokines, changes in jejunal microbial composition, lower concentrations of several jejunal short-chain fatty acids (SCFAs), and altered hypothalamic inflammatory- and appetite-related transcripts. Factorial analysis of growth-performance and serum-cytokine endpoints identified prominent stocking-density effects, whereas density × CGA interactions were limited to selected growth variables and were not consistently detected across outcomes. Accordingly, isolated HD-HDCGA pairwise differences were interpreted as treatment-combination differences unless supported by a significant interaction. Jejunal propionate and butyrate showed numerical upward trends in HDCGA compared with HD, but the pairwise differences were not significant. The transcriptional findings are consistent with possible involvement of AMPK/NF-κB- and MAPK-related regulation; however, direct pathway activation was not demonstrated. Collectively, these findings support coordinated associations among stocking density, dietary CGA, jejunal microbial ecology, local SCFA profiles, and hypothalamic transcription without establishing a causal microbiota-gut-brain signaling mechanism.