Simple Summary High density (HD) rearing seriously hinders the healthy development of broilers. Chlorogenic acid, a natural antioxidant food and drug source, may relieve the HD oxidative stress. Therefore, the present study was to investigate the effects of CGA supplementation on growth performance, antioxidant index in serum, relative mRNA expression of jejunum, and cecum microflora of broilers expose to HD stress. A total of 476 7-day-old AA+ broilers were divided into normal density (ND group, 14 broilers/m(2)) and high density (HD group, 22 broilers/m(2)), and CGA supplementation (ND or HD + CGA) group. The results showed that HD stress decreased the serum antioxidant capacity, inhibited the development of intestinal villi and the expression of tight junction genes, and thus affected the intestinal permeability during the rapid growth period from 21 to 35 days. Meanwhile, with the progression of HD oxidative stress, adverse changes occurred in intestinal microorganisms, leading to intestinal inflammation, and finally affected the growth performance on day 42. Additionally, dietary CGA can inhibit the oxidative stress from serum to intestinal tract, improve intestinal integrity, and enhance beneficial flora of HD broilers. Thus, dietary supplementations of an antioxidant such as CGA can effectively prevent oxidative stress in HD broilers. Chlorogenic acids (CGA) are widely used as feed additives for their ability to improve growth performance and intestinal health in poultry. However, whether dietary CGAs could reverse the impaired intestinal condition caused by high stocking density (HD) in broiler chickens is unknown. We determined the effect of dietary CGA on growth, serum antioxidant levels, jejunum barrier function, and the microbial community in the cecum of broilers raised under normal (ND) or HD conditions. HD stress significantly decreased growth and body weight, which was restored by CGA. The HD group showed increased serum malondialdehyde, an oxidative byproduct, and decreased SOD and GSH-Px activity. CGA reduced malondialdehyde and restored antioxidant enzyme activity. HD stress also significantly decreased jejunal villus length and increased crypt depth. Compared with ND, the expression of tight-junction genes was significantly decreased in the HD group, but this decrease was reversed by CGA. HD also significantly upregulated TNF-alpha. Compared with ND, the cecal microbiota in the HD group showed lower alpha diversity with increases in the harmful bacteria Turicibacter and Shigella. This change was altered in the HD + CGA group, with enrichment of Blautia, Akkermansia, and other beneficial bacteria. These results demonstrated that HD stress decreased serum antioxidant capacity, inhibited the development of jejunal villi, and downregulated expression of tight-junction genes, which increased intestinal permeability during the rapid growth period (21 to 35 days). Dietary CGA enhanced antioxidant capacity, improved intestinal integrity, and enhanced beneficial gut bacteria in chickens raised under HD conditions.
