摘要The consequences of feeding broiler chickens with reduced protein(RP)diets for gut health and barrier function are not well understood.This study was performed to elucidate the effect of reducing dietary protein and source of protein on gut health and performance parameters.Four experimental diets included 2 control diets with standard protein levels either containing meat and bone meal(CMBM)or an all-vegetable diet(CVEG),a medium RP diet(17.5％in growers and 16.5％in finisher),and a severe RP diet(15.6％in grower and 14.6％in finisher).Off-sex Ross 308 birds were assigned to each of the 4 diets and performance measurements were taken from d 7 to 42 post-hatch.Each diet was replicated 8 times(10 birds per replicate).A challenge study was conducted on additional 96 broilers(24 birds per diet)from d 13 to 21.Half of the birds in each dietary treatment were challenged by dexamethasone(DEX)to induce a leaky gut.Feeding birds with RP diets decreased weight gain(P＜0.0001)and increased feed conversion ratio(P＜0.0001)from d 7 to 42 compared with control diets.There was no difference be-tween CVEG and CMBM control diets for any parameter.The diet containing 15.6％protein increased(P＜0.05)intestinal permeability independent of the DEX challenge.Gene expression of claudin-3 was downregulated(P＜0.05)in birds fed 15.6％protein.There was a significant interaction between diet and DEX(P＜0.05)and both RP diets(17.5％and 15.6％)downregulated claudin-2 expression in DEX-challenged birds.The overall composition of the caecal microbiota was affected in birds fed 15.6％pro-tein having a significantly lower richness of microbiota in both sham and DEX-injected birds.Proteo-bacteria was the main phylum driving the differences in birds fed 15.6％protein.At the family level,Bifidobacteriaceae,Unclassified Bifidobacteriales,Enterococcaceae,Enterobacteriaceae,and Lachnospir-aceae were the main taxa in birds fed 15.6％protein.Despite supplementation of synthetic amino acids,severe reduction of dietary protein compromised performance and intestinal health parameters in broilers,evidenced by differential mRNA expression of tight junction proteins,higher permeability,and changes in caecal microbiota composition.