Effects of Dietary Standardized Ileal Digestible Lysine and Amylose/Amylopectin Ratio on Intestinal Morphology, Barrier-Related Gene Expression, and Cecal Microbiota in Broilers Fed Low-Protein Diets

This study investigated the effects of dietary standardized ileal digestible (SID) lysine levels and amylose to amylopectin ratios on the intestinal health of broilers fed an 18.5% crude protein diet from 22 to 42 days of age. A total of 540 healthy male Ross 308 broilers were randomly assigned to nine treatments in a 3 × 3 factorial design consisting of three SID lysine levels (1.00%, 1.20%, and 1.40%) and three AM/AP ratios (0.19, 0.29, and 0.41), with six replicates of 10 birds each. Ileal morphology, intestinal barrier function and inflammation-related gene expression, and the composition of cecal microbiota were evaluated. Significant interactions between lysine level and AM/AP ratio were observed for Occludin, ZO-1, Claudin-1, and TNF-α expression, with the highest expression in the 1.40% lysine + 0.41 AM/AP group and the lowest in the 1.00% lysine + 0.19 AM/AP group. The VH/CD ratio showed a significant interaction, with the highest value in the 1.20% lysine + 0.19 AM/AP group and the lowest in the 1.40% lysine + 0.41 AM/AP group. IL-18 and IL-10 were primarily affected by the main effects of lysine and AM/AP ratio. The expression levels of both IL-10 and IL-18 increased with increasing lysine level and increasing starch AM/AP ratio. Dietary SID lysine level and AM/AP ratio interactively regulate the expression of barrier-related genes, inflammatory status, intestinal morphology, and cecal microbiota, potentially contributing to enhanced intestinal health in broilers. However, because microbial metabolites were not measured, the functional significance of the observed microbiota alterations remains speculative. In broilers fed an 18.5% CP diet, a combination of 1.20% SID lysine with an AM/AP ratio of 0.19 was identified as the optimal strategy for maintaining intestinal morphology from 22 to 42 days of age.