Low crude protein with moderate net energy and digestible lysine improves gut health and nutrient transport in broilers.

This study used response surface methodology based on a Box-Behnken design to investigate the individual and interactive effects of dietary net energy (NE), crude protein (CP), and standardized ileal digestible lysine (SID Lys) on intestinal health in male broilers from 22 to 35 days of age. A total of 936 22-day-old Cobb 500 male broilers were assigned to 13 dietary treatments varying in NE (2,200, 2,290, and 2,380 kcal/kg), CP (18%, 19.5%, and 21%), and SID Lys (1.08%, 1.14%, and 1.20%). At 35 days of age, intestinal morphology and the jejunal mRNA expression of barrier-related and nutrient transporter genes were measured. A significant NE × SID Lys interaction was observed for duodenal villus height (P < 0.05). Under low-NE conditions, villus height was quadratically affected by increasing SID Lys, whereas the opposite trend was observed under high-NE conditions. MUC2 mRNA expression was affected by NE × SID Lys and CP × SID Lys interactions (P < 0.05), and increased MUC2 mRNA expression in response to SID Lys was observed only under low-NE or high-CP conditions. Nutrient transporters expression was also influenced by a CP × SID Lys interaction, and increased LAT1 and B⁰AT mRNA expression was observed when high CP was combined with high SID Lys or when low CP was combined with low SID Lys (P < 0.05). GLUT2 mRNA expression was significantly decreased by increasing NE levels (P < 0.05). Response surface optimization indicated that a combination of 2,290 kcal/kg NE, 18% CP, and 1.08-1.14% SID Lys effectively supported intestinal morphology, barrier function, and nutrient transport in 22- to 35-day-old broilers fed low-protein diets. These findings indicate that moderate reductions in CP and SID Lys, when balanced with appropriate NE levels, may maintain intestinal health without compromising key physiological functions, and that the nutritional optima for intestinal health differs from those for maximum growth performance.