Reduced Crude Protein Nutrition in Poultry: Mechanistic Links Between Nitrogen Metabolism, Gut Microbial Ecology, and Intestinal Physiology.

Reducing dietary crude protein (CP) has emerged as an effective nutritional strategy to improve nitrogen utilisation efficiency and reduce environmental nitrogen losses in poultry production. Beyond these practical benefits, dietary protein concentration and digestibility influence nitrogen flow, microbial substrate availability, and metabolic interactions within the gastrointestinal tract, thereby shaping host-microbiota function. This review synthesises current evidence on the mechanistic links between reduced crude protein nutrition, nitrogen metabolism, gut microbial ecology, and intestinal physiology in poultry. Quantitative evidence indicates that responses depend strongly on the magnitude of CP reduction and dietary formulation. For example, a 2-percentage-point CP reduction with balanced lysine, methionine, and threonine improved growth and intestinal morphology, whereas a 3-percentage-point reduction has been associated with decreased duodenal and ileal villus height; in contrast, a 1.5-percentage-point reduction produced no significant morphological impairment. Recent evidence further shows that a 2% lower-protein diet can increase ileal microbial richness and diversity, while bacteriophage supplementation significantly improves nitrogen digestibility (p = 0.045), and dietary protein level itself affects nitrogen digestibility (p = 0.017). Collectively, moderate CP reduction supported by adequate digestible amino acids and highly digestible ingredients can decrease undigested nitrogen flow and proteolytic fermentation while maintaining intestinal function. However, excessive protein restriction, poor amino acid balance, or reduced ingredient digestibility may disrupt microbial metabolism and compromise intestinal integrity. These findings position reduced crude protein nutrition as a precision nutritional strategy in which the magnitude of protein reduction, amino acid adequacy, digestibility, and microbial substrate availability collectively determine productive, intestinal, and environmental outcomes.