Differential effects of dietary protein sources on nitrogen metabolism and ileal microbiota in pigs correlated with amino acid release rates
Dietary protein source is one of the primary factors influencing host nitrogen metabolism and intestinal microbiota. However, conventional diet formulations primarily focus on overall nutritional levels, overlooking the differences in amino acid release rates among various protein sources. This study investigates how different protein sources affect nitrogen metabolism and gut microbiota in finishing pigs. The corn-soybean meal diet served as the control (SBM). Fish meal and rapeseed meal isonitrogenously replaced 50% of soybean meal to formulate FM and RPM diets. Twenty-four finishing pigs (DLY; 53.40 ± 2.53 kg) were housed individually in metabolic cages, randomly assigned to three groups (n = 8), and fed for 8 weeks. Feeding the three diets had no significant effect on the growth performance of finishing pigs. Compared with SBM and FM, RPM diet significantly increased fecal nitrogen, but reduced urinary nitrogen, blood urea nitrogen and blood ammonia levels, reduced free amino acids in the small intestinal chyme and plasma, downregulated expressions of amino acid transporters, and showed slower in vitro amino acid release, collectively indicating less amino acid release into the small intestinal lumen and consequently less amino acids into the circulation. In addition, compared with the SBM and FM groups, RPM exhibited increased ileal microbial diversity, characterized by lower relative abundances of Streptococcus, Staphylococcus and higher relative abundances of Lactobacillus, Turicibacter. Additionally, microbial cell amino acids and microbial cell protein concentrations were significantly reduced in the RPM group. Correlation analysis showed that the amino acid release rate was negatively correlated with fecal nitrogen, the relative abundance of Lactobacillus and Turicibacter, while positively correlated with urinary nitrogen, blood urea nitrogen, the relative abundance of Streptococcus and Staphylococcus, microbial cell protein, and total microbial cell amino acid content. Dietary protein sources altered host nitrogen metabolism and restructured the ileal microbiota, with the rate of amino acid release appearing to be a key factor associated with these changes.