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Dietary protein source is a key determinant of nitrogen metabolism and gut microbiota in pigs. Protein sources differ not only in nutrient composition but also in amino acid release kinetics, which may influence amino acid availability, nitrogen utilization, and intestinal microbial communities.
Objective: This study evaluated the effects of replacing soybean meal with fish meal or rapeseed meal on nitrogen metabolism and gut microbiota in finishing pigs.
Method: Twenty-four growing barrows were randomly assigned to one of three isonitrogenous and isoenergetic diets containing soybean meal, fish meal, or rapeseed meal as the main protein source for 56 days. Growth performance, nitrogen balance, blood biochemistry, free amino acids, intestinal gene expression, short-chain fatty acids, microbial protein, and ileal microbiota (16S rRNA sequencing) were evaluated.
Results: Replacing 50% of soybean meal with fish meal or rapeseed meal did not affect growth performance. However, pigs fed the rapeseed meal diet showed increased faecal nitrogen excretion and reduced urinary nitrogen, blood urea nitrogen, and blood ammonia levels compared with soybean meal and fish meal diets. The rapeseed meal diet also reduced free amino acid concentrations in intestinal chyme and plasma, decreased amino acid transporter expression, and showed slower in vitro amino acid release, indicating a lower rate of amino acid absorption into circulation. Additionally, rapeseed meal increased ileal microbial diversity, with a lower abundance of Streptococcus and Staphylococcus and higher abundance of beneficial bacteria such as Lactobacillus and Turicibacter. Microbial cell protein and microbial amino acid concentrations were reduced in the rapeseed meal group. Amino acid release rate was negatively associated with faecal nitrogen and Lactobacillus and Turicibacter abundance, while positively associated with urinary nitrogen, blood urea nitrogen, Streptococcus, Staphylococcus, microbial protein, and microbial amino acid content.
Conclusion: Different dietary protein sources modify nitrogen metabolism and reshape ileal microbiota composition in finishing pigs. The rate of amino acid release from dietary protein appears to be a key factor linking protein source with nitrogen utilization and microbial responses.
Wang W, Zhao W, Pan L, Yu K, Wang J, Zhu W. Differential effects of dietary protein sources on nitrogen metabolism and ileal microbiota in pigs correlated with amino acid release rates. Journal of Animal Science and Biotechnology. 2026; 17(1): 145. https://doi.org/10.1186/s40104-026-01461-4