Dietary Arginine Supplementation Modulates Nitrogen Utilization, Serum Biochemistry, and Gut Microbiota in Growing Beagles Under Low- and Normal-Protein Diets

This study investigated the effects of dietary protein levels and arginine supplementation on growth performance, nitrogen metabolism, serum biochemical indices, inflammatory markers, gut microbiota, and fecal metabolomic profiles in growing beagles. Sixty healthy beagles were assigned to six dietary treatments in a 2 × 3 factorial design: two crude protein levels (normal, 22%; low, 18%) and three arginine supplementation levels (0%, 0.266%, 0.532%) over 45 days. Growth performance and nutrient digestibility were not significantly affected by diet. Low-protein diets reduced nitrogen intake and excretion while increasing biological value, indicating improved nitrogen utilization. Serum total cholesterol, blood urea nitrogen, and creatinine were influenced by protein and arginine levels, and inflammatory markers TNF-α and IL-6 were modulated by their interaction. Gut microbiota composition was altered at the taxonomic level but not at the level of the overall structure; arginine supplementation increased microbial richness under normal protein conditions. Fecal metabolomic analysis showed that arginine mainly affected amino acid metabolism, mineral absorption, and bile acid pathways, whereas protein reduction impacted lipid metabolism, bile secretion, and energy-related pathways. Overall, reducing dietary protein to 18% improved nitrogen utilization without impairing growth. Among the six dietary treatments, the normal-protein diet supplemented with moderate arginine (22% CP + 0.266% L-arginine, 1.10% total arginine) was associated with the most balanced overall physiological response. Metabolomic findings from the four selected dietary groups provided additional evidence that dietary protein and arginine modulated distinct metabolic pathways, although these findings should be interpreted within the scope of the selected-group comparison. These results provide a practical basis for optimizing amino acid balance and nitrogen utilization in growing dogs and may contribute to the development of more precise and sustainable pet food formulations.