Multi-omics Integration Reveals L-arginine as a Key Mediator Linking Dietary Protein to Weight Change in Cats: An Exploratory Study

Cats exhibit an insufficient endogenous capacity for arginine synthesis, making dietary protein a core factor in nitrogen homeostasis. In this study, we investigated whether protein level-induced body weight change is mediated by host-microbiome interactions and serum metabolites. A 180-day trial was conducted on 30 cats fed three isocaloric diets with varying crude protein levels (LP: 28%, MP: 36%, HP: 46%). Fecal shotgun metagenomic sequencing, serum short-chain fatty acid analysis, untargeted metabolomics, and targeted amino acid profiling were integrated and then subjected to causal mediation analysis. The HP diet improved protein digestibility (HP: 93.29% vs. LP: 89.36%, P = 0.027) and attenuated body weight loss (HP: -3.43% vs. -14.52%, P = 0.031) without compromising fecal scores (all groups: 2.5-3.0 throughout the trial) or intestinal barrier integrity, as indicated by unchanged serum DAO, D-LA, and LPS levels. The MP group significantly enhanced humoral immunity, as evidenced by higher serum IgA, IgG, and IgM concentrations (P < 0.05 vs. LP and HP), while the HP group boosted antioxidant capacity, with significantly increased SOD and T-AOC activities (P < 0.05 vs. LP and MP). High protein increased the abundance of Phocaeicola and decreased that of Megasphaera. Serum arginine biosynthesis pathways and branched-chain amino acids were significantly enriched in MP and HP groups compared with the LP group (P < 0.05 for both comparisons). Crucially, multi-omics network and causal mediation analysis identified L-arginine as a statistically confirmed key regulatory node in the protein-weight relationship, as evidenced by a significant indirect effect (ACME = 0.208, P = 0.006) and a non-significant direct effect (ADE = 0.31, P = 0.23). These results suggested that L-arginine may serve as a key regulatory node mediating this process linking dietary protein to body weight in cats, providing a novel mechanistic target for precision nutritional interventions in feline weight management.