The Gut–Kidney Axis in Feline Chronic Kidney Disease: Nutritional Modulation of the Microbiome and Uremic Toxin Control

Chronic kidney disease (CKD) is common in older cats, and nutritional management remains the intervention with the strongest clinical evidence. Interest has expanded from conventional control of phosphorus and uremic signs to the gut–kidney axis, in which renal dysfunction may alter the intestinal environment while microbial metabolism generates solutes that accumulate as kidney function declines. This PRISMA-ScR-guided scoping review and critical narrative synthesis evaluates feline evidence on CKD-associated dysbiosis, gut-derived uremic solutes, bile acid metabolism, and microbiome-directed nutrition. The original search covered January 2021 to April 2026, with inclusion of earlier studies and targeted source verification through July 2026. Eligibility was organized into two evidence strata: core feline evidence and contextual evidence. The final revised evidence map comprised 49 unique sources (28 empirical feline sources and 21 contextual sources spanning comparative, methodological, guideline, regulatory, or safety evidence); non-feline evidence was used only for mechanistic, methodological, safety, or translational interpretation and was not pooled with feline clinical outcomes. Cats with CKD have shown lower fecal microbial richness and diversity and higher circulating indoxyl sulfate in small cross-sectional cohorts, but causality remains unproven. Recent work also identified altered secondary bile acids and lower fecal ursodeoxycholic acid; however, Peptacetobacter hiranonis is principally linked to bai-mediated 7α-dehydroxylation, whereas ursodeoxycholic acid formation requires distinct hydroxysteroid dehydrogenase reactions. Complete therapeutic renal diets improve clinical outcomes, but their benefits reflect multiple simultaneous modifications and cannot yet be decomposed into a microbiome-specific effect. Evidence for isolated prebiotics, probiotics, postbiotics, synbiotics, fecal microbiota transplantation, and precision nutrition algorithms is preliminary or absent in feline CKD. The field is constrained by small cohorts, confounding, reliance on 16S rRNA sequencing, compositional data, and limited interlaboratory reproducibility. Current clinical practice should therefore prioritize a palatable complete renal diet, adequate energy and protein intake, muscle condition monitoring, hydration, and constipation management, while microbiome-directed products remain adjunctive or investigational.