Traditional Chinese medicine and plant metabolites for rheumatoid arthritis via modulating gut microbiota: a scoping review evaluating the transition from correlation to causality.

Abstract

<h4>Background</h4>Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic synovitis. The "gut-joint axis" proposes gut microbiota and metabolites modulate RA inflammation via mucosal and systemic immune responses. Botanical drugs (Traditional Chinese Medicine, TCM) and plant metabolites offer multi-target potential. However, most studies remain descriptive, demonstrating concurrent microbial shifts but lacking causal designs to verify mechanistic necessity.<h4>Objectives</h4>This scoping review examines TCM and plant metabolite interventions on RA gut microecology (2015-2025), focusing on the "microbiota-metabolite-immune" axis. It aims to classify evidence based on causal design rigor and identify steps to advance research from correlation to causality.<h4>Methods</h4>We searched PubMed, Embase, and Web of Science (2015-2025). Studies reporting RA outcomes and gut microbiota changes following TCM interventions were included. We established a hierarchical classification system based on design rigor: antibiotic depletion (ABX), fecal microbiota transplantation (FMT), metabolite rescue, and blocking. Evidence was stratified: Level A (Closed-loop: ABX + FMT + rescue/blocking), Level A+ (plus <i>in vitro</i> blocking), Level B (Partial: ABX/FMT alone), and Level C (Correlational).<h4>Results</h4>Of 25 included studies (24 animal, 1 clinical), only 2 were Level A, 1 Level A+, 3 Level B, and 19 Level C. While TCM improved RA phenotypes and altered microbiota, complete closed-loop verification remains rare. Short-chain fatty acids (SCFAs) show promise but inconsistent trends due to heterogeneity. Bile acids and tryptophan metabolites correlate with reduced inflammation, yet their mechanistic necessity remains largely untested.<h4>Conclusion</h4>Botanical drugs and plant metabolites demonstrate potential in modulating gut microbiota to improve RA. However, definitive causal links remain underexplored. Future research should prioritize "shortest closed-loop" strategies, including targeted quantification, rescue, and necessity validations. Longitudinal designs and systemic immune metrics are essential to transition from correlations to translatable mechanisms.