Currently, achieving sustained clinical remission in rheumatoid arthritis (RA) remains a challenge.Vitamin D (VD) exhibits significant immunomodulatory potential in rheumatoid arthritis (RA); however, whether it ameliorates RA progression by modulating the gut microbiota via the "gut-joint axis" remains incompletely understood.Our objective is to investigate the specific mechanisms underlying VD-mediated RA amelioration and to clearly determine whether VD exerts its protective effects by remodeling the gut microbiota and provide novel intervention targets and therapeutic options for the clinical management of RA.

We established a collagen-induced arthritis (CIA) mouse model and depleted the gut microbiota using an antibiotic (ABX) cocktail. Groups included CIA, VD-treated, ABX-treated, and ABX+VD+sodium butyrate-supplemented mice. Joint damage and systemic inflammation were evaluated via clinical scores, H&E staining, and ELISA. Concurrently, 16S rRNA sequencing and gas chromatography-mass spectrometry (GC-MS) analyzed shifts in gut microecology and butyrate levels.

Gut microbiota and butyrate are essential for the anti-arthritic efficacy of Vitamin D.VD significantly alleviated CIA severity, mitigating synovial hyperplasia, bone erosion, and serum pro-inflammatory factors (IL-6, TNF-α, MMP-3). However, antibiotic-induced microbiota depletion diminished VD's anti-arthritic efficacy. Sodium butyrate co-supplementation effectively restored VD's therapeutic effects in ABX-treated mice. Taxonomic analysis revealed that VD reversed CIA-induced gut dysbiosis by precisely suppressing opportunistic and mucosa-damaging pathogens , while enriching butyrate-producing commensals, prominently the Eubacterium_xylanophilum_group.

In conclusion, the present study demonstrates that Vitamin D supplementation effectively alleviates clinical arthritis severity and mitigates histopathological damage in CIA mice. This protective effect is highly dependent on the modulation of the "gut-joint axis"—specifically through the precise suppression of opportunistic pathogens and the robust enrichment of butyrate-producing symbiotic bacteria (e.g., the Eubacterium_xylanophilum_group), thereby restoring gut microbial homeostasis. Our findings provide novel microecological evidence supporting the therapeutic potential of VD in RA and establish a solid theoretical foundation for developing innovative RA treatment strategies that integrate targeted gut microbiome modulation with nutritional intervention.