Rheumatoid arthritis is a chronic systemic autoimmune disease that arises from complex interactions among genetic susceptibility, environmental factors, and immune dysregulation. Growing evidence indicates that microorganisms residing in the oral cavity and gastrointestinal tract, together with dietary factors, play a central role in shaping inflammatory and autoimmune responses in rheumatoid arthritis, forming an interconnected microbiome–immune–nutrition axis. Alterations in the composition and function of oral and intestinal microbial communities are associated with disruption of mucosal barrier integrity, activation of innate and adaptive immune pathways, increased differentiation of proinflammatory T lymphocyte subsets, and loss of immune tolerance that promotes autoantibody production. In addition, microbially derived metabolites, particularly short-chain fatty acids, provide a mechanistic link between microbial ecology, immune regulation, and bone metabolism. Diet represents a key upstream modulator of this axis. Dietary patterns rich in anti-inflammatory nutrients support microbial diversity and immunoregulatory metabolite production, whereas diets high in processed foods and saturated fats favor proinflammatory microbial profiles. Accumulating clinical evidence suggests that nutritional strategies and microbiome-targeted dietary interventions may reduce systemic inflammation and disease-related comorbidities when used alongside standard pharmacological treatments. Taken together, the microbiome–immune–nutrition axis represents a modifiable and clinically meaningful target in rheumatoid arthritis, emphasizing the need for interdisciplinary research and well-designed clinical trials to translate these insights into personalized approaches for disease management. The aim of this review is to integrate current mechanistic and clinical evidence on the interactions between the microbiome, immune system, and nutrition in rheumatoid arthritis, with a focus on their pathogenic relevance, therapeutic potential, and implications for personalized, diet-based interventions.
