Gut Microbiota Dysbiosis and Gut-Nasal Axis Alterations in Preschool Children with Allergic Rhinitis and Functional Constipation.

Background: The comorbidity of allergic rhinitis (AR) and functional constipation (FC) in preschool children represents an emerging clinical entity; yet its underlying microbial mechanisms remain inadequately elucidated. This study sought to comprehensively characterize the gut microbiota (GM) profile and functional alterations in children with concurrent AR and FC (ARFC), with specific emphasis on the gut-nasal axis. Methods: In this cross-sectional analysis, fecal samples from 32 ARFC, 22 AR, and 21 healthy control (HC) children underwent 16S rRNA gene sequencing (V3-V4 regions). Microbial α-and β diversity, taxonomic composition, functional pathways, and correlations with clinical parameters were systematically analyzed. Results: The ARFC group exhibited significant GM dysbiosis, including elevated α-diversity (Shannon index: 5.2 ± 0.3 vs HC 4.5 ± 0.4; P = 0.014) and distinct β-diversity (PERMANOVA P = 0.001). Taxonomic analysis revealed elevated enrichment of Proteobacteria (7.92% vs HC 1.94%; P = 0.001) and depletion of Bacteroidetes (40.06% vs HC 50.72%; P = 0.049). Pathogen genera, including Klebsiella and Escherichia/Shigella, were elevated, while butyrate-producing genera (Faecalibacterium and Ruminococcus) were reduced. Paradoxically, Bifidobacterium abundance was higher in ARFC than AR (4.21% vs 1.80%; P = 0.018), suggesting a potential compensatory immunomodulatory response. Functional prediction indicated impaired carbohydrate and lipid metabolism alongside enhanced xenobiotic degradation. Haemophilus abundance positively correlated with constipation severity (ρ = 0.52 and P = 0.008) and rhinorrhea severity (ρ = 0.56 and P = 0.003). Conclusion: ARFC comorbidity is characterized by a distinct GM signature featuring pathogenic expansion, metabolic dysfunction, and compensatory Bifidobacterium enrichment, which may modulate immune responses. Loss of butyrate-producing bacteria may disrupt the gut-nasal axis, highlighting potential microbial and therapeutic targets for integrated therapeutic strategies in this dual symptom condition. These findings provide a foundation for microbiota-based interventions targeting both allergic and gastrointestinal manifestations.