Gut microbiome composition and predicted functions relate to growth and behavior in a Japanese preschool cohort

Early childhood is a period of rapid brain maturation and gut microbiome assembly, when emerging behavioral difficulties can shape later mental health and learning trajectories. Microbiota–gut–brain communication has been implicated in neurodevelopment through microbial metabolites and immune signaling. However, most pediatric evidence comes from high-risk or clinically referred cohorts, and gut microbiome–related correlates of typical behavioral variation in community-based preschool children remain poorly defined. In a cross-sectional sample of typically developing Japanese preschool children, we observed exploratory nominal associations between behavioral variation within normative ranges and gut microbiome composition and predicted functions. Internalizing domains showed candidate links with taxa and predicted pathways related to inflammatory potential and nucleotide biosynthesis, whereas somatic complaints and withdrawn behavior showed nominal associations with lower predicted respiratory and fermentative activity. Sleep-related difficulties showed multiple representative nominal pathway-level associations, including pathways related to methyl-donor and heme biosynthesis, while externalizing domains showed candidate links with predicted cell-envelope and carbohydrate-remodeling pathways. In contrast, age, height, and weight tracked expected maturation-related microbiome features, indicating that behavioral associations were not simple proxies of growth. Together, these findings provide an exploratory profile of microbiome–behavior correlations in a low-risk Japanese preschool cohort and highlight pathway-level candidates that may interface with neurodevelopment.