Paullin, Trillitye R.

Introduction Eczema and food allergy commonly emerge during infancy and are linked to changes in the gut microbiome, yet it remains unclear when microbiome differences associated with allergic disease first appear during development. Methods We analyzed age-stratified shotgun metagenomic data from 97 children aged 4–36 months, including physician-confirmed cases of eczema or food allergy and non-allergic controls, excluding recent antibiotic or probiotic exposure. Microbial taxa, functional pathways, and composite microbiome metrics were evaluated across three developmental stages: early infancy (4–6 months), mid-infancy (6–12 months), and toddlerhood (12–36 months). Results Differences between allergic and non-allergic children were minimal before 6 months of age but became more apparent during mid-infancy and persisted into toddlerhood. Allergic conditions were associated with reduced abundance of fiber-fermenting and butyrate-producing taxa, enrichment of facultative and inflammation-associated microbes, lower microbiome maturation scores, and shifts in metabolic and inflammatory functional capacity. Discussion These findings suggest that gut microbiome divergence associated with allergic disease becomes more apparent during mid-infancy, highlighting a developmentally relevant period for understanding early immune disruption. The results support further longitudinal and interventional studies aimed at clarifying whether earlier microbiome-targeted strategies may help modify progression along the atopic march.