Seeding the Future: How Feeding Mode Shapes the Infant Gut Microbiota

Early life represents a critical developmental programming window during which nutrition and microbial exposures shape long-term physiological function. Feeding mode is a major determinant of infant gut microbiota assembly and metabolic activity. This narrative review synthesizes current evidence comparing breastfeeding (BF) and formula feeding in relation to microbial composition, functional capacity, and immune programming during the preweaning and early postweaning periods. BF may support a relatively stable, bifidobacteria-dominated microbiota enriched in pathways involved in carbohydrate utilization, vitamin biosynthesis, and immune modulation. Human milk oligosaccharides, secretory IgA, lactoferrin, and milk-associated microbes collectively guide microbial succession, enhance barrier integrity, and support immune tolerance. In contrast, formula-fed infants typically exhibit greater microbial diversity, earlier transition toward adult-like profiles, and increased abundance of facultative anaerobes, alongside the enrichment of pathways related to bile acid and amino acid metabolism. Microbiota patterns in formula-fed infants are further influenced by formula composition, including protein load, lipid structure, and supplementation with prebiotics, probiotics, and human milk oligosaccharide analogues. Although advances in formula design have reduced compositional gaps, functional differences in microbial stability and immune programming persist. Recognizing early infancy as a sensitive programming window underscores the need for microbiome-informed nutritional strategies and longitudinal, multi-omics research to clarify causal mechanisms and optimize early-life interventions.