Breastfed infants receiving formula supplementation show altered lipid and gut microbiota profiles at 3 months of age.

BACKGROUND & AIMS: Exclusive breastfeeding offers numerous health benefits. Despite advancements in formula, significant differences compared with breast milk remain. We aimed to assess how milk feeding type and volume at 3 months affect the infant plasma lipidome and gut microbiota. METHODS: Infants were classified into exclusive breastfeeding (EBF), mixed feeding (MF), or exclusive formula feeding (EFF) groups based on feeding data collected prospectively across two European cohorts (n = 519). Lipidomics and shotgun metagenomics were applied to plasma and stool samples, respectively. RESULTS: Feeding type explained major variation in both lipidomic and microbial profiles. Plasma lipids showed distinct signatures across groups, particularly in sphingomyelins and diacylglycerols. Microbiota diversity and species richness increased with formula exposure. Formula rich in intact whey protein was linked to higher S. thermophilus abundance in the infant gut. Random forest classification of feeding type using either lipidomic or gut microbiota features achieved high discriminatory accuracy (AUROC >0.90) in training and validation datasets. CONCLUSION: Early nutrition is a critical determinant of the lipidome and gut microbiome during the breastfeeding phase.