Infant diet and maternal gestational weight gain predict early metabolic maturation of gut microbiomes

Commensal gut bacterial communities (microbiomes) are predicted to influence human health and disease^1,2. Neonatal gut microbiomes are colonized with maternal and environmental flora and mature toward a stable composition over 2-3 years^3,4. To study pre- and postnatal determinants of infant microbiome development, we analyzed 402 fecal metagenomes from 60 infants aged 0-8 months, using longitudinal generalized linear mixed models (GLMMs). Distinct microbiome signatures correlated with breastfeeding, formula ingredients, and maternal gestational weight gain (GWG). Amino acid synthesis pathway accretion in breastfed microbiomes complemented normative breastmilk composition. Prebiotic oligosaccharides, designed to promote breastfed-like microflora⁵, predicted functional pathways distinct from breastfed infant microbiomes. Soy formula in six infants was positively associated with Lachnospiraceae and pathways suggesting a short-chain fatty acid (SCFA)-rich environment, including glycerol to 1-butanol fermentation, which is potentially dysbiotic. GWG correlated with altered carbohydrate degradation and enriched vitamin synthesis pathways. Maternal and postnatal antibiotics predicted microbiome alterations, while delivery route had no persistent effects. Domestic water source correlates suggest water may be an underappreciated determinant of microbiome acquisition. Clinically important microbial pathways with statistically significant dietary correlates included dysbiotic markers^6,7, core enterotype features⁸, and synthesis pathways for enteroprotective⁹ and immunomodulatory^10,11 metabolites, epigenetic mediators¹, and developmentally critical vitamins¹², warranting further investigation.