Breastfeeding Shapes the Gut Microbiota and Its Structure Is Associated with Weight Gain Trajectories in Mexican Infants

<b>Background:</b> Rapid weight gain in early infancy increases the risk of childhood obesity, while exclusive breastfeeding can protect against it, depending on breastmilk composition, maternal diet, and infant gut microbiota. <b>Objective:</b> The objective of this study was to analyze the association between maternal diet, breastmilk components, infant gut microbiota, and weight gain in the first year of life of Mexican breastfed infants. <b>Methods:</b> This longitudinal study included 27 mothers with exclusively breastfed infants (≥5 months of age). We evaluated maternal diet and breastmilk composition at 5 months postpartum (pp), the infant fecal microbiota at 5 and 12 months pp using 16S rRNA gene sequencing, and weight gain as normal, rapid or slow weight gain (NWG, RWG or SWG) in periods 1 (0-5.5 months) and 2 (5.5-12 months). <b>Results:</b> Infants with NWG in periods 1 and 2 made up 51% and 56%, respectively. In period 1, ingested breastmilk protein content was higher for NWG infants than for infants with SWG (<i>p</i> = 0.01), and the protein content was negatively correlated with maternal BMI (<i>r</i> = -0.42, <i>p</i> = 0.02). The genera <i>Veillonella</i> (19.5%), <i>Bifidobacterium</i> (19.5%), and <i>Escherichia-Shigella</i> (16.8%) dominated the microbiota at 5 months. At 12 months, <i>Bacteroides</i> predominated, and the first two genera remained. Breastmilk fat correlated with <i>Veillonella</i> abundance (<i>r</i> = -0.50, <i>p</i> = 0.02) and oligosaccharides with Lachnospiraceae (<i>r</i> = 0.73, <i>p</i> = 0.03) at 5 months. There was a trend of a higher abundance of <i>Bifidobacterium</i> in NWG infants than in other infants in period 1, while infants with RWG and SWG had a higher abundance of <i>Ruminococcus gnavus</i> (<i>p</i> = 0.03) in period 1 and <i>Alistipes</i> in period 2 (<i>p</i> = 0.01), respectively. <b>Conclusions:</b> Breastfeeding shaped the gut microbiota of exclusively breastfed infants, and its structure was associated with infant weight gain trajectories.