This project will examine specific factors in breast milk that may impact infant's gut microbiome development, and thereby impact their adiposity and weight gain in early life. An important early-life experience that could meaningfully impact the development of the gut microbiome, as well as future obesity risk, is breastfeeding. Breast milk has been shown to contain certain macronutrients (human milk oligosaccharides; HMOs) that not only vary greatly among women, but recent studies suggest that these factors are significant predictors of infant weight gain and adiposity. Examining early onset obesity during infancy is important because accelerated weight gain and adiposity in the first few years of life are major predictors for later obesity, and because neuronal, metabolic, and gut microbiome systems are especially vulnerable by programming from nutritional and maternal factors during this stage of life.

This study will be conducted in Los Angeles. The investigators will recruit two-hundred-and-forty mother-infant pairs. Mothers and infants will be monitored during the first 36 months of life, with sampling of breast milk (while the mother is still breast feeding) and infant and maternal stool at 1, 6, 12, 18, and 24 months of age (or as close as possible to each month). Only an infant stool sample will be collected at 36 months of age. The main outcomes of this study will be infant body weight-for-length growth during this period, as well as body composition (fat and lean mass), which will be measured by skin-fold thickness using a caliber, height, weight, and waist circumference measurements at 1, 6, 12, 18, 24, and 36 months of infant age (or as close as possible to each month). The investigators will also assess maternal and infant diet using 24-hour recalls, and maternal feeding style and infant eating behaviors using questionnaires.