Overweight and obesity are important risk factors for type 2 diabetes, and they begin very early in life. The objective of this study was to compare the impact of formula versus solid foods later in infancy, brief formula exposure (“just one bottle”) in the hospital on infant weight gain and the microbes in their gut. Longitudinal data provided the opportunity to look at the relative importance of microbiota profiles in early and late infancy (3 and 12 months, respectively).