Mother-infant dyads who participated in the MiLC Trial. These were healthy, lactating women 18 years or older. Women had no signs of breast infection and self-reported that they nor their infants had signs/symptoms of acute illness 7 days before participating in the study. We also restricted participation to women who pumped with electric breast pumps, who could donate 1 oz of their milk on two consecutive pumping sessions from the same breast, and women whose infants had not consumed formula within the past 2 weeks. A full description of eligibility criteria can be found on the original protocol (ClinicalTrials.gov Identifier: NCT03123874).