Leftover Infant Milk After Bottle Feeding: Parental Practices and Microbiological Findings

ImportanceCurrent guidelines from the World Health Organization, Centers for Disease Control and Prevention, and Academy of Breastfeeding Medicine recommend discarding all milk remaining in bottles immediately after infant feeding. However, these recommendations lack supporting microbiological evidence from studies of actual infant feeding, imposing substantial financial and emotional burden on the 78 million families worldwide who bottle-feed their infants. ObjectiveTo determine (1) the financial, emotional, and time burden associated with bottle feeding and parental milk disposal practices, and (2) bacterial growth in leftover human milk and formula under different storage conditions. Design(1) Cross-sectional online survey (January 2023-February 2024) and (2) prospective microbiological cohort study. Setting(1) Online survey, (2) infants recruited in Hannover, Germany Participants(1) Survey respondents (n=1056; 99% mothers) and (2) healthy, full-term, bottle-fed infants (n=44; 17 humanmilk, 27 formula) aged 0-12 months. Main Outcomes and MeasuresParental burden scores, milk disposal frequency, and bacterial colony-forming units (CFU)/ml in milk samples before feeding, immediately after feeding, and at 4, 8, and 24 hours post-feeding at 4{degrees}C and 20{degrees}C. ResultsAmong surveyed parents, 46% discarded leftover milk daily, yet 84% reported they would keep milk longer if deemed safe. In microbiological testing, median bacterial burden in humanmilk increased from 4200 CFU/ml (range 300-350,000) pre-feeding to 24,600 CFU/ml (range 1900-29,004,400) post-feeding, but showed no significant further increase at 4 hours (p=0.82) or 8 hours (p=0.64) when stored at either 4{degrees}C or 20{degrees}C. Formula showed similar stability: median CFU/ml increased from 0 (range 0-10,700) to 11,700 (range 1900-630,000) post-feeding, with no significant change at 4 hours (p=0.91) or 8 hours (p=0.73) at either temperature. Significant bacterial growth occurred only after 24 hours at 20{degrees}C (p<0.001). Conclusions and RelevanceBacterial burden in leftover infant milk remained stable below concerning thresholds for 8 hours when refrigerated and 4-8 hours at room temperature, challenging current guidelines that mandate immediate disposal. Evidence-based guideline revision could reduce financial burden and milk waste for families around the globe without compromising infant safety. Key PointsO_ST_ABSQuestionC_ST_ABSHow long is it safe to offer leftover milk in a bottle to an infant that has previously drunk from it? FindingsThe number of bacteria in leftover human milk or formula did not significantly increase from 0 to 8h post-feeding in milk bottles sampled from 44 infants, regardless of whether the milk was kept at room temperature or refrigerated. MeaningLeftover milk may be safely reoffered beyond the limits of the current guidelines.