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

Abstract Importance Current 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. Objective To 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 Measures Parental 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°C and 20°C. Results Among 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°C or 20°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°C (p