Human Milk Sodium Monitoring Across the Continuum of Lactation: From Secretory Activation to Established Lactation

Background: Human milk sodium (Na) concentration is a biomarker of lactation physiology and secretory activation, but conventional measurement methods are impractical for routine breastfeeding support. Portable Na analyzers may enable real-time home monitoring. Objective: The purpose of this study was to characterize longitudinal changes in human milk Na concentrations during the first 3 postpartum months and to evaluate the feasibility of longitudinal home Na monitoring throughout established lactation. Methods: Nine healthy lactating mothers intending to breastfeed were recruited at a breastfeeding management clinic in Tokyo, Japan, between August 2025 and December 2025. Participants performed home-based Na measurements from both breasts from birth through approximately 100 days postpartum, with the planned monitoring schedule defined as 90 postpartum days for adherence assessment. Each measurement required 2-3 drops of milk and was completed within 1 min. Longitudinal Na trajectories and Na elevation episodes were evaluated. Results: Na concentrations were highest immediately after birth and declined rapidly during the first postpartum week. Six of nine participants (66.7%) reached concentrations below 413.8 ppm within the first 7 postpartum days, and five of nine (55.6%) reached concentrations below 220 ppm by postpartum day 8. During established lactation, 62 breast-level Na elevation episodes were identified, with substantial interindividual variation in frequency and duration. Some episodes occurred without overt breast symptoms. Frequent home monitoring detected transient physiological fluctuations that would likely have been missed by conventional intermittent sampling. Conclusions: Longitudinal home milk Na monitoring was feasible in this small pilot cohort and enabled characterization of Na fluctuations from secretory activation through established lactation. These findings are preliminary and hypothesis-generating, and larger prospective studies are required to determine the clinical utility of longitudinal Na monitoring for breastfeeding assessment.