The relation between breast milk sodium to potassium ratio and maternal report of a milk supply concern at 7 days postpartum (1016.9)

It was previously shown that 27% of breastfeeding primparas report a milk supply concern at 7 days postpartum (d7). We examined whether milk Na:K, as a biomarker of progress toward mature milk production, is predicted by maternal report of a milk supply concern at d7. We used flame photometry to determine Na and K concentrations in d7 milk from the Early Lactation Success cohort, and defined elevated Na:K (↑Na:K) as >75%tile. Of 406 mothers interviewed, 257 provided a milk sample, including 62 (24%) who reported a milk supply concern. Na:K ranged from 0.24 to 13.60 (median, 0.62) and was strongly correlated with formula use (Spearman r=0.21, p=0.0009). Subsequent analyses were restricted to the 197 mothers exclusively breastfeeding (EBF) at d7 (Na:K 75%tile=0.80). ↑Na:K occurred in 15/36 (42%) vs. 33/161 (20%) of mothers with vs. without a milk supply concern, respectively (p=0.008). This relation strengthened after adjustment for ethnicity: OR (95%CI), 2.8 (1.3‐6.0); and AOR (95%CI), 3.5 (1.5‐7.9). For nearly half of exclusively breastfeeding mothers, their report of milk supply concern is corroborated by elevated Na:K.