RESUMEN
OBJECTIVES: To describe natremia in healthy term newborns and determine whether there is a relationship between blood sodium and feeding patterns. METHODS: All normal newborns, admitted to the nursery between January and March 2004 were eligible for this prospective cohort study. Inclusion criteria were: > or =37 weeks of gestational age, birth weight > or =2500 g, Apgar scores > or =7 at 5 and 10 min and normal physical examination. A capillary blood sample was taken at 48+/-12 h of life. RESULTS: Blood samples from 126 newborns were analyzed. Mean gestational age was 39.6 weeks, birth weight was 3414 g and weight loss at 48 h of life was 6.5% of birth weight. Mean capillary blood sodium was 141 mmol/L (SD 3.4). Exclusively, breast-fed newborns had statistically higher mean blood sodium (141 mmol/L, SD 3.0) than the non-exclusively breast-fed+formula fed group (139 mmol /L, SD 3.7). There was a significant linear association between blood sodium and the quantity of milk supplements received as well as between blood sodium and weight loss. CONCLUSIONS: Most newborns have blood sodium values within a narrower range than previously described in the literature. We also demonstrate that the exclusively breast-fed infants appear to have marginally but statistically higher values of blood sodium than non-exclusively breast-fed and formula-fed infants.