AIMS To compare the effects of early and delayed sodium supplementation on body composition and body water compartments during the first two weeks of postnatal life.

RESULTS Twenty four infants received early, and 22 delayed, sodium supplementation. There were no significant differences between the groups in body water compartments on day 1. In the delayed group, but not the early group, there was a significant loss of total body water during the first week (delayed −44 ml/kg, p=0.048; early 6 ml/kg, p=0.970). By day 14 the delayed, but not the early group, also had a significant reduction in extracellular fluid volume (delayed −53 ml/kg, p=0.01; early −37 ml/kg, p=0.2). These changes resulted in a significant alteration in body composition at the end of the first week (total body weight: delayed 791 ml/kg; early 849 ml/kg, p=0.013). By day 14 there were once again no significant differences in body composition between the two groups.

CONCLUSIONS Body composition after preterm birth is influenced by the timing of introduction of routine sodium supplements. Early sodium supplementation can delay the physiological loss of body water that is part of normal postnatal adaptation. This is likely to be of particular relevance to babies with respiratory distress syndrome. A tailored approach to clinical management, delaying the introduction of routine sodium supplements until there has been postnatal loss of body water, is recommended.