Randomised controlled trial of postnatal sodium supplementation on body composition in 25 to 30 week gestational age infants

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. Methods-Preterm infants of 25-30 weeks' gestation were stratified and rando mly assigned according to gender and gestational age, to receive a sodium i ntake of 4 mmol/kg/day beginning either on the second day after birth or wh en weight loss of 6% of birthweight had been achieved. Daily sodium intake, total fluid intake, energy intake, urine volume, and urinary sodium excret ion were recorded. Total body water was measured by (H2O)-O-18 dilution on days 1, 7, and 14, and extracellular fluid volume by sodium bromide dilutio n on days 1 and 14. Results-Twenty four infants received early, and 22 delayed, sodium suppleme ntation. There were no significant differences between the groups in body w ater 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 (del ayed -44 ml/kg, p=0.048; early 6 ml/kg, p=0.970). By day 14 the delayed, bu t not the early group, also had a significant reduction in extracellular fl uid volume (delayed -53 ml/kg, p=0.01; early -37 ml/kg, p=0.2). These chang es resulted in a significant alteration in body composition at the end of t he first week (total body weight: delayed 791 ml/kg; early 849 ml/kg, p=0.0 13). By day 14 there were once again no significant differences in body com position between the two groups. Conclusions-Body composition after preterm birth is influenced by the timin g of introduction of routine sodium supplements. Early sodium supplementati on can delay the physiological loss of body water that is part of normal po stnatal adaptation. This is Likely to be of particular relevance to babies with respiratory distress syndrome. A tailored approach to clinical managem ent, delaying the introduction of routine sodium supplements until there ha s been postnatal loss of body water, is recommended.