A comparative study of the response to an oral NaCl and NaHCO3 load in newborn preterm and full term infants

Previous recommendations on salt tolerance of newborn infants have mainly been based on knowledge of the capacity of newborn infants to excrete NaCl. Since tolerance to NaHCO3 can hardly be predicted from knowledge of tolerance of NaCl, a comparative study of Na+ excretion following equivalent doses of NaCl and NaHCO3 has been carried out. Forty-nine full term infants aged 0-2 days and 3-7 days were given either an oral load of NaCl or NaHCO3. Ten preterm infants (gestational age 30-35 weeks, postnatal age 2-18 days) were given both NaCl and NaHCO3 after a 2-day interval. The average urinary Na+ excretion 1-5 hr after the load was higher in all full term infants studied, but the difference was significant only in 3-to 7-day-old infants. In 7 of the 10 preterm infants the urinary Na+ excretion was higher following the NaHCO3 load than following the NaCl load. The mean difference between the tests averaged 1 mmol Na+/1.73 m2/hr and was significant. A somewhat higher daily allowance of NaHCO3 (15 mmol/kg/24 hr) than of NaCl (12 mmol/kg/24 hr) is therefore recommended.