Urea kinetics in neonates receiving total parenteral nutrition

Urea kinetics were measured on 10 occasions in eight neonates who had not received an oral intake from birth and were maintained on total parenteral nutrition. After a prime/intermittent oral dose of 15N15N-urea over 14 hours urine was collected every three to four hours, urea isolated, and kinetics determined from the plateau level of enrichment in urea, measured by isotope ratio mass spectrometry. The total parenteral nutrition provided 393 kJ (94 kcal)/kg/day and 360 mg nitrogen/kg/day. Urea production was mean (SD) 84 (44) mg nitrogen/kg/day, or 50% of intake. Urinary excretion of urea, 39 (16) mg nitrogen/kg/day, was 40% of production. Therefore 54% of urea production was salvaged through the lower bowel, 45 (35) mg nitrogen/kg/day. It is concluded that even in infants who have never had a regular dietary intake the microflora of the lower bowel is sufficiently developed to salvage urea nitrogen for further metabolic interaction, however it is not clear whether the rate of salvage is adequate to satisfy the metabolic demand.