Urea production and recycling in neonates

Urea represents 15% of the nitrogen in breast milk, but its functional significance is unclear. We have proposed that the urea may be utilised as a valuable source of nitrogen in neonates. We report the first measurements of urea kinetics in newborn, full-term infants. Six neonates received a continuous nasogastric infusion of 15N15N-urea over 24 hours. Urine was collected every 2 hours and the isotopic enrichment in urea was measured by mass spectrometry. Urea kinetics were calculated by the method of Jackson. The urea production rate was 17.3 mmol/kg/d (range, 15.6 to 19.2), but the urinary urea excretion rate, 3.5 mmol/kg/d (range, 1.2 to 4.9), was only 20% of production. Hence, 80% of urea produced was hydrolysed in the colon and the nitrogen made available for further metabolism. These data show that urinary urea excretion is a poor indication of urea production in the newborn and that there is substantial salvaging of urea nitrogen by the colon.