Urea and its bioavailability in newborns.

urea synthesised in the body.2 The discrepancy could result not only from the hydrolysis of urea in the gut but also from excretion of urea in the skin. A significant proportion of N released from hydrolysis of urea in the gut is also known to be recycled into urea or excreted as ammonia in the breath. In healthy adults with a normal protein intake about 20% of synthesised urea is not excreted in urine, and an insignificant amount of urea N is incorporated into protein.3 Whether such a salvage of N occurs in newborns is not known. Data on breast fed infants, both pre-term and full term, have shown that most of the urea ingested is not bioavailable that is, it is not hydrolysed in the gut.4 Studies in healthy pre-term and full term newborns show that the rate of urea synthesis measured by isotopic tracers ranges between 3 mg and 6 mg N/kg/hour, which is similar to the rate of urinary urea N excretion measured in a number of balance studies.5 6 In contrast, Wheeler et al observed very high rates of urea N synthesis (about 17-3 mmol N/kg/day or 20 mg N/kg/hour) in six neonates after major abdominal surgery.7 However, the rate of urinary urea N excretion was similar to that reported by others. Thus the discrepancy between synthesis and excretion was astronomical (80%). Interestingly, the recycled N