Postnatal development of urea- and ammonia-excretion in urine of very-low-birth-weight infants small for gestational age.

In 12 very-low-birth-weight (VLBW) infants with intrauterine growth retardation and in 14 VLBW-infants appropriate for gestational age (AGA) fed a human milk (HM) formula (HM enriched with 6 g freeze dried HM per 100 ml) the renal excretion of urea and ammonia was studied on the 10th, 21st and 42nd days of life. The lowest excretion of urea was found in both groups on the 10th day of life. Up to the 42nd day of life the excretion raised significantly more in the AGA- than in the small for gestational age (SGA)-infants. In contrast to the urea excretion the excretion of ammonia was highest on the 10th day of life in both groups, but the excretion was significantly higher in the SGA-infants if compared to the AGA-infants. In the AGA-infants excretion of ammonia decreased with postnatal age whereas in the SGA-infants the high excretion remained up to the 42nd day of life. The data suggest that in VLBW-infants the urea synthesizing capacity is decreased and develops within the first weeks of postnatal life. The postnatal development is delayed in SGA-infants when compared to AGA-infants. The differences are more pronounced with increasing degree of intrauterine growth retardation.