Evidence for functional immaturity of the ornithine-urea cycle in very-low-birth-weight infants.

In a randomized, prospective study the response of serum urea concentration to different protein intakes was studied in two groups of preterm infants with different gestational ages on the 8th and the 21st day of life. 16 very-low-birth-weight (VLBW) infants (GA: 29-31 weeks) and 17 low-birth-weight (LBW) infants (GA: 31-33 weeks) were fed either with fresh human milk or with fortified human milk. The protein intake varied between 2.1 and 3.3 g/kg/day. On the 8th day of life the VLBW infants did not respond to higher protein intakes by increased serum urea concentrations, although a clear correlation was found between protein intake and serum alpha-amino-nitrogen concentrations in these infants. The more mature LBW infants responded to a high protein intake by increased serum urea concentrations on the 8th day of life. On the 21st day of life protein intake correlated to both serum urea and alpha-amino-nitrogen concentrations in all infants studied. These findings are consistent with a somewhat limited capacity of the immature infant for expansion of urea production for some time after birth. This immaturity should be considered when metabolic responses to protein intake are studied and evaluated in such infants.