Rates of Urea Synthesis in the Human Newborn: Effect of Maternal Diabetes and Small Size for Gestational Age

Abstract

ABSTRACT. The rates of urea synthesis and glucose turnover were quantified during fasting using ["N2]urea and [6,62H2]glucose tracers with the prime constant rate infusion technique in 33 infants: 19 normal infants, 10 infants of diabetic mothers (IDM), and four small for gestational age (SGA) infants born at term gestation. All infants were studied during fasting 4 h after their last feed. Eleven normal infants and six IDM were studied soon after birth before any feeding. The rate of urea synthesis in normal infants was 5.84 + 2.0 mg of nitrogen (N)/kg* h-' or 3.5 rmol of urea/kg*min-'. The rate was slightly higher in IDM (7.09 + 3.0 mg N/kg-h-') and lower in SGA infants (4.59 f 1.22 mg N/kg-h-'); however, the differences were not statistically significant. No differences in urea synthesis were observed between infants studied soon after birth and those studied after initiation of feeding. The rate of appearance of glucose was lower in IDM infants studied during the first 6 h after birth (IDM 19.62 f 2.14 rmol/kg*min-', normal infants 24.03 f 4.05 rmol/ kgwmin-I, p = 0.01). However, rate of appearance of glucose in IDM infants studied between 17 and 20 h after birth was similar to that in normal infants. Rate of appearance of glucose was lower (not significantly) in SGA infants (17.7 f 3.3 pmol/kg*min-') as compared with normal infants. No correlation between rates of urea synthesis and glucose turnover was observed. These data show that newborn infants during fasting have an obligatory rate of protein oxidation of -0.87 g/kg*d-' and that maternal diabetes has no effect on it. The slightly lower rate of protein oxidation in SGA infants may be related to increased N assimilation. (Pediatr Res 34: 801-804, 1993) Abbreviations N, nitrogen IDM, infants of insulin-dependent diabetic mothers SGA, small for gestational age AGA, appropriate for gestational age Ra, rate of appearance Measurements of urea N excretion are performed to quantify nitrogen balance and rates of protein oxidation in infants and children. However, limited data are published in literature on the rates of urea synthesis or excretion in the human newborn. Previous data suggested that the rates of urea N excretion in the newborn infant may be much lower than those in the adults as (2) showed that the rates of urea N excretion in the urine of newborn infants were higher than previously suggested. Nevertheless, the urea excretion rate was much less than that estimated for the human fetus at term gestation by Gresham et al. (3). Inasmuch as urinary urea does not reflect the excretion of the total urea synthesized because of loss of urea by skin and possibly because of hydrolysis in the gut, the measurements of urea N excretion in urine will underestimate the actual rates of urea synthesis and therefore protein oxidation (4, 5). In fact, the rates of protein oxidation, as measured by oxidation rates of essential amino acids such as leucine (6, 7), suggest a much higher rate of protein oxidation in full-term newborn infants (-1.4 g/kg. d-I) as compar...

Postnatal Development of Urea Synthesis Capacity in Preterm Infants with Intrauterine Growth Retardation

“…A low capacity for urea synthesis of preterm infants with IUGR was also found by Van Goudoever et al [11]. In contrast, in a study with term infants, the urea synthesis was not affected by IUGR [12], indicating an intrauterine development of this function [8]. An other aspect is the severity of IUGR.…”

“…After application of 15 N-glycine, Van Coudoever et al [11] found a very low 15 N enrichment of urinary urea in preterm infants with IUGR; even the nitrogen balance was high when compared to preterm infants without IUGR. In contrast, Kalhan [12] could not find any influence of IUGR on the urea synthesis rate in term infants.…”

Mechanisms to conserve glucose in lactating women during a 42-h fast

“…were measured using GC-combustion-isotope ratio mass spectrometry and GC-MS, respectively (29). The [ 15 N2]urea enrichment was determined using the 2-pyrimidinol-tert-butyldimethylsilyl derivative as described previously (11,27). Plasma ␣-[1-…”

Comparison of Total Body Urea Production Potential with Total Body Carbamoyl Phosphate Synthetase (CPS-1) Activity in Newborn Piglets Infused with Alanine at 50% of Resting Energy Expenditure for 36 Hours

“…Estimates of in vivo urea production in newborn infants are difficult, given risk involved and the issue of parental consent. Thus, it is not surprising that few such studies (Kalhan 1993) have been done.…”