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The rates of urea synthesis and glucose turnover were quantified during fasting using [15N2]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[middle dot]h-1 or 3.5 [mu]mol of urea/kg[middle dot]min-1. The rate was slightly higher in IDM (7.09 /- 3.0 mg N/kg[middle dot]h-1) and lower in SGA infants (4.59 /- 1.22 mg N/kg[middle dot]h-1); 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 /- 2.14 [mu]mol/kg[middle dot]min-1, normal infants 24.03 /- 4.05 [mu]mol/kg[middle dot]min-1, 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 /- 3.3 [mu]mol/kg[middle dot]min-1) 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[middle dot]d-1 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.

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