Bioavailability of Urea Nitrogen for the Low Birthweight Infant

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Abstract
Previous studies have demonstrated increased retention (40%) of dietary urea nitrogen by term infants recovering from infection compared to healthy infants (13%), possibly due to a higher requirement for nitrogen. Since low birthweight infants also have a high requirement for nitrogen, the bioavailability of urea nitrogen was investigated in low birthweight infants using 15N, 15N‐urea. Four low birthweight infants (gestational age = 30 ±2.2 weeks [mean ± SD, birthweight = 1.4 ±0.3 kg) were fed formula enriched with 15N, 15N‐urea. Metabolic balance studies (72 hours) were performed with urine and fecal collections. Nitrogen was quantitated by Kjeldahl analysis and 15N, 15N‐urea by gas chromatography/mass spectrometry. Mean nitrogen intake was 489 ± 32 mg/kg/d, with 75 % nitrogen absorption and 62% nitrogen retention. Maximum urinary enrichment was 8%. Cumulative 15N, 15N‐urea excretion was 72%, resulting in 28% retention. Thus the bioavailability of urea nitrogen for low birthweight infants appears to be intermediate between compromised and normal term infants.