Bioavailability of urea nitrogen for the low birthweight infant

Previous studies have demonstrated increase 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.