A NEW STABLE-ISOTOPE TRACER TECHNIQUE TO ASSESS HUMAN NEONATAL AMINO-ACID SYNTHESIS

The amino acid (AA) synthetic ability and requirements of human infants are undefined. A stable isotope tracer technique was employed in neonates to assess conversion of uniformly labeled C-13 glucose into biochemically nonessential AA (NEAA). Ten neonates (5 males, 5 females) were studied at a mean age of 7 +/- 2.0 (SEM) days. The mean gestational age was 35.5 +/- 1.1 weeks, and the mean weight at time of study was 2,191 +/- 181 g. Six infants were fed enterally, and four received only intravenous 10% dextrose (D10W). Blood samples were obtained before, and 30, 60, and 120 minutes after an orogastric bolus of D-[U-C-13]glucose (100 mg/kg). The conversion of glucose carbon into seven NEAA was assessed by measuring their isotopic enrichments in plasma, using gas chromatography/mass spectrometry (GC/MS), and was expressed as mole percent excess (MPE), with detectable MPE defined as greater than or equal to 0.2. The isotopic enrichment of plasma glucose also was measured using GC/MS. Free plasma AA concentrations were assayed using an automated AA analyzer and expressed in micromoles per liter. The mean glucose enrichment was 9.33 +/- 1.8 MPE (range, 5.82 to 13.48). Detectable C-13-labeling of the NEAA was observed as follows: Glu in 100% of infants; Gly, 100%; Ala, 90%; Ser, 80%; Asp, 70%; Cys, 60%; and Pro, 60%. Detectable Pro enrichment was observed in none of three premature infants on D10W. Free plasma Cys concentration was markedly lower than normal (19.8 v 86 mu mol/L). The number of isotopically enriched NEAA in the plasma of premature infants was 5.1 +/- 0.5, compared with 6.7 +/- 0.3 in that of full-term infants (t = 2.69, p < .05). Test NEAA synthesis was correlated with the following clinical variables: study weight (r = .68), gestational age (r = .30), study age (r = .24), enteral feeding (r = .24), and gender (r = 0). By stepwise multiple linear regression analysis, only study weight independently predicted NEAA enrichment (F = 6.72, P < .05). These results show that a new stable isotope tracer technique can be employed safely and effectively for the study of human neonatal amino acid metabolism. Furthermore, the data suggest that low-weight neonates are unable to synthesize Cys and Pro, AA considered to be biochemically nonessential in adults. Optimal nutritional strategies for neonates may need to address these apparent deficiencies. Copyright (C) 1995 by W.B. Saunders Company.