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

The amino acid (AA) synthetic ability and requirements of human infant s are undefined. A stable isotope tracer technique was employed in neo nates to assess conversion of uniformly labeled C-13 glucose into bioc hemically nonessential AA (NEAA). Ten neonates (5 males, 5 females) we re 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 onl y intravenous 10% dextrose (D10W). Blood samples were obtained before, and 30, 60, and 120 minutes after an orogastric bolus of D-[U-C-13]gl ucose (100 mg/kg). The conversion of glucose carbon into seven NEAA wa s assessed by measuring their isotopic enrichments in plasma, using ga s chromatography/mass spectrometry (GC/MS), and was expressed as mole percent excess (MPE), with detectable MPE defined as greater than or e qual to 0.2. The isotopic enrichment of plasma glucose also was measur ed using GC/MS. Free plasma AA concentrations were assayed using an au tomated AA analyzer and expressed in micromoles per liter. The mean gl ucose enrichment was 9.33 +/- 1.8 MPE (range, 5.82 to 13.48). Detectab le C-13-labeling of the NEAA was observed as follows: Glu in 100% of i nfants; Gly, 100%; Ala, 90%; Ser, 80%; Asp, 70%; Cys, 60%; and Pro, 60 %. Detectable Pro enrichment was observed in none of three premature i nfants 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 sy nthesis was correlated with the following clinical variables: study we ight (r = .68), gestational age (r = .30), study age (r = .24), entera l feeding (r = .24), and gender (r = 0). By stepwise multiple linear r egression analysis, only study weight independently predicted NEAA enr ichment (F = 6.72, P < .05). These results show that a new stable isot ope tracer technique can be employed safely and effectively for the st udy of human neonatal amino acid metabolism. Furthermore, the data sug gest that low-weight neonates are unable to synthesize Cys and Pro, AA considered to be biochemically nonessential in adults. Optimal nutrit ional strategies for neonates may need to address these apparent defic iencies. Copyright (C) 1995 by W.B. Saunders Company.