15N natural isotope abundance (NIA) is systematically higher in infants' hair than in that of their mothers at birth. This study aimed to investigate this difference in plasma pools. We compared 15N NIA values for plasma amino acid (AA) pools (free + protein-bound) in the umbilical cord artery (UCA) and vein (UCV) and in the maternal vein (MV) at birth. This preliminary study included 7 mother-infant dyads. Whole plasma was treated (HCl) to hydrolyze protein. Following derivatization, AAs were separated using gas chromatography and compound-specific 15N NIA values were measured on-line using an isotope ratio monitoring mass spectrometer. 15N NIA plasma AA pools in the UCA and UCV were highly correlated to the MV, r 2 > 0.89 and r 2 > 0.88 (both P < 10-4) respectively. The full model found a significant effect of sampling compartment (P = 0.02) and AA type (P < 0.0001) on 15N NIA plasma AA values. 15N NIA plasma AA was 0.74 ‰ higher (P = 0.01) in the MV than in the UCA. This study indicates that a decrease in 15N NIA for plasma AA pools occurs in the fetal-placental unit. Trial registration: ClinicalTrials.gov identifier: NCT00607061.
Keywords: Amino acids; cord blood; fetus; metabolism; natural isotope abundance; nitrogen-15; pregnancy.