Background Present postnatal nutritional strategies for premature infants often fail to sustain adequate growth. To improve neonatal nutrition, we should explore normal fetal amino acid (AA) metabolism as this is a largely unknown area.

Objective To quantify human fetal leucine (leu) kinetics (incl reversible transamination to its ketoacid αKIC and subsequent oxidation) and methionine (met) kinetics (incl transmethylation to homocysteine (hcys) and subsequent conversion to cysteine).

Methods Eight healthy pregnant women undergoing cesarean section at term received continuous stable isotope infusions of [1-13C,15N]leu and [1-13C,methyl-D3]met prior to surgery. Umbilical blood flow was measured using ultrasound. After birth, umbilical blood was collected and analyzed for leu, αKIC, and met enrichments and concentrations using gas-chromatography mass-spectrometry.

Results Metabolic rates are shown in the table. Conversion of µmol to protein reveals a net fetal uptake of 4.1±1.6 or 2.5±1.8 g/kg/d and a protein breakdown rate of 10±4 or 8±3 g/kg/d based on leu or met kinetics, respectively.

Conclusion Although fetuses were born at term, they still showed considerable AA uptake. The metabolic uptake of available leu and met (AA usage) was low (35 and 22%), implying a large nutritional reserve capacity of placentally delivered nutrients. Whereas mature fetuses show high leu deamination and αKIC reamination, hcys remethylation to met is low.