Alanine production by the human fetus at term gestation.

In a previous study, we demonstrated the complete dependence of the human fetus on the mother for its glucose needs. As alanine is considered the major glucogenic amino acid synthesized endogenously and an important source of urea nitrogen, in the present study we examined whether the human fetus at term gestation can produce alanine. 5 normal pregnant women, undergoing elective cesarean section, were given a constant infusion of [2,3-13C2]alanine in trace quantities for a period of 4 h prior to and during surgery. Isotopic steady state was achieved in the maternal blood by 1.5 h and maintained through anesthesia and surgery. The 13C enrichment (mol% excess) of alanine was measured in the peripheral blood of the mother and in the simultaneously obtained umbilical arterial and venous blood at delivery. Even though the umbilical venous and arterial concentrations of alanine were similar, a 42% decrease in 13C enrichment of alanine occurred between umbilical vein and artery, 1.00 +/- 0.23 and 0.58 +/- 0.15%, respectively (mean +/- SD). These data suggest that the human fetus at term gestation following an overnight maternal fast produces alanine endogenously. This may serve to transfer nitrogen from fetal muscle to the liver for urea synthesis.