An update of fetal and placental metabolism: carbohydrate and amino acids.

This paper presents a review of fetal and placental metabolism, particularly as it relates to carbohydrate and amino acid metabolism. The strengths of simultaneous application of the Fick principle methodology for the determination of net substrate fluxes with tracer methodology for estimating rates of fetal and placental utilization of compounds is discussed and illustrated. The interconversion of nutrients, both within the fetus and in the placenta, is reviewed with particular emphasis on potential cycling of nutrients between the placenta and fetal liver. Recent work defining the changes in fetal and placental metabolism at mid gestation is also discussed. This work is used as an example illustrating the importance of defining a wide gestational age window for study of ontogenetic changes in fetal metabolism. Finally, recent clinical studies utilizing cordocentesis or periumbilical blood sampling for evaluating the biochemical environment of the human fetus at mid gestation are reviewed and compared to data obtained under chronic steady-state conditions in fetal sheep.