ABSTRACT
Fetal Substrate Utilization Studies using fetal sheep demonstrate that the fetus utilizes amino acids, volatile fatty acids (VFAs), glucose, and lactate as major metabolic substrates, with amino acids supplying 40% to 50% of required calories. Lactate is normally present in fetal plasma in concentrations higher than in the dam, and is derived from both fetal and placental glucose oxidation. Roughly half of the glucose, VFAs, lactate, and amino acids are used to provide energy. Under normal circumstances, free fatty acids (bound to albumin) seem to be less utilized by the fetus, which may be due to a reduced capacity for transport across the placenta. However, their maternal hepatic products, ketone bodies, are water-soluble and, therefore, cross the placenta with ease. These are used by the fetus as energy substrates and building blocks for needed macromolecules.
