Fuel metabolism in the mammalian fetus

The mammalian fetus receives energy fuels from its mother through the placenta. The placental transfer of substrates depends upon the placental permeability to the substrate and upon the difference of concentration of the substrate in maternal and fetal blood. The fetus uses the substrates for 3 purposes : synthesis of new tissues (growth), oxidative metabolism and building of energy stores. Since the 02 consumption per Kg of body weight is relatively constant among the different species, the partitioning of substrates in anabolic and catabolic pathways is related to the rate of fetal growth. Glucose and lactate account for a large part of the oxidative needs of the fetus, but amino acids catabolism plays also an important role. During maternal starvation, ketone bodies can be used as oxidative substrates by the fetus of monogastrics. The fetus has a low capacity to oxidize free fatty acids, even in species in which the placental transfer of these substrates occurs rapidly. Free fatty acids are used as precursors of complex lipids or are stored in fetal adipose tissues or liver. Several recent observations suggest that gluconeogenesis could occur in the fetus of ruminants, and thus could allow the transformation of lactate and amino acids into glucose before utilization by individual tissues of the fetus (brain, skeletal muscle, heart). The hormones secreted by the fetus has been shown to play an important role in inducing the storage of glycogen in fetal liver (glucocorticoids and a pituitary hormone) and the accumulation of triglycerides in fetal adipose tissue (insulin).