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Extract: The relative utilization rates of glucose and oxygen by the fetal brain were evaluated by measuring simultaneously arteriovenous differences of glucose (Δ glucose) and oxygen (Δ oxygen) across the cerebral circulation of 10 fetal sheep. The mean ± 1 SEM Δ glucose was 0.197 ± 0.012 mM, and the Δ oxygen was 1.11 ± 0.08 mM. Preductal arterial blood was obtained from the brachiocephalic artery; cerebral venous blood was obtained from the fetal sagittal sinus up to 6 days following surgery. A glucose/oxygen quotient was calculated to determine the fraction of cerebral oxygen consumption required to metabolize the glucose utilized by the brain to CO2 and H2O. This quotient was 1.06 with 95% confidence limits of 1.12 and 1.02. These data indicate that glucose can supply all the aerobic metabolic requirements of the fetal brain. Speculation: Although glucose seems to be the primary substrate of cerebral metabolism, the amount of glucose acquired by the fetal lamb via the umbilical circulation can satisfy, at most, one-half of the total fetal aerobic needs. Thus the fetal brain seems much more dependent on glucose than do other fetal organs, and may be particularly vulnerable to periods of hypoglycemia.

Glucose has been considered the principal metabolic fuel of the mammalian fetus, but sound experimental evidence is lacking. In this study, the relative contribution of glucose to aerobic metabolism across the umbilical and cerebral circulations of fetal lambs was evaluated. The fetal vessels catheterized were : femoral artery and vein, umbilical vein, subclavian artery and periorbital vein. The arteriovenous differences in glucose and oxygen contents (Δ glucose and Δ oxygen) were determined from simultaneous samples across both circulations. The Δ glucose was expressed as the O2 stoichiometrically required to metabolize glucose to CO2 and H2O. In six chronic, unstressed, unanesthtized animals, the Δ glucose/Δ oxygen ratio across the umbilical circulation during control periods was 0.47 ± 0.04 (mean ± s.e.m.). Fetal infusions of porcine insulin increased the ratio by 90%. We have attempted to compare metabolism in a single organ of the fetus, the brain, with that of the whole fetus (i.e. umbilical circulation) in acute experimetns in seven fetuses. The ratio across the cerebral circulation was 1.5 ± 0.03 and across the umbilical circulation was 0.38 ± 0.03. These data indicate that (a) under normal conditions, glucose supplies a maximum of 50% of the metabolic fule of the fetal lamb, (b) exogenous insulin does affect fetal glucose uptake, and (c) in the acute preparation, the fetal brain utilizes more glucose than can be accounted for by aerobic metabolism.