Contribution of fructose and lactate produced in placenta to calculation of fetal glucose oxidation rate

We examined the rate of production of [¹⁴C]fructose and [¹⁴C]lactate from [U-¹⁴C]glucose by the placenta and the contribution of ¹⁴CO₂ from fetal oxidation of these metabolic products to the calculation of glucose oxidation rate in fetal sheep. During fetal tracer infusions (n = 16), oxidation of fructose contributed 16 ± 3% of total fetal CO₂ production; oxidation of lactate accounted for 3.3 ± 0.1%. Thus 80% of total fetal CO₂ production resulted from direct oxidation of carbon atoms in glucose; the 'direct' glucose oxidation fraction was 0.46 ± 0.04. During maternal tracer infusion (n = 15), CO₂ production from fructose was 21 ± 3, 20 ± 3, and 30 ± 4% and from lactate was 16 ± 3, 13 ± 3, and 11 ± 4% in hypo-, normo-, and hyperglycemic animals, respectively; the direct glucose oxidation fraction was 0.40 ± 0.04, not different from the fraction obtained with the fetal tracer infusion. Fetal oxidation of substrates derived from glucose metabolism in the placenta contributes significantly to fetal CO₂ production. Fetal oxidation of placental products of a metabolic substrate tracer should be considered in studies of fetal oxidative metabolism.