Fructose disposal and oxidation rates in the ovine fetus

Fructose disposal and oxidation rates were measured in fetal lambs receiving a constant intravenous infusion of D-[U-14C]fructose. Approximately 60% of the infused tracer entered the placenta, but loss of fructose into the maternal circulation was negligible. Fructose was metabolized to lactate and CO2 in both the placenta and fetus, whereas there was no detectable conversion to glucose. In well-fed ewes the fetal disposal and utilization rates of fructose were 2.4 +/- 0.17 and 0.97 +/- 0.09 mg/min. kg, respectively. The umbilical excretion rate of CO2 originating from the oxidation of fetal fructose was 18.1 +/- 1.3 mumol/min. kg or 5.3% of total fetal CO2 production. This excretion rate is one-fifth of the CO2 excretion rate from fetal glucose carbon. In four ewes comparison of fructose metabolism in the fed and fasted states showed a significant decrease of fructose production and oxidation with fasting. Although fructose is present in high concentrations in the fetal blood of ungulates, its contribution to fetal oxidative metabolism is relatively small in comparison to glucose.