Six male subjects were studied during running exercise (120 min, 69% maximal oxygen consumption) with ingestion of a placebo or 3.5 g/kg of [¹³C]glucose (~2 g/min). Indirect respiratory calorimetry corrected for urea excretion in urine and sweat, production of ¹³CO₂ at the mouth, and changes in plasma glucose ¹³C/¹²C were used to compute energy substrate oxidation. The oxidation rate of exogenous glucose increased from 1.02 at minute 60 to 1.22 g/min at minute 120 providing ~24 and 33% of the energy yield (%En). Glucose ingestion did not modify protein oxidation, which provided ~4-5%En, but significantly increased glucose oxidation by ~7%, reduced lipid oxidation by ~16%, and markedly reduced endogenous glucose oxidation (1.25 vs. 2.21 g/min between minutes 80 and 120, respectively). The oxidation rate of glucose released from the liver (0.38 and 0.47 g/min, or 10-13%En at minutes 60 and 120, respectively), and of plasma glucose (1.30-1.69 g/min, or 34 and 45%En and 50 and 75% of glucose oxidation) significantly increased from minutes 60 to 120, whereas the oxidation of muscle glycogen significantly decreased (1.28 to 0.58 g of glucose/min, or 34 and 16%En and 50 and 25% of glucose oxidation). These results indicate that, during moderate prolonged running exercise, ingestion of a very large amount of glucose significantly reduces endogenous glucose oxidation, thus sparing muscle and/or liver glycogen stores.