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The purposes of this study were 1) to investigate the effect of carbohydrate (CHO) ingestion on endogenous glucose production (EGP) during prolonged exercise, 2) to study whether glucose appearance in the circulation could be a limiting factor for exogenous CHO oxidation, and3) to investigate whether large CHO feedings can reduce muscle glycogen oxidation during exercise. Six well-trained subjects exercised three times for 120 min at 50% maximum workload while ingesting water (FAST), a 4% glucose solution (LO-Glc), or a 22% glucose solution (HI-Glc). A primed continuous intravenous [6,6-²H₂]glucose infusion was given, and the ingested glucose was enriched with [U-¹³C]glucose. Glucose ingestion significantly elevated CHO oxidation as well as the rates of appearance (R_a) and disappearance. R_a glucose equaled R_a of glucose in gut (R_a gut) during HI-Glc, whereas EGP was completely suppressed …