Glucose kinetics during prolonged exercise in highly trained human subjects: Effect of glucose ingestion

1. The objectives of this study were (1) to investigate whether glucose ing estion during prolonged exercise reduces whole body muscle glycogen oxidati on, (2) to determine the extent to which glucose disappearing fi om the pla sma is oxidized during exercise with and without carbohydrate ingestion and (3) to obtain an estimate of gluconeogenesis. 2. After an overnight fast, six well-trained cyclists exercised on three oc casions for 120 min on a bicycle ergometer at 50 % maximum velocity of O-2 uptake and ingested either water (Fast), or a 4 % glucose solution (Lo-Glu) or a 22% glucose solution (Hi-Glu) during exercise. 3. Dual tracer infusion of [U-C-13]-glucose and [6,6-H-2(2)]-glucose was gi ven to measure the rate of appearance (R-a) of glucose, muscle glycogen oxi dation, glucose carbon recycling, metabolic clearance rate (MCR) and non-ox idative disposal of glucose. 4. Glucose ingestion markedly increased total R-a especially with Hi-Glu. A fter 120 min R-a and rate of disappearance (R-d) of glucose were 51-52 mu m ol kg(-1) min(-1) during Fast, 73-74 mu mol kg(-1) min(-1) during Lo-Glu, a nd 117-119 mu mol kg(-1) min(-1) during Hi-Glu. The percentage of R-d oxidi zed was between 96 and 100% in all trials. 5. Glycogen oxidation during exercise was not reduced by glucose ingestion. The vast majority of glucose disappearing from the plasma is oxidized and MCR increased markedly with glucose ingestion. Glucose carbon recycling was minimal suggesting that gluconeogenesis in these conditions is negligible.