Influence of Ingested Glucose Dose on Oxidation, Endogenous Carbohydrate Utilization and Cycling Time-Trial Performance

Abstract

PURPOSE: To determine dose response effect of glucose ingestion on exogenous glucose oxidation, endogenous glucose utilization and cycling time-trial performance. METHODS: Ten male cyclists/triathletes ingested a placebo or glucose drinks delivering 15, 30, and 60 g/hr during 120 minutes of constant load cycling at 75% VO2 peak. Glucose drinks were extrinsically labeled with 1.8 mg/g U-13C-glucose and a 20-km time-trial followed each constant load ride. RESULTS: Relative to placebo, glucose ingestion improved (p < 0.01) time-trial performance with no statistical difference between glucose doses. Blood glucose and insulin were highest when ingesting 60 g/hr while free fatty acids were the lowest. Insulin and free fatty acid responses for placebo and the 15 g/hr trial were virtually identical. Exogenous glucose oxidation rates during the second hour of constant load cycling (mean ± SE) were 0.21 ±0.01, 0.39 ±0.02 and 0.62 ± 0.03 g/min for 15, 30 and 60 g/hr ingestion rates, each being significantly different from each other (p < 0.001). For glucose ingestion trials, endogenous glucose oxidation rate was highest when consuming 15 g/hr (2.69 ± 0.11 g/min) versus 30 and 60 g/hr (2.36 ± 0.12 g/min and 2.42 ±0.13 g/min, respectively p < 0.01). CONCLUSIONS: Ingesting glucose at a rate of between 15 and 60 g/h resulted in improved cycling time-trial performance but an intake of 15 g/hr placed a greater demand on endogenous carbohydrate stores to support energy metabolism.