The Impact of Increasing Carbohydrate Intake Doses on Exogenous Carbohydrate Oxidation, Substrate Utilization, and Exercise Performance

Abstract

This study assessed: (1) the impact of increasing carbohydrate dosages on carbohydrate oxidation and (2) the impact of increasing carbohydrate dosages on exercise performance. Twelve male cyclists/triathletes ingested a placebo or glucose drinks delivering 15, 30, and 60 ghr-1 during 120 minutes of constant load cycling at ~75% VO2 PEAK. Glucose drinks were extrinsically labeled with 1.8 mgg-1 U-13C-glucose and a 20-km time-trial followed each constant load ride. Blood glucose and insulin were highest when ingesting 60 ghr-1 while free fatty acids were the lowest. Insulin and free fatty acid responses for placebo and the 15 ghr-1 trial were virtually identical. Exogenous glucose oxidation rates during the last 30 minutes of the constant load cycling (mean ± SE) were 0.26 ± 0.05, 0.44 ± 0.04 and 0.66 ± 0.07 gmin-1 for 15, 30 and 60 ghr-1 ingestion rates, each being significantly different from each other (p = 0.05). Liver glucose oxidation rate was highest when consuming 15 ghr-1 (0.63 ± 0.13 gmin-1) followed by 30 ghr-1 (0.51 ± 0.12 gmin-1) and 60 ghr-1 (0.42 ± 0.08 gmin-1), all significantly different from one another at a p = 0.05 level. There was also significant reduction in muscle glycogen oxidation during the last hour of the 2-hour constant load ride with no significant differences between the 15, 30, and 60 trials. Relative to placebo, glucose ingestion improved time-trial performance (p = 0.05) with no statistical difference between glucose doses. These findings suggest the ingestion of glucose at increasing rates, between 0 ghr-1 and 60 ghr-1, reduce the demand on liver glucose and carbohydrate ingestion rates as low as 15 ghr-1 can improve cycling time-trial performance.