Responses to varying rates of carbohydrate ingestion during exercise

The purpose of this study was to determine how the ingestion of carbohydrate at varying rates influences physiological, sensory, and performance responses to prolonged exercise at 65-75% VO2max. Ten subjects ingested either a water placebo (WP) or carbohydrate solutions formulated to provide glucose at the rates of 26, 52, and 78 g, h-1 during 2 h of cycling exercise in a cool (10 degrees C) environment. Beverages were administered in a double-blind, counterbalanced design. A 4.8 km performance test followed each 2 h session. The average time required to complete the performance test was less with the carbohydrate feedings than with WP: mean (+/- SE) for WP = 505.0 +/- 18.7 s. 26 g.h(-1) = 476.0* +/- 8.8 s. 52 g.h(-1) = 483.8 +/- 12.7 s. 78 g.h(-1) = 474.3* +/- 19.1 s; *P less than 0.05 vs WP. Carbohydrate feeding resulted in higher plasma glucose and insulin, and lower free fatty acid concentrations than did WP. Changes in plasma osmolality, plasma volume, rectal temperature, lactate, heart rate, respiratory exchange ratio, ratings of perceived exertion, and sensory responses were similar among beverage treatments. Compared with WP, ingestion of the glucose beverages minimized changes in plasma ACTH and cortisol. In summary, carbohydrate feeding at the rates of 26 and 78 g.h(-1) was associated with improved exercise performance. The data further indicate that a dose-response relationship does not exist between the amount of carbohydrate consumed during exercise and exercise performance.