Jentjens, R. L., Bridge, M. W., Wagenmakers, A. J., & Jeukendrup, A. E. (2001). Reduced oxidation of ingested carbohydrates during exercise in the heat. Medicine and Science in Sports and Exercise, 33(5), Supplement abstract 19.

The oxidation rate of ingested carbohydrate (CHO) was compared in hot (35+ degrees Celsius) and cool (16+ degrees Celsius) conditions. Trained cyclists (N = 9) performed (90 min of cycling at 67% VO2max) in each condition one week apart. Ss ingested 8 ml/kg of an 8% glucose solution at the onset of exercise followed by 3 ml/kg every 15 minutes thereafter. [U/13C]-glucose was added to the solution to enable exogenous glucose to be tracked.

Heart rate and rectal temperature were significantly higher in the hot compared to the cool condition. Exogenous glucose was significantly lower in the hot than cool condition.

It was concluded that muscle glycogen use increased, and the oxidation rate of ingested CHO decreased, during exercise in the heat compared to exercise in a cool environment.

Implication. Ingested CHO use declines in exercise as the environmental temperature increases.

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