Slivka, D., Cuddy, J., Hailes, W., & Ruby, B. (2008). Effects of caffeine and carbohydrate use on exercise performance, substrate oxidation, and salivary cortisol. ACSM 55th Annual Meeting Indianapolis, Presentation Number, 2034.

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This study determined the effects of caffeine and carbohydrate supplementation on substrate use, exercise performance, and salivary cortisol response. Males (N = 7) completed four randomly assigned trials including two hours of cycling at 512% of VO2max followed by a simulated 20-km time trial. Biopsies and saliva samples were obtained before and after the two-hour ride. The four trials consisted of mixing the conditions - placebo/placebo, placebo/carbohydrate, caffeine/placebo, and caffeine/carbohydrate. Diet and exercise were controlled for two days before each trial to ensure Ss were in negative energy balance thus adding to the physical stress.

Muscle glycogen use was similar among all trials. Total fat oxidation during the two-hour ride was higher in caffeine/placebo and caffeine/carbohydrate trials than in the placebo/carbohydrate trial. Cortisol response did not reach statistical significance in any condition. Time-trial performance was better in placebo/carbohydrate and caffeine/carbohydrate trials compared to the placebo/placebo trial.

Implication. When carbohydrate is ingested, fat oxidation is reduced unless supplemented with caffeine. Carbohydrate is needed to improve performance.

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