Title: Effects of Caffeine and Carbohydrate Use on Exercise Performance, Substrate Oxidation and Salivary Cortisol
Abstract: The ingestion of carbohydrate (CHO) and caffeine (CAF) during exercise is a common nutritional strategy used during endurance exercise. Caffeine has been proposed to increase sustained exercise capacity by increasing fat oxidation and sparing muscle glycogen. However, the addition of carbohydrate to caffeine supplementation may alter these substrate use patterns. PURPOSE:To determine the effects of caffeine and carbohydrate supplementation on substrate use, exercise performance, and salivary cortisol response. METHODS:Male participants (n=7, 25+/-3 yr, 74+/-13 Kg) completed four randomly assigned trials including 2 hours of cycling at 51+/-2% of VO2 max followed by a simulated 20km time trial. Biopsies and saliva samples were obtained before and after the two-hour ride. The four trials consisted of PLA/PLA, PLA/CHO, CAF/PLA, and CAF/CHO. Diet and exercise were controlled for two days before each trial to ensure that participants were in negative energy balance thus adding to the physical stress. RESULTS: Muscle glycogen use was similar among all trials. Total fat oxidation during the two-hour ride was higher in CAF/PLA (88.3+/-25.0 g) and CAF/CHO (79.4+/-20.2 g) than in PLA/CHO (71.8+/-19.1 g, p<0.05). Cortisol response did not reach statistical significance but demonstrated a trend for higher levels in the PLA/ PLA and CAF/PLA trials. Additionally, time trial performance was better in PLA/CHO (171+/-60 watts) and CAF/CHO (192+/-43 watts) compared to PLA/PLA (129+/-84 watts, p<0.05). CONCLUSIONS: These data demonstrate that when carbohydrate is ingested fat oxidation is reduced unless supplemented with caffeine. The ingestion of carbohydrate shows a trend toward diminished cortisol response. Thus, the observed increase in fat oxidation during CAF/CHO may be stimulated by mechanisms independent of cortisol induced lipolysis. Supported by Air Force Research Laboratories, FA8650-06-1-679