Oral presence of carbohydrate and caffeine: independent and combined effects on endurance performance
Background: Carbohydrate ingestion and mouth-rinsing has been shown to improve exercise performance during high-intensity, short duration endurance exercise. The precise mechanisms underlying the ergogenic effects remains unidentified, but have been partially attributed to central effects being mediated via the detection of carbohydrate in the mouth by oral ‘energy-receptors’. Similarly, caffeine ingestion and delivery via the buccal cavity has been shown to improve endurance exercise performance, with ergogenic effects attributed to centrally mediated effects on adenosine receptor sites. However, no study has investigated whether providing carbohydrate and/or caffeine late in exercise can improve performance when under realistic conditions of exercise-induced fatigue. Aim: To determine the independent and combined effects of carbohydrate and caffeine chewing gum on self-paced cycling time-trial performance under the influence of exercise-induced fatigue. Further, the study aimed to examine the possibility of the chewing gums’ contents mediating central mechanistic effects on subconscious motor output (pacing) during time-trial performance. Method: Using a double-blind, repeated measures, cross over design, eleven male competitive cyclists (Mean ± SD: age: 32.2 ± 7.5 yr, body mass: 74.3 ± 6.8 kg; V̇O2peak: 60.2 ± 4.0 ml•kgˉ1•minˉ1) performed 90-min constant-load cycle at 80% of their second ventilatory threshold (207 ± 30 W) followed by a 20-km time-trial under each condition. At the beginning and at every 25% of the total distance (5, 10, 15 km) during the time-trial, participants were given one piece of chewing gum to chew for 3 km (~5 min) containing either: placebo (PLA: artificial sweeteners), carbohydrate (CHO: ~1.8 g sucrose per piece + artificial sweeteners), caffeine (CAF: 50 mg per piece + artificial sweeteners), and carbohydrate and caffeine (CHO+CAF: ~1.8 g sucrose + 50 mg caffeine per piece + artificial sweeteners). Power output and heart rate were recorded continuously throughout the trial. Blood glucose and lactate samples were obtained before and after the time-trial, whilst perceptual measures on completion only. Data were analysed using an MS Excel spreadsheet designed for statistical analysis. The uncertainty in the effect was expressed as 90% confidence limits and a smallest worthwhile effect of 1.0% for power output was assumed. Results: No substantial alterations in time-trial performance were observed with CHO (Mean ± SD: 271 35 W), CAF (273 40 W), or CHO+CAF (270 37 W) compared with PLA (270 37 W). However, mean power output had a tendency to be improved in the first two quarters of the 20-km time-trial with CHO (Mean ±90%CL: 1.6 ±3.1 and 0.8 ±2.0%); whilst in CAF and CHO+CAF trials, mean power output was substantially enhanced in the final quarter (4.2 ±3.0 and 2.0 ±1.8%). No differences in heart rate (ES <0.2) were observed between trials. Blood lactate concentrations on completion of the time-trials were substantially higher for CAF (ES 90%CL: 0.90 1.09 and 0.85 0.55, respectively), and CHO+CAF (0.78 1.14 and 0.72 0.62, respectively) than PLA and CHO trials. Differences in changes in blood glucose between experimental trials were small (+1.1-1.7 mmol/L-1) and appeared unrelated to performance. Conclusion: Endurance performance, under conditions of fatigue and reduced glycogen, was not altered by the oral presence of carbohydrate and caffeine in chewing gum, either independently or combined. However, results support the theories of central activation and manipulation of the anticipatory regulation strategy in response to oral carbohydrate and/or caffeine by demonstrating subconsciously altered motor output during the time-trial. Specifically, carbohydrate appeared to facilitate an immediate increase in power output, whilst caffeine exhibited ergogenic effects later in exercise. Practically, the results suggest that 1) utilising an oral carbohydrate gum, in combination with prior ingestion of small amounts of carbohydrate early in exercise (<90min), or 2) the delivery of caffeine, via a chewing gum, in absence or presence of supplemental carbohydrate, may facilitate improvements in endurance performance by increasing central drive and thus, re-setting the internal pacing strategy to allow a higher power output at one’s self-selected ‘maximal’ work rate.