Journal Title: Med Sci Sports Exerc
Year: 2010 [Epub ahead of print]
Background: The benefits of carbohydrate (CHO) ingestion during prolonged exercise have been well established. Recent studies suggest that CHO ingestion also has a positive effect on short (1 hour), intense (> 75% maximal oxygen [O2] consumption [VO2max]) endurance performance and time-trials. However, in these trials there is no clear metabolic advantage to providing exogenous CHO, leading investigators to speculate that performance benefits from CHO ingestion are related to its effects on the central nervous system. In a recent study, placement of either glucose or maltodextrin in the mouth was shown to activate regions in the brain associated with reward. This “central effect” may explain reports showing that placement of CHO in the mouth has a positive effect on cycling time-trial performance.
Hypothesis:The authors of the current study hypothesized that mouth-rinsing and ingestion of a CHO-electrolyte solution would have a positive effect on a 1-hour running performance trial.
Subjects:10 experienced male runners accustomed to training and/or competitions lasting ≥ 1 hour participated in the study. Mean ± standard deviation (SD) age, height, body mass, and VO2peak were 26 ± 6 years, 1.81 ± 0.06 m, 74.2 ± 5.7 kg, and 65.0 ± 4.4 mL•kg/min, respectively.
Experimental design: Randomized, double-blind (with regard to the ingestion treatments), crossover
Treatments and protocol: All exercise trials were carried out on a motorized treadmill equipped with an ultrasonic feedback-controlled radar modulator that spontaneously regulated belt velocity relative to the changing position of the runner on the treadmill. During each trial subjects were instructed to run as fast as possible. The study began with a habituation trial in which subjects ingested water in place of a CHO drink, and then ran for 1 hour on the treadmill. Subjects participated in 3 exercise trials (1-hour run) spaced 7 days apart after an overnight (14- to 15-hour) fast. Subjects were randomly assigned to ingest either a 6.4% carbohydrate-electrolyte solution (CHO) or an electrolyte-matched, carbohydrate-free placebo solution (P), or to mouth-rinse (R) with the CHO solution. For the 2 ingestion treatments, the subjects ingested 8 mL solution/kg 30 min before the 1-hour run, 25 mL solution immediately before the run, and 2 mL solution/kg every 15 min during the run. Subjects were instructed to rinse the last mouthful of each drink for 5 sec before swallowing to match oral contact time with the R treatment. The feeding schedule for the CHO ingestion treatment was designed to provide subjects with approximately 60 g CHO/hour. For the R treatment, the subjects mouth-rinsed a 25-mL volume of each solution for 5 sec before expectorating the solution, and the rinsing occurred at the same time intervals as the 2 ingestion treatments. Ratings of perceived exertion (RPE) and fingertip blood samples were provided by subjects in each trial. The prospective threshold value for a beneficial or negative effect of CHO on performance was set at 1% of the mean distance covered in the 3 trials.
This study found that ingestion of a CHO-electrolyte solution significantly improved 1-hour running performance compared with just mouth-rinsing with the same CHO-electrolyte solution or ingestion of an equivalent volume of a non-carbohydrate placebo solution. This is in contrast with previous studies that have shown equivalence of mouth-rinsing versus ingestion of carbohydrate. Unexpectedly, no accompanying changes in oxygen uptake, heart rate, or blood lactate concentrations were observed. One possible explanation for this result is that the methods of monitoring physiologic changes in this study were not sensitive enough to detect the physiologic effects of small changes in running speed. Future studies are needed to determine the relative contributions of “central” and “peripheral” effects of carbohydrate-electrolyte solutions on exercise performance.