The influence of hypotonic carbohydrate electrolyte solutions on muscle metabolism and exercise capacity with regards to intermittent high intensity shuttle running
Performance and exercise capacity in prolonged continuous exercise has been shown to be enhanced by the ingestion of dilute (-6%) carbohydrate-electrolyte (CHO-E) solutions during exercise. There is a wealth of literature on the effects of CHO-E solutions on prolonged, continuous exercise however this is not the case for highintensity intermittent exercise. One reason for this is the difficulty of replicating the demands of multiple-sprint sports within the laboratory. The development of the Loughborough Intermittent Shuttle Running Test (LIST), a protocol that simulates the physiological demand of multiple-sprint sports, has allowed for further study in this area. Sports drinks contain a variety of different types of carbohydrate, including synthetic polymers of maltodextrins. The rationale for these is two-fold; firstly there is some evidence to suggest that these glucose polymers of low osmolality may empty faster than isoenergetic glucose solutions especially at dilute (-6%) concentrations. Secondly CHO-E solutions formulated from maltodextrins with low dextrose equivalents are less acidogenic and have lower cariogenic potential than simple glucose solutions, thus making them preferable for tooth health. Therefore the purpose of this thesis was to examine the effects of a 6.4% hypotonic synthetic polymer maltodextrin CHO-E solution on muscle metabolism and physiological performance during prolonged high-intensity intermittent running (LIST). This thesis contains four experimental studies. From the results of study 1 it was concluded that the ingestion of a 6.4% hypotonic CHO-E solution provided no ergogenic benefit to running capacity during the LIST despite blood and hormonal data suggesting that the solution was effective at delivering glucose. As isotonic CHO-E solutions have shown to have ergogenic properties during LIST running the purpose of study 2 was to compare isoenergetic hypotonic and isotonic CHO-E solutions during LIST running. The results from this study suggested that performance, as measured by exercise capacity and sprint speed, was not affected by either treatment. In study 3 the protocol was modified to extend the exercise duration so that a greater demand was placed on the subjects' glycogen reserves. The results from this study further confirmed that exercise capacity in LIST running was not influenced by CHO provision during exercise however the habitual CHO intake of the subjects may have masked any ergogenic properties of the CHO-E solution and led to increased performance in the control trials. In the final study the subjects' pre-trial exercise and dietary CHO intake was manipulated to increase endogenous glycogen concentrations. The results showed that in these subjects the ingestion of a 6.4% hypotonic CHO-E solution increased exercise capacity during the LIST by 21%. Muscle biopsy analyses revealed a lower net muscle glycogen utilisation in the CHO trial post-90 min (p=0.07). There is evidence in the literature to suggest that there is an ergogenic effect of CHO ingestion during the LIST. However the results presented in this thesis suggest that when endogenous glycogen stores are moderate the ingestion of a CHO-E solution may suppress lipid oxidation without a concomitant increase in CHO oxidation and thus LIST capacity is not enhanced. In contrast, when endogenous glycogen concentrations are elevated through dietary manipulation there is an ergogenic benefit from the ingestion of hypotonic CHO-E solutions during LIST exercise.