Impact of Prolonged Exercise in the Heat and Carbohydrate Supplementation on Performance of a Virtual Environment Task

Military Medicine, Feb 2008 by Bailey, Stephen P, Holt, Craig, Pfluger, Kent C, Budde, Zina La, Afergan, Daniel, Stripling, Roy, Miller, Paul C, Hall, Eric E

CHO supplementation during prolonged exercise has been shown to "spare" Uver glycogen"; however, the impact of CHO supplementation on the breakdown of muscle glycogen remains unclear.15 CHO supplementation during moderate intensity exercise has repeatedly been shown to prevent the drop in blood glucose that is typically seen after 1 hour or more of continuous exercise when a water placebo is consumed.18,19

CHO supplementation may also improve physical performance through central mechanisms. Evidence suggesting that CHO supplementation influences physical performance through central mechanisms is provided by three different types of experiments. CHO feeding during relatively short (60 minutes or less) high-intensity exercise (>75% of VO^sub 2max^) consistently has been shown to have an ergogenic effect despite the fact that it is extraordinarily unlikely that an exogenous CHO source could influence energy production within the muscle.20 Furthermore, a unique experiment by Carter et al.21 found that exercise performance of a task approximately 60 minutes in length was slightly (2.8%), but significantly improved when subjects simply rinsed their mouth with a CHO solution. CHO supplementation during prolonged exercise has also been shown to indirectly influence the availability of various amino acids to brain, ultimately impacting production of various neurotransmitters that could influence exercise performance.22,23

Improvement of exercise performance in the heat subsequent to CHO feeding is theorized to result from attenuation of water loss and improved availability of glucose for energy production. Fluid loss equivalent to 2% of body weight has consistently been shown to negatively influence exercise performance.24 In this investigation, changes in body weight were greater during the hot trials than during the normal environment trials; however, there were no differences between drink conditions. Furthermore, the decreases in body weight were consistently less than 2% during all conditions. Similar findings were found for heart rate and core temperature. Both of these variables were negatively affected by exercise in the heat; however, heart rate never exceeded 80% of maximal heart rate and core temperature never approached 39°C. All of these physiologic findings suggest that it is unlikely that dehydration played a significant role in the changes in performance observed here.

Quantitative performance during the VE task was best when subjects consumed CHO during the normal environment. This is demonstrated by the greater number for kills and lower number of failures seen under these conditions as compared to the other conditions (Figs. 2 and 3). CHO supplementation during exercise in the hot environment also resulted in a fewer number of failures than when the subject consumed the water placebo (Fig. 2). Considering the lack of differences observed in the physiologic variables (body weight, heart rate, and core temperature) it is not likely that the positive influence of CHO feedings was the result of differences in hydration status. Rather, it is much more likely that these differences are subsequent to differences in glucose availability.

As mentioned previously, the CHO feeding regime used in this investigation has been shown to maintain blood glucose levels during exercise in NORM and HOT environments.19,25 Decrements in cognitive function following strenuous military field exercises and prolonged physical activity in a laboratory setting have previously been found to be associated with decreases in glucose availability. Furthermore, CHO feeding during prolonged exercise has been shown to be an effective strategy for attenuating these negative changes in cognitive function. Although assessment of cognitive function during these military exercises has used procedures that approximate function during combat (complex decision making, marksmanship, etc.)1,10 this is the first investigation where changes in performance in a virtual warfare environment are negatively affected by prolonged exercise and heat stress. Furthermore, the use of CHO feedings during these stressors was effective at attenuating these negative changes.