There have been many different tests and training regimens put together to see what affects that carbohydrate intake have on athletic performance. No matter how the tests have been put together, they always involve the ingestion of carbohydrates either before, during, or even after an exercise. It has been observed that very little attention has been devoted to see whether high carbohydrate diets affect moderate exercise lasting between 20-60 minutes. The purpose of this investigation is to examine the effects of a high versus low carbohydrate diet during moderate duration (45-minute) strenuous cycling exercise (2). Athletes who compete after an overnight fast may fatigue prematurely because fasting reduces liver glycogen stores. Therefore, athletes are advised to avoid prolonged fasting and to ingest a meal high in carbohydrates and low in fats, proteins, and fiber about 3-4 hours before exercise. The purpose of this study is to determine whether a moderate amount of carbohydrate food ingested 3 hours before exercise, improves endurance running capacity when compared to running without the meal and ingesting only water during the exercise (1). Dehydration and depletion of the bodies limited carbohydrate stores are the two most important influences on endurance capacity during prolonged exercise. Therefore, the purpose of this study is to examine the effects of rehydration per se and dehydration plus CHO ingestion during 4 hours of recovery on subsequent endurance running capacity (3).
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Twelve trained male cyclist volunteered to participate in the investigation. The men all trained regularly, competed in road cycling or mountain bike races, were non-smokers, and reported a history free of endocrine, cardiovascular, renal, and thermoregulatory disorders. The subjects performed a 45 minute cycling trial at 82 plus or minus 2% of VO2peak after a 6-day diet and exercise control. The 7 the day trial was repeated under the two randomly assigned dietary trials: high carbohydrate and low carbohydrate on electrically braked cycle ergometer (2). Ten male recreational or club level runners participated in this study. The subjects were required to run to exhaustion at 70% VO2max on a motorized level treadmill on three different occasions separated by a week. On one occasion, a high carbohydrate meal was consumed 3 hours before exercise, and 6.9% carbohydrate-electrolyte solution was ingested during exercise. On another occasion the high-carbohydrate meal was consumed 3 hours before exercise, but during the exercise the subject drank only water. On another occasion subjects drank 10 ml of placebo solution 3 hours before they started running and during the exercise they drank only water (1). Nine endurance-trained male subjects volunteered for the study. The subjects were required to complete two experiments in which two treadmill runs were performed, separated by at least 7 days. On the day of the experiment, subjects reported to the laboratory 8 hours after a 12-hour overnight fast. Their nude body mass was measured before the exercise. Following a 5 minute warm-up at a running speed corresponding to 60% VO2max the treadmill speed was increased to a pace equivalent to 70% VO2max of the subject. In the first trial the subjects ran for 90 minutes, and immediately after, a 4 hour rehydration recovery period started. In the second trial subjects were required to run at 70% of VO2max for as long as possible as a measure of their endurance capacity. To ensure max effort during the second trial the subjects were given strong verbal encouragement and were allowed to reduce the treadmill to walking pace on two occasions for duration of 2 minutes. The treadmill was increased to prescribed speed and when the subjects were unable to keep up with prescribed speed the trial was terminated. Each subject was prescribed a volume of fluid equivalent to 200% of body mass lost during trial one in both experimental trials. The fluid ingested was either a 6.9% CHO-electrolyte solution or a CHO and electrolyte free sweetened placebo (3).
As a result of the 6-day exercise and diet manipulation, pre-exercise muscle glycogen concentration during the high carbohydrate trial was significantly greater than the low carbohydrate trial. At the of the high-carb trial blood glucose was significantly higher (p>.05) than the pre-exercise level, while the blood glucose for the low-carb trial didn't change. Also, plasma triglycerides mirrored the blood glucose having increased for the high-carb trial and none at all for the low-carb trial (2). The results on endurance capacity after ingesting high-carb meal and 6.9% carbohydrate solution during exercising were obvious. Exercise time to exhaustion was longer in the (high-carb + carb solution) and the (high-carb + water) trial compared with the (placebo + water). Exercise time was also longer in the (high-carb + carb solution) rather than (high-carb + water) trial. There were no differences in oxygen uptake or heart rates measured between the three trials. However, the rates of perceived exertion were higher in the (placebo + water) trial as compared with the (high-carb + carb solution) trial at 60 minutes of exercise, but didn't differ from the (high-carb + water) trial. Also, the respiratory exchange ratios were higher in the two meal trials compared with the control (placebo + water) trial during the first hour of exercise (3).
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The different tests were done to equate the effects of carbohydrate intake on athletic performance. The results of the first experiment showed that a high-carb diet induced 45% higher muscle glycogen levels than the low-carb diet. However, the high-carb versus low-carb diets showed no difference in rating of perceived exertion, heart rate, oxygen uptake, or respiratory quotient. Therefore, low-carb diets may decrease glycogen levels, but don't induce any response that wasn't observed during high-carb diet, when total energy uptake was adequate (2). In the second experiment the main findings were that the consumption of the high carb meal 3 hours before exercise improved endurance running capacity by 9% when compared with the performance of the no meal trial. Furthermore, when the 6.9% carbohydrate-electrolyte solution was ingested during exercise, the endurance capacity was even greater (22%) than during the no food trial. Also, the endurance capacity during meal plus carbohydrate solution trial was 12% greater than during the meal trial alone (1). The main findings of the third experiment was that rehydration was achieved by drinking the equivalent of 170% of the body mass lost in the first run as either a 6.9 % CHO-electrolyte solution or CHO free sweetened placebo during a 4 hour recovery. Furthermore, endurance capacity during the post-recovery run was significantly greater after drinking the CHO-electrolyte solution than after drinking an equal volume of placebo solutions (3).
1. Chryssanthopoulos C, William C, Nowitz A, Kotsiopoulou C, Vleck V. The effect of
a high carbohydrate meal on endurance running capacity. Int J Sport Nutr. 2002; 12(2):157-171.
2. Kavouras SA, Troup JP, Berning JR. The influence of low versus high carbohydrate
diet on a 45 min strenuous cycling exercise. Int J Sport Nutr. 2004;14(1):62-72.
3. Wong SH, William C, Adams N. Effects of ingesting a large volume of carbohydrate-
electrolyte solution on rehydration during recovery and subsequent exercise capacity. Int J Sport Nutr. 2000;10(4):375-393.