Intense exercise (i.e.; above 60% VO2max) can be maintained for prolonged periods provided sufficient carbohydrate is available for energy and the heat generated from muscle metabolism does not cause excessive hyperthermia and/or dehydration due to sweating. It is clear that people should ingest carbohydrate during prolonged exercise (i.e.; longer than 1–2 h), which causes fatigue because of an inadequate supply of blood glucose and that fluids should also be ingested in an attempt to offset dehydration and reduce hyperthermia. Ingestion of approximately 30–60 g of carbohydrate (i.e.; glucose, sucrose, or starch) during each hour of exercise will generally be sufficient to maintain blood glucose oxidation late in exercise and delay fatigue. Since the average rates of gastric emptying and intestinal absorption can reach 1 l·h-1 for water and solutions containing up to 8% carbohydrate, exercising people can be supplemented with both carbohydrate and fluids at relatively high rates (over 60 g·h-1 of carbohydrate and 1 l·h-1 of fluid). Therefore, when sweat rate is not high (i.e.; less than 1 l·h-1), the addition of carbohydrate to fluids, and vice versa, does not prevent adequate supplementation of each, especially if large volumes are consumed to keep the stomach somewhat full and thus increase gastric emptying. Therefore, in most situations there are no trade-offs between fluid and carbohydrate. However, the scientific literature contains surprisingly little direct data regarding the extent to which fluid replacement during exercise should match sweat rate to offset cardiovascular drift and hyperthermia when sweat rate approaches or exceeds 1 l·h-1 and hyperthermia is common. Therefore, it is not possible at this time to develop scientifically sound recommendations regarding the optimal fluid replacement regimen, which may vary from person to person because of the tremendous interindividual variability in gastric emptying rate. It remains to be determined whether the benefits of high rates of fluid ingestion during continuous exercise (i.e.; 600–1,200 ml·h-1) outweigh the interruption of exercise and the discomfort it may cause.
©1992The American College of Sports Medicine