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  1. Lim, Chin Leong PhD, MBA, MS(PE), BS(PE)


Abstract: The physiological mechanisms of thermoregulation during physical exercise affect fluid homeostasis. Although there is consensus on the science of fluid homeostasis during exercise, there is, however, disagreement on the appropriate volume of fluid to ingest before, during, and after exercise. In 1996, the American College of Sports Medicine recommended ingestion of 500 mL of fluid 2 hours before an event, replacement of fluid loss at a rate equal to sweat rate, and consumption of the maximum amount of water that can be tolerated during exercise. In practice, these guidelines were translated into what is called volume-driven exercise hydration (ie, drinking by a fixed volume), with some scientists recommending drinking 2 L of water before exercise and 1.2 L/h of water during exercise in hot conditions. However, there is a considerable interindividual variation in sweat response, such that recommending a single volume of water intake during exercise may not be appropriate. Besides the fact that ingesting a high volume of fluid during exercise may not be tolerable by athletes, volume-driven exercise hydration may increase the risk of hyponatremia. The American College of Sports Medicine revised its position stand on fluid replacement during exercise in 2007. The revised guidelines agree that it is not possible to recommend a single volume of fluid replacement to suit everyone because of the variety of factors (body weight, environment, and training status) that influence fluid intake requirement during exercise. (This guideline was not published at the time of the symposium.) Dr Lim recommends drinking enough water to maintain a clear urine color before and after exercise and to prevent a fluid deficit of more than 2% of body weight during exercise. There is ongoing debate on whether drinking by thirst is sufficient to meet fluid intake requirement during exercise, but there is consensus that drinking about 0.4 to 0.8 L/h of fluid during exercise is sufficient to meet fluid replacement requirement for most people. Exercise and sports participants should establish their own fluid intake requirement by measuring body weight changes before and after exercise over a few training sessions. Dr Lim concludes that as the debate continues over an ideal model of exercise hydration, such a model needs to be aligned with both the science of fluid homeostasis and the practices and experience of athletes.