Thirst is not a good short-term regulator of fluid balance. Humans frequently lose up to 2% of their body weight as water before the thirst mechanism is activated. The actual point at which the thirst mechanism is activated varies considerably between individuals. Some athletes are closely attuned to their anticipated fluid needs and develop the habit of drinking before they become dehydrated. However, the majority of individuals do not feel compelled to drink until they have become moderately dehydrated, even though fluids may be available. These individuals are called 'voluntary dehydrators.' Voluntary dehydrators frequently replace only approximately two-thirds of their short-term fluid losses.
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