Over the past few decades a notion that a loss of at least 2% of body weight, as a result of hypohydration, impairs exercise performance has been established, although observations from real sports events show that a large number of very good athletes finish lighter by as much as 10% without indices of deterioration of performance and/or heat stress. More recent research shows that larger hypohydration (-4% BW) is well tolerated by humans and it does not impair performance provided that one is not thirsty and drinks ad-libitum. Hydration recommendation need to prevent dangerous hyponatremia and drastic reductions in body weight as a result of hypohydration. It appears that ad-libitum drinking strategy is sufficient for events where fluids are available at all times and when the heat stress is not too high, whereas in cases these criteria are not met, use of personalised hydration plan is recommended. Addition of electrolytes appears not to affect performance but is recommended for those with high electrolyte concentrations in the sweat and those whose thirst sensation is impaired.

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