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Endurance and team-sport athletes competing at the 2021 Tokyo Olympics Games can expect hot and humid conditions. Ensuring euhydration prior to an event and minimising dehydration during competition will be important, to ensure an optimal physiological state and competitive advantage. Contention exists, however, regarding a primary recommendation provided to athletes: drink to thirst or plan to drink. The debate is centred on the level of dehydration that can be incurred prior to its effects influencing physiological responses and performance. Whole-body sweat rate during exercise ranges from 0.5 to 2.0 L hour-1 with some athletes (~2%) sweating substantially more (>3.0 L hour-1). Gradual reductions in body mass of 2%-5% can occur if body water losses are not replenished, resulting in marked decrements in plasma (=10%) and blood (=6%) volume. Such levels of dehydration lead to a state of hyperosmotic hypovolaemia that is proportional to the decrement in total body water. Hyperosmolality during exercise reduces sweat rate, and thus evaporative heat loss, for any given core temperature.6 As a result, the magnitude of hyperthermia experienced during endurance exercise under heat stress is exacerbated. Progressive dehydration and hyperthermia also intensify tachycardia and compromise the ability to maintain cardiac output
© Copyright 2021 British Journal of Sports Medicine. BMJ Publishing Group Ltd of the BMA. Alle Rechte vorbehalten.