Obligatory urine losses persist even in the dehydrated state, because of the need for elimination of metabolic waste products. The volume of fluid consumed after exercise-induced or thermal sweating must therefore be greater than the volume of sweat lost if effective rehydration is to be achieved. This contradicts earlier recommendations that after exercise athletes should match fluid intake exactly to the measured body mass loss. Studies have investigated the effect of drink volumes equivalent to 50%, 100%, 150% and 200% of the sweat loss consumed after exercise-induced dehydration equivalent to approximately 2% of body mass.
To investigate the possible interaction between beverage volume and its sodium content, a relatively low-sodium drink (23 mmol/l) and a moderately high-sodium drink (61 mmol/l) were compared. Subjects could not return to euhydration when they consumed a volume equivalent to, or less than, their sweat loss, irrespective of the drink composition. When a drink volume equal to 150% of the sweat loss was consumed, subjects were slightly hypohydrated 6 hours after drinking when the test drink had a low sodium concentration, and they were in a similar condition when they drank the same beverage in a volume of twice their sweat loss. With the highsodium drink, enough fluid was retained to keep the subjects in a state of hyperhydration 6 hours after drink ingestion when they consumed either 150 or 200% of their sweat loss. The excess would eventually be lost by urine production or by further sweat loss if the individual resumed exercise or moved to a warm environment.
Whilst other studies have also shown the importance of drinking a larger volume of drink than the sweat volume lost, an interaction between sodium intake, volume intake and whole-body rehydration has not always been reported.