Summary
Maintenance of fluid balance is a major
concern for all athletes competing in events
held in hot climates. This paper reviews recent
work relating to optimisation of fluid replacement
after sweat loss induced by exercising in
the heat. Data are taken from studies undertaken
in our laboratory. Issues investigated
were drink composition, volume consumed,
effects of consuming food with a drink, effects
of alcohol on rehydration effectiveness, voluntary
intake of fluid, and considerations for
women related to the menstrual cycle. The
results are presented as a series of summaries of
experiments, followed by a discussion of the
implications. The focus of this review is urine
output after ingestion of a drink; fluid excreted
in urine counteracts rehydration. Also included
are data on the restoration of plasma volume
losses. Ingestion of large volumes ofplain water
will inhibit thirst and will also promote a
diuretic response. If effective rehydration is to
be maintained for some hours after fluid ingestion,
drinks should contain moderately high
levels of sodium (perhaps as much as 50-60
mmol/l) and possibly also some potassium to
replace losses in the sweat. To surmount ongoing
obligatory urine losses, the volume consumed
should be greater than the volume of
sweat lost. Palatability of drinks is important in
stimulating intake and ensuring adequate volume
replacement. Where opportunities allow,
the electrolytes required may be ingested as
solid food consumed with a drink. There are no
special concerns for women related to changes
in hormone levels associated with the menstrual
cycle. Ingestion of carbohydrate-electrolyte
drinks in the post-exercise period restores
exercise capacity more effectively than plain
water. The effects on performance of an
uncorrected fluid deficit should persuade all
athletes to attempt to remain fully hydrated at
all times, and the aim should be to start each
bout of exercise in a fluid replete state. This will
only be achieved if a volume offluid in excess of
the sweat loss is ingested together with
sufficient electrolytes.