Introduction
As described in the previous chapters, the sweat-
ing mechanism is effective in limiting the rise in
body temperature that occurs during exercise
but, if the exercise is severe and prolonged and
the climatic conditions hot and humid, the de-
hydration that results will inevitably have an
adverse effect of exercise capacity. Sweat losses
equivalent to 2–5% of body mass are often
incurred in the course of endurance events, and
if these are not replaced, the dehydration that
ensues may precipitate circulatory collapse and
heat illness. Fluid replacement is therefore
important in situations where some degree of
sweating is unavoidable: prolonged hard exer-
cise in extreme conditions may increase the total
daily water requirement from about 2.5 l to some-
thing in excess of 12–15 l. Even though this
amounts to about 25–30% of total body water
content for the average individual, such condi-
tions can be tolerated for prolonged periods
provided that the sweat losses are replaced.
The choice of rehydration beverage will vary,
depending on the circumstances, and requires an
awareness of the extent of water and electrolyte
losses and of substrate utilization by the working
muscles as well as some understanding of the
psychological and physiological factors that
influence the rehydration process.
Sweat losses in exercise
The physics of heat exchange between the
human body and the environment have been
described in several excellent reviews (e.g. Nadel
1988). Sweating is an effective mechanism of
heat loss when heat loss by physical transfer
cannot prevent a rise in core temperature. The
heat required to evaporate 1 kg of sweat from the
skin surface is approximately 2.6 MJ (620 kcal),
allowing high rates of heat loss from the body to
be achieved, provided only that sweat secretion
is possible and that evaporation can occur.
Although high temperature poses a threat to the
athlete by adding to the heat load and reducing
heat loss by physical transfer, high humidity,
which prevents the evaporation of sweat, is
more of a challenge: heat loss is limited, leading
to hyperthermia, and high sweat rates occur
without effective heat loss, leading to dehydra-
tion. The combination of hyperthermia and
hypohydration will reduce exercise performance
and may lead to potentially fatal heat illness
(Sutton 1990).
Several different factors will interact to deter-
mine the sweat rate during exercise. The major
determinants are the metabolic heat load and
the environmental conditions of temperature,
radiant heat load, humidity and wind speed, but
there is a large interindividual variability in the
sweating response even in standardized condi-
tions. Although the sweat loss incurred on a
daily basis by an athlete during training will be
determined largely by the training load (inten-
sity, duration and frequency of training sessions)
and weather conditions, there will also be an
effect of the amount and type of clothing worn, of