1954 Empire Games marathon in Vancouver,
and Gabriella Andersen-Scheiss at the 1984
Los Angeles Olympics. Such problems are rarely
encountered in cooler conditions. None the
less, high rates of evaporation require high
rates of sweat secretion onto the skin surface, and
the price to be paid for the maintenance of
core temperature is a progressive loss of water
and electrolytes in sweat. If not corrected,
this dehydration will impair exercise perfor-
mance (Chapter 16) and may itself become
life-threatening.
Water balance
The body’s hydration status is determined by the
balance between water input and water losses
from the body. As with all nutrients, a regular
intake of water is required for the body to main-
tain health, and deficiency symptoms and over-
dosage symptoms can both be observed. Water is
the largest single component of the normal
human body, accounting for about 50–60% of the
total body mass. Lean body tissues contain about
75% water by mass, whereas adipose tissue
consists mostly of fat, with little water content.
The body composition, and specifically the fat
content, therefore largely determines the normal
body water content. For a healthy lean young
male with a body mass of 70 kg, total body water
will be about 42 l. Losses of only a few per cent of
total body water will result in an impaired exer-
cise tolerance and an increased risk of heat ill-
ness, and yet the sweating rate can reach 2–3 l · h–1
in extreme situations.
Sweat losses for various sporting and occupa-
tional activities are well categorized, but the vari-
ability is large because of the different factors
that affect the sweating response (Rehrer &
Burke 1996). Even at low ambient temperatures,
high sweat rates are sometimes observed when
the energy demand is high, as in marathon run-
ning, so it cannot be concluded that dehydration
is a problem only when the ambient temperature
and humidity are high: marathon runners com-
peting in cool temperatures (10–12°C) typically
lose between 1% and 5% of body mass during a
race (Maughan 1985). The sweat loss is, however,
closely related to the environmental conditions,
and large fluid deficits are much more common
in the summer months and in tropical climates.
Body mass losses of 6 l or more have been
reported for marathon runners in warm weather
competition (Costill 1977). This corresponds to a
water deficit of about 8% of body mass, or about
12–15% of total body water, and this is sufficient
to give cause for concern.
It is well established that women tend to sweat
less than men under standardized conditions,
even after a period of acclimatization (Wyndham
et al.1965). It is likely, however, that a large part
of the apparent sex difference can be accounted
for by differences in training and acclimation
status. There is a limited amount of information
on the effects of age on the sweating response,
and again levels of fitness and acclimation are
confounding factors, but the sweating response
to a standardized challenge generally decreases
with age (Kenney 1995). These observations
should not, however, be interpreted as suggest-
ing an inability of older people to exercise in the
heat, nor should they be taken to indicate a
decreased need for women or older individuals
to pay attention to fluid intake during exposure
to heat stress. Rather, because of the reduced sen-
sitivity of the thirst mechanism in older individ-
uals (Kenney 1995), there is a need for a greater
conscious effort to increase fluid intake. There
are some differences between children and
adults in the sweating response to exercise and in
sweat composition. The sweating capacity of
children is low, when expressed per unit surface
area, and the sweat electrolyte content is low
relative to that of adults (Meyer et al.1992), but
the need for fluid and electrolyte replacement is
no less important than in adults. Indeed, in view
of the evidence that core temperature increases
to a greater extent in children than in adults at
a given level of dehydration, the need for fluid
replacement may well be greater in children
(Bar-Or 1989). There may also be a need to limit
the duration of children’s sports events, or to
provide for specified rest periods, when the tem-
perature and humidity are high.