Introduction
Hard physical exercise poses a formidable chal-
lenge to the body’s ability to maintain its internal
environment within the range that allows
optimum function. In sport, however, both in
training and competition, these homeostatic
mechanisms are under constant threat, and
fatigue is the result of a failure to stay within the
zone of optimum functioning. It may be an
excess acidosis resulting from lactic acid forma-
tion, a change in the extracellular potassium
concentration causing a decrease in the muscle
excitability or a rise in the temperature of the
tissues as a result of a high rate of metabolic
heat production. Some increase in body tempera-
ture is normally observed during exercise, and
may even have beneficial effects by increasing
the rate of key chemical reactions and altering
the elastic modulus of tissues, but high tempera-
tures are detrimental to exercise performance
and may be harmful to health. Sweating is the
normal physiological response invoked to limit
the rise in body temperature by increasing
evaporative heat loss, but the loss of signifi-
cant amounts of sweat results in dehydration
and electrolyte depletion if the losses are not
replaced. Some understanding of the regulatory
processes involved in the control of body tem-
perature and of fluid and electrolyte balance is
therefore fundamental to the design of drinks
intended for use during exercise and to an under-
standing of how and when these drinks should
be used.
Temperature regulation in exercise
The temperature of the skin can vary widely,
depending on the environmental temperature,
but the temperature of the deep tissues must be
maintained within only a few degrees of the
normal resting level of about 37°C. For this to be
the case, the rate of heat gain by the body must be
balanced by the rate of heat loss: any imbalance
will result in a change in body temperature. All
chemical reactions occurring in the body are rela-
tively inefficient, resulting in a large part of the
chemical energy involved appearing as heat. The
rate of heat production is therefore directly pro-
portional to the metabolic rate. The resting meta-
bolic rate for a healthy adult with a body mass of
70 kg is about 60 W. In a warm climate, this is suf-
ficient to balance the rate of heat loss, but in cold
weather the insulative layer surrounding the
body must be increased to reduce the rate of heat
loss. In other words, more or thicker clothes are
worn when it is cold. Alternative strategies are to
raise the ambient temperature (by turning up the
thermostat on the heating system if indoors) or to
increase the metabolic rate, thus increasing the
rate of heat production.
The metabolic rate increases in proportion to
the rate of energy turnover during exercise: in
activities such as walking, running, swimming
or cycling at a constant speed, the energy
demand is a function of the rate of movement. In
walking or running, where the body mass is
moved against gravity at each step, body mass
and speed will together determine the energy