Sports Dieititians Australia explains the importance of
hydration for performance.
ater is essential for the human body.
It is required to maintain blood
volume, regulate body temperature and
allow muscle contractions to take place.
During exercise, the body maintains its
optimal body temperature through
sweating. Heat is removed from the body
when beads of sweat on the skin evaporate,
which results in a loss of body fluid. Sweat
production (and therefore fluid loss)
increases with a rise in ambient
temperature and humidity, as well as with
an increase in exercise intensity. So while
sweat loss during exercise is essential for
body temperature regulation, it can lead to
As dehydration increases, there is a gradual
reduction in physical and mental
performance. There is an increase in heart
rate and body temperature, and an
increased perception of how hard the
exercise feels, especially when exercising in
the heat. Impaired skill level can also occur,
along with mental fatigue. Studies show
that loss of fluid equal to two per cent of
body mass is sufficient to cause a detectable
decrease in performance (that’s a 1.4kg loss
in a 70kg athlete).
Dehydration of greater than two per cent
loss of body weight increases the risk of
nausea, vomiting, diarrhoea and other
gastro-intestinal problems during and
Dehydration also reduces the rate of fluid
absorption from the intestines, making it
more difficult to reverse the fluid deficit.
You may end up feeling bloated and sick if
you delay fluid replacement. It is
impossible to ‘train’ or ‘toughen’ your body
to handle dehydration.
Can you drink too much?
Drinking more fluid than is comfortable (in
any conditions) has the potential to interfere
with your performance. In cool weather or
when the exercise pace is gentle, the rate of
sweat loss may be quite low. It is unnecessary
and potentially dangerous to drink at rates
that are far greater than sweat losses.
Over-hydration during exercise is called
hyponatraemia (dilute levels of sodium in the
bloodstream). Symptoms include headaches,
disorientation, coma, and in severe cases,
death. It is important to note though that this
is relatively rare and dehydration is a much
more common issue for athletes.
Estimating your fluid loss
Knowing your sweat rate can give you an
indication of how much you should be
drinking during exercise. Sports dietitians
routinely measure an athlete’s sweat rate
during training and competition in a range
of environmental conditions, to provide
them with the information required to
design an individual fluid plan. A simple
strategy to work out your individual fluid
loss is as follows:
• Weigh yourself in minimal clothing, as
close to the start of exercise as possible.
Ideally you should empty your bladder
Sports Dietitians Australia (SDA)
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To find out more about dehydration and sport, check out the full fact sheet online at
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• Commence exercise session.
• Weigh yourself at the end of your
session, again in minimal clothing,
ensuring you towel off any excess sweat
from your body, pass urine and void
your bowels if necessary.
• Your weight change during exercise
reflects your total fluid loss (i.e. the
difference between your sweat losses
and fluid intake). Other minor losses
come from breathing, spitting, vomiting
and other insignificant sources.
• Repeat this procedure under different
training conditions to get a good
understanding of your individual fluid
needs, for example in hot versus cold
temperatures, high versus low intensity
• Remember that weight loss during
exercise is primarily water loss (not fat
loss), and needs to be replaced soon
after finishing exercise.
• Aim to match your sweat rate and fluid
loss with fluid intake as closely as
• Get to know your fluid loss by weighing
yourself before and after training
sessions and competition.
• Ensure that you drink at a rate that is
• Practice your competition fluid intake
plan in training sessions.
• Water is an excellent fluid for low
intensity and short duration sports.
• Sports drinks are ideally suited to high
intensity and endurance sports.
• Drink alcohol sensibly and assess the
detrimental effects on your recovery. N