Hydration during
sport and exercise
Adhering to good hydration practices
is essential before, during and after
exercise, writes Giles Warrington
D
uring exercise there is a dramatic increase
in core body temperature resulting from
mechanical heat produced by the working
muscles, coupled with the chemical reaction associated with the sharp rise in energy turnover. This
accumulating heat must in some way be removed
from the body to prevent thermal stress.
The human body can only tolerate a small
increase in core temperature, of about three
degrees centigrade and, unless appropriately
regulated, will result in increased thermal stress,
fatigue, reduced work capacity, heat injury and in
extreme cases heat stroke.
Depending on the magnitude of thermal
stress, a wide variety of symptoms of heat injury
have been identified including: cramps; extreme
fatigue; dizziness; disorientation vomiting;
headache; cessation of sweating; hot dry skin;
rapid pulse and respiration; hypertension; and
unconsciousness.
The body’s main defence mechanism for regulating temperature during exercise is through the
evaporation of sweat, which accounts for about
80% of all heat loss. This will result in significant
losses in body water which must be replaced to
avoid dehydration. Unless fluid losses associated
with sweating are adequately replaced, this will
impact on physiological function resulting in the
cardiovascular system not being able to meet the
dual demands of thermoregulation and oxygen
delivery to the working muscles.
In this regard a fundamental difference
between the hydration needs of someone who is
physically active and that of the general sedentary population, are that those in training require
additional fluid intake to cover losses through
sweating in order to effectively regulate body
temperature.
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Importance of hydration
training session lasting around 60 minutes!
Consumption of water is vital for supporting
life, regulating key bodily functions and maintaining
good health. The human body is made up of about
60-65% water, which is a key component of the
various cells, muscle and other tissues. While the
human body can exist without food over extended
periods, it is only possible to survive without water
for a matter of days. Good hydration practices are
critical for maintaining normal physiological and
cognitive function and optimising performance
during participation in sport and exercise.
Adequate hydration before, during, and after
exercise is essential. Despite this, hydration and
maintaining fluid levels is a factor often overlooked in training and competition. Individuals
engaging in regular exercise, particularly in warm
conditions, must consume fluids in excess of their
perceived needs. Despite this many athletes typically only replace about 50% of sweat loss during
exercise. It is important to be aware that good
hydration practices do not just apply to serious
athletes, but also to anyone engaging in regular
physical activity and exercise.
Avoiding dehydration
Dehydration is a condition in which the body
contains an insufficient volume of water to meet
its various needs. By exercising, even in a mildly
dehydrated state, the body has to work that
much harder to regulate body temperature. As a
consequence sub-maximal heart rate and perception of effort during exercise will be higher than
normal, indicating increase physical stress. Not
only will physiological and cognitive function be
impaired, but work capacity and performance will
also decrease.
Research has shown that fluid losses of as
little as 2% of body weight, which are commonly
experienced, will impair physiological and cognitive function, leading to a marked negative impact
on work capacity and endurance performance, as
well as concentration and decision making. This
may have significant implication for those involved
in prolonged activities or events with a high skill
component.
The bottom line is that those engaging in
regular physical training, should create a plan to
get into the habit of making sure that they are
well-hydrated before commencing exercise, as
well as drinking enough fluid during and after
exercise to balance potential water losses.
Daily fluid needs
For the average healthy person under normal
conditions, daily fluid requirements will typically
be in the region of around 2-3 litres, (guidelines vary) which includes the water content in
foods consumed. For those engaging in regular
physical activity, fluid intake needs will be greater
and should match body water losses through
sweating.
Scientific evidence across a range of activities
suggests that sweat rates can vary considerably and can be affected by a number of factors
including: age; gender; fitness levels; exercise
intensity and duration; as well as environmental
conditions. Typical sweat losses during physical
activity and sports participation will be in the
region of 0.5-2 litres per hour, which equates to a
0.5-2kg loss in body weight. For the average 70kg
individual who sweats heavily, this may result
in a fall in body weight of up to 2-3% during a
Specific hydration guidelines
The importance of maintaining good hydration
practices during sport and exercise participation
cannot be overstated. Exercise performance will be
optimised when fluid balance is maintained and is
impaired even with moderate levels of dehydration
of as little as 2% of body weight. Considerable
variability exists between individuals with regards
to their specific fluid intake requirements and is
influenced by factors such as size, sweat rates,
environmental conditions and different types
physical activities. Specific circumstances therefore
need to be taken into account when developing a
personal hydration plan.
Monitoring hydration status
Thirst is not a reliable indicator of hydration
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be carefully planned and should reflect fluid losses
during training. This is why self-monitoring of
hydration status is important.
To ensure you are fully hydrated after training
and competition, you should apply the 150%
rule: for every 1kg of weight loss you should
consume about 1.5litres of fluid. Including a small
amount of sodium in a post-exercise drink will
speed up the rehydration process by maintaining
the drive to drinking, as well as promoting fluid
retention. In addition, drinks containing 4-8%
carbohydrate will enhance the recovery process by
replenished the bodies depleted glycogen stories.
What to drink? Sports drinks are commonly used
by athletes and are available in many different
brands, flavours and types.
Despite scientific evidence supporting the use
of sports drinks, consumption of plain water still
has an important role to play in any overall hydration strategy. Isotonic sports drinks supply some
electrolytes (sodium and potassium) and a small
amount of carbohydrate (typically 4-8%). Enhancing drink palatability may play an important role
in promoting fluid consumption. Palatability is
affected by factors such as temperature, concentration, sodium content and flavouring. Drink
temperatures should typically be around 10-15ºc.
Flavour preference will vary greatly therefore it
is best to try different formulations to see what
works best for you.
status. By the time you are thirsty you are already
experiencing the first signs of dehydration. There
are other practical methods you can use to measure hydration. One quick and simple method is to
weigh yourself, in minimal clothing, before and
after exercise. Additionally, you can also monitor
urine colour and volume to avoid dehydration
your urine should be pale and frequent.
Before exercise
Current fluid intake guidelines developed by
the American College of Sports Medicine recommend that you slowly drink about 5-7ml of fluid
per kg of body (ie. a 70kg individual would take
in at least 350-500ml) in the four hours prior to
exercise. If there is no urine production or the
urine colour is dark, an additional fluid intake
of about 3-5ml per kg body weight should be
consumed about two hours prior to the event.
The inclusion of a small amount of sodium or a
salty snack will help to maintain thirst as well as
the retention of the fluids consumed.
During exercise
The goal of drinking during exercise is to
prevent excessive dehydration and underperformance, however, this takes planning. It is often not
possible or practical to maintain fluid balance
during exercise but as a good rule of thumb,
optimum hydration would be achieved by trying
to consume in the region of 150-250ml every 15
to 20 minutes during the activity.
While plain water may be sufficient for
moderate duration activities, it is generally
recommend to consume drinks containing 4-8%
carbohydrate for intense activities lasting longer
than 60 minutes in duration. In relation to hydration strategies for competition, these should be
practised in training to see what works best for
each individual.
Conclusion
Physical activity even in moderate climates
can result in high sweat rates and subsequent
losses in both fluids and electrolytes. If body
water losses resulting from sweating are not
adequately replaced, this will result in dehydration
and impaired physiological function, which will
ultimately impact on athletic performance as well
as increasing the risk of thermal stress and heat illness. Adhering to good hydration practices should
therefore be emphasised both before, during and
after exercise. n
Giles Warrington is a sport and exercise
physiologist at Dublin City University
After exercise
In most cases, those engaging in exercise
do not consume sufficient fluids to match water
losses through sweating. As a result training sessions are often completed with some degree of
dehydration. Rehydration after exercise needs to
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