When we exercise, our muscles only use about 25 per cent of the
energy for work, with the rest released as heat - which is why
exercise makes you hot! The main way the body is kept cool is by
sweating. Heat from the working muscles is transferred to the
blood. The blood flow to the skin is increased, and heat is lost by
evaporation - sweating. Sweat comes from the water in blood, so we
need to replace this vital fluid to prevent dehydration. If we
exercise while dehydrated, our temperature can rise quickly and
cause heatstroke, which is potentially fatal.
5(i). Fluid Needs
In general, we need to drink about two litres of fluid a day to be
properly hydrated. However, it's quite likely that exercise will
increase our fluid needs. The more you sweat, the more you need to
drink to replace the lost fluid. Some people naturally sweat
heavily, but even small losses can cause fatigue. Plus, the fitter
you are, the more effectively you keep your body cool - so the more
you sweat! Training harder, longer or in hot and humid surroundings
will also make you sweat more.
During exercise we lose on average 500-1000ml of fluid an hour.
The easiest way to get a better estimate of how much fluid you lose
is to weigh yourself before and after exercising - preferably naked
so as not to include the sweat absorbed within your clothing. Each
kg of weight loss is equivalent to a litre of fluid loss. However,
you will lose more fluid as urine, so you should drink at least 1.5
litres of fluid for every kg of weight lost. The colour of your
urine is also a good indicator of fluid loss - if it's pale and
plentiful, you're well hydrated, but if it's dark and sparse, you
need more fluid. A loss of just 2 per cent in your body weight may
affect your ability to exercise; a 4 per cent loss can cause
exhaustion. If you're competing, for every 1 per cent drop in body
weight there's about a 5 per cent drop in performance, which could
mean the difference between coming first and last!
If you keep exercising without replacing fluid, you will become
increasingly dehydrated. You will no longer be able to keep your
body cool, your temperature will rise, you will feel lightheaded
and nauseous and, ultimately, get fatigue or heatstroke. The only
way to prevent this is to start off well hydrated and stay that
way!
5(ii). Drinking Schedule
It is vital that you drink plenty of fluid to avoid compromising
your health. To succeed, you need to plan your drinking strategies
and get into the habit of drinking, so that your body can gradually
adapt to increased fluid intakes. Don't leave it to chance, take
your beverage choice with you, and keep it with you while you work
out - see the 'Hydration and Exercise' box for the best way to stay
hydrated during exercise.
It is unlikely that you will drink too much water - not drinking
enough is usually the problem. The only time drinking plain water
may cause a problem is if you're sweating very heavily for a
prolonged period of time. In this situation, a sports drink
containing sodium would be better than plain water to prevent low
blood sodium levels (hyponatraemia).
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Hydration and Exercise
Before exercising always start every exercise session well
hydrated. Drink 300-500ml of fluid in the 15 minutes prior to your
workout.
During exercise aim to drink 150-250ml every 15 minutes to
offset fluid losses - drinking smaller volumes more frequently
minimises stomach discomfort. Remember, the more you sweat, the
more you need to drink.
After exercise how much fluid you need depends on how much you
lost, but you'll probably need at least 500ml. Try to drink 1.5
litres of fluid for every kg of weight lost during exercise, or
keep drinking until you pass light-coloured urine.
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5(iii). Which Fluid?
Which fluid you opt for depends on how hard you exercise, and for
how long. However, choose a flavour you like to encourage you to
drink more. If you're exercising at a low-to-moderate intensity for
less than an hour, then water is great. If you find it difficult to
drink large quantities of plain water, try adding some juice or
squash, which will also provide you with some carbohydrates to help
restock glycogen stores. If you work out continuously for more than
an hour, then a sports drink would probably be a good idea.
Sports drinks help maintain better fluid levels, plus the added
carbohydrates provide vital glucose to help avoid fatigue. Most
sports drinks are five to eight per cent carbohydrate, which makes
them 'isotonic' - a similar concentration to blood - and,
therefore, quickly absorbed.
In addition, sports drinks contain sodium to stimulate sugar and
water absorption, and replace the sodium lost in sweat. This added
sodium is particularly useful if you're a 'salty sweater' - where
your sweat is opaque, tastes salty, and leaves white marks on your
clothes. The sodium has the added benefit of encouraging you to
drink more. In fact, the drive to drink is present for several
hours following exercise (it stops when you eat). However, when
your mouth is moistened with fluid, your body automatically signals
your brain to stop drinking. This inhibition can happen before the
body's fluid levels have been completely restored. This means that
even if you don't feel thirsty, you're not necessarily well
hydrated, so it's important to keep drinking fluid throughout the
day.
Although alcohol in moderation is fine, it's not a good idea to
drink it just before exercise. You also need to rehydrate properly
before drinking alcohol after exercise. Alcohol before exercise not
only has a detrimental effect on co-ordination skills and exercise
performance, but also increases the risk of injury. Furthermore,
alcohol can cause dehydration and slow down recovery from
injury.
