Q - Tanya Griffin, final year student from Westminster University kindly looked at the question often asked – should I drink water or a sports drink. The following is her answer:
A - Water makes up 55 to 60 % of our body weight. We lose lots of water every day by sweating, breathing and in our urine and faeces. When we lose water we become dehydrated (not enough water in the body). Dehydration has been defined as 1% or greater loss of bodyweight as a result of fluid loss. This can be a problem for athletes by affecting their performance; therefore an athlete should aim to minimise dehydration.
At the moment there are lots of sports drinks on the market that claim to
give the athlete energy to exercise for longer, and prevent or slow down dehydration.
The body needs to cool down during exercise and so we sweat to get rid of
this heat. We also breathe harder, which means that we also lose water vapour
this way. A large part of blood, called plasma, is made of water. Dehydration
causes the amount of water in the blood to decrease i.e. it lowers plasma
volume. The amount of water in an athlete’s blood i.e. their hydration
status, is of great importance because, all else being equal, it could decide
if the athlete wins or loses an event. As dehydration affects the blood, it
also causes other systems to change i.e. it increases the heart rate and amount
of blood pumped out of the heart every minute (cardiac output). A study found
that during two hours of exercise, as the amount of fluid drank became less,
the subject’s heart rate increased, and the amount of blood pumped out
of the heart per minute decreased. The higher heart rate shows us that dehydration
is making the heart work harder and faster to keep up with the needs of the
body.
Sports people will often dehydrate ‘voluntarily’ because the amount
they drink is not enough to match the amount of fluid lost from the body.
It is often quoted that people will not voluntarily replace more than 50%
of their fluids losses. Marathon runners can lose between 1000 to 1500 ml
of sweat an hour, but they only drink about 500 ml. Research has shown that
a person can dehydrate by about 3 % and not have any major effects on strength,
however there is no critical amount of dehydration that can be tolerated without
physiological consequence – cardio vascular function and the ability
to regulate temperature will be impaired. Even small loses will affect exercise
capacity if not performance. The more dehydrated a person becomes, the more
they will struggle to produce sweat i.e. their body is trying to conserve
water. However, as sweating is the body’s way of regulating it’s
core, an inability to sweat will make the temperature of the body increase.
Heat stress, or hyperthermia, is potentially life threatening. A cardiovascular
physiologist, LB Rowell once wrote that,
“Perhaps the greatest stress ever imposed on the human cardiovascular
system (except for severe haemorrhage) is the combination of exercise and
hyperthermia.”
It has been advised that to put less stress on the heart, and keep the body’s
temperature in the normal range, people should drink about 30 –60 minutes
before exercising. We now know that drinking enough before, during and after
exercise is important, but what should we drink? By taking carbohydrates like
glucose or other sugars into our bodies as solid or liquid is known to make
the persons exercise performance better. Sports people may not want to eat
a large carbohydrate meal just before an event; therefore sports drinks are
a convenient source of carbohydrate for them. Adding carbohydrate to the sports
drink has shown to enable the person to exercise for longer i.e. improve endurance
capacity.
Other work however has shown that there was no major difference between the
heart rate; sweat rate or how tired the person feels (perceived exhaustion)
when people drank sports drink, water or an artificially sweetened drink.
The flavour, temperature and ingredients of a drink all affect how much is
drank, (particularly with children). Work showed that when boys exercised
they drank more of the flavoured sports drink, and flavoured water than plain
water.
Sports drinks also have the added benefit because salts, called electrolytes,
are added to their ingredients. These salts. Particularly sodium, have a number
of important functions:
- Helps stimulate the drive to drink more fluid. The effect of moisture in the mouth switches off the thirst reflex before an athlete has fully hydrated. -
Electrolytes will help maintain the desire to keep drinking. |
Overall the work that has been done on sports drinks shows that benefits for a sports person are mixed. This is due to a number of factors but includes the intensity and the duration of the exercise itself and so effects of the sports drink will vary from sport to sport. Exercise that takes less than30minutes is unlikely to be improved by taking a sports drink, or even water; vigorous exercise lasting longer than 60 minutes if carried out in hot conditions or exercise lasting more than 90minutes will see benefits from a sports drink. We do know that the flavour of the drink improves palatability and therefore acceptance - a sports drink is only good if it is drunk. Carbohydrates, like glucose, when added to a sports drink enable the body to exercise for longer by calling less on the muscle's store of carbohydrate. Of course a sports drink containing carbohydrate can also provide fuel to rebuild the stores of muscle carbohydrate and help normalise blood sugar (blood glucose). Whether the average person going to the gym, with the intention of weight loss really needs a sports drink or whether they would be better off with tap water is debatable.
Text by Tanya Griffin
Edited by Jacqueline Boorman
Editor’s note: see also the effects of maintaining blood glucose and
the use of a carbohydrate-containing beverage on the immune system. See also
the table below comparing the physiological effects that different drinks
can have on the body.
This advice should not replace that of your doctor or usual state registered dietitian. You should always discuss medical matters with your GP.