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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.

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 is pale and plentiful, you are well hydrated, but if it is 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 is 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!

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).