How much?

The more you sweat, the more fluid you lose, and the more you need to drink to replace the fluid lost. Some people naturally sweat more than others. Plus the fitter you are, the more effectively you keep your body cool - so the more you sweat! Training harder and longer, and/or in hotter and more humid surroundings, will also make you sweat more.

On average, we lose 1 litre of fluid for each hour we exercise. The easiest way to work out how much fluid you lose is to weigh yourself before and after exercise. Each kg of body weight loss is equivalent to a litre of fluid loss. However, you will lose further fluid as urine, so to compensate for this try to drink 1.5 litres of fluid for every kg of weight lost. Another way to check is by the colour of your urine - if it's pale and plentiful you're well-hydrated, but if it's dark and in short supply you'd better start drinking!

A loss of just 2% in your body weight - that's 1.4kg or 1.4 litres if you weigh 70kg - will affect your ability to exercise. Plus, if you're competing, for every 1% drop in body weight there's a 5% drop in performance - that could mean the difference between coming first or last!

If you keep exercising without replacing the fluid lost, you will become more and more dehydrated. You will no longer be able to keep your body cool, your body temperature will start to rise, you will begin to feel nauseous and light-headed, and ultimately you will end up with fatigue or heat stroke. The only way to prevent this is to start off well-hydrated, and stay that way!

Before, during and after exercise

The more you sweat, the more fluid you lose, the more you need to drink to replace the fluid lost. As always, prevention is better than cure - start your exercise session well-hydrated. Try to drink 300-500ml of fluid in the 15 minutes prior to your work-out. During exercise, aim to drink 150-250ml every 15 minutes to offset fluid losses - remember the more you sweat, the more you need to drink. The sooner you get into the habit of drinking during exercise the better.

After exercise, how much fluid you need depends on how much you lost, but you'll probably need at least 500ml - use the guidelines above and either weigh yourself or check out your urine! Whatever you do - drink! Do not wait till you feel thirsty - this probably means you are already dehydrated.

What's more, it is unlikely that you will drink too much water - not drinking enough is usually the problem! The only time it 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 the occurrence of low blood sodium levels (hyponatraemia).

Which fluid?

Which fluid you opt for depends on how hard and how long you exercise. You should find a flavour you like though - let's face it, if you don't like the taste you won't drink enough! If you're exercising at a low to moderate intensity for less than an hour then water is great.

If you work-out continuously for more than an hour, then a sports drink would be a good idea. Not only will it help maintain better fluid levels, but the added carbohydrates will provide the vital glucose to help avoid fatigue.

Most sports drinks are 5 - 8% carbohydrate - that's 5 - 8g of carbohydrate in every 100ml. This makes them 'isotonic' - a similar concentration to blood - and therefore quickly absorbed. In addition, sports drinks contain sodium to speed up fluid absorption and replace sweat losses.

Finally, a word of warning about alcohol! Yes, alcohol in moderation is fine, but not just before exercise and not until you've properly re-hydrated afterwards.

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 we know we want to avoid that - as well as slowing down recovery from injury. All in all, not such a good idea in the exercise environment.