Whatever sport you are taking part in, it's important to give your body the right amount of fluids.
What does your body need?
Your body is finely regulated, with water making up two thirds of your weight. This water accounts for 85 percent of your blood, 80 percent of your muscle, and a quarter of your bone. It's important that your body stays hydrated - keeps the right level of water - to allow vital chemical reactions to happen, and nutrients to be transported to organs and tissues.
You lose water all the time, when you urinate, sweat, or even breathe. You need to replace what you lose. When you exercise, you lose more water, so you need to drink more fluids to stay hydrated.
You also need to replace minerals (sodium (salt) in particular) that you lose. During strenuous exercise, you will lose one to five grams of salt per litre of sweat.
Day-to-day fluid needs
The media encourages us all to drink at least two litres (about eight glasses) of water a day, but it needn't be as strict as this. Fluids from food and other drinks count too, so instead it's best to aim to drink enough fluids from a varied diet and a range of drinks.
Drinking while exercising
During strenuous exercise, the water you lose through sweating can range from half a litre to two litres per hour, so you need to replace the fluid and salts you have lost.
Everyone is different; some people sweat more than others, and some lose more salt in their sweat. Because of this individual variation, it's not possible to give exact advice on how much you should drink during a session. As a general rule, you should drink 120 to 150ml of cold fluid every 10 to 15 minutes (cooler fluids because they are normally more palatable and help to lower your core body temperature).
Don't wait until you feel thirsty - drink before you start, during and after exercise.
Remember that you will also need to take some energy on board if you exercise for more than an hour and a half. Try a banana or dried fruit before or during a long session. The amount of extra energy you need will depend on your fitness, weight, genetics and metabolic rate. The key is to know your own body.
Signs of dehydration
One way to know whether you are drinking enough is to trust your body: if you feel thirsty you need to drink. You will feel unwell if you are dehydrated. Dehydration can give you a headache and leave you feeling tired, constipated and sick.
There's a common belief that that dark urine suggests dehydration, with pale straw-coloured urine being ideal. While this may be a helpful indicator, it can be hard to judge.
Drinking too much water
Drinking a lot more water than you need can lead to a dilution of the salts in the body known as hyponatraemia. Symptoms of hyponatraemia include confusion, disorientation, worsening headache, feeling sick, vomiting, impaired co-ordination and muscle cramps. If it becomes very severe, it can cause a coma, heart failure and death.
Publication date: June 2009