Keeping hydrated for exercise

Published by Bupa’s Health Information Team, December 2010.

It might surprise you to know that when you exercise you can lose up to 500ml of fluid in just half an hour – that’s nearly as much liquid as you get in a pint of milk. You mainly lose this through sweating and in the air that you breathe out. If you don’t keep your fluid levels topped up, you will quickly become dehydrated, which can affect your health and performance. Make sure you get the right amount of fluid before, during and after exercise and you will perform at your best.

This article explains the importance of keeping hydrated and how you can prevent dehydration.

Before you start exercise

Making sure you’re well hydrated before exercising is very important, especially if you’re in a warm environment. If you’re dehydrated before you even start, your core temperature will rise faster and your heart will have to work harder than usual. This can have a negative affect on your performance and can even lead to serious conditions such as heat exhaustion or heat stroke.

Provided you keep yourself topped up with fluid during the day, and you haven’t exercised for eight to 12 hours, then you should be hydrated enough to exercise at any time of the day. The easiest way to test if you’re well hydrated is to check the colour of your urine. It should be pale – the darker it is, the more dehydrated you are. Some factors can change the colour of your urine, such as vitamin supplements, and therefore this test may not be reliable. If your urine is dark or you think you’re dehydrated, you should slowly drink fluids at least four hours before exercise. If you don’t pass urine or it’s still dark then drink more fluids around two hours before exercise. This will allow enough time for the fluids to be absorbed into your body. You may need to experiment with different amounts and timings of fluid to get it right.

During exercise

If you’re dehydrated by just two percent it can have a negative affect on your performance so it’s important to keep your fluids topped up.

It’s best to drink early on and at regular intervals while you’re exercising. The amount you need to drink will depend on how much you sweat, how long you exercise for and of course, whether you actually get a chance to drink during your exercise. If you plan to exercise for longer than one hour, sports drinks that contain carbohydrate or even just squash can help you keep going as the sugar provides extra fuel and the salts will help to keep you hydrated. You might like to try drinking a sports drink that contains 60 to 80g carbohydrate every hour. This will help to prevent you from getting injured and keep you on track with your training.

If you’re exercising for less than an hour, cool water is probably the best drink to keep you hydrated.

It’s important that you don’t wait until you feel thirsty before you drink – by then, it’s too late and you will already be dehydrated.

After exercise

Once all the hard work is over, you’ll no doubt be ready for something to drink. Not only will this be refreshing, but it’s important to restore your fluid levels to normal as this helps your muscles to recover. You can work out how much fluid you lose in a typical exercise session and how much you will need to drink by weighing yourself and comparing it with how much you weighed before exercising. For every kg of body weight you lose you need to drink around 1.5 litres of fluid. The quicker you start to replace the fluid, the sooner you will recover from the exercise. Sports drinks or just water with a pinch of salt can help to restore your fluid levels. Try having something salty to eat – it might sound odd but this will naturally make you more thirsty so you will drink more and therefore rehydrate more quickly. Don’t drink alcohol or caffeinated drinks straight after exercise because they are diuretics (remove water from your body by increasing the amount of urine your kidneys produce).

Drinking too much

Although you may be so parched during and after exercise that you feel you could drink gallons of water, it’s important to bear in mind that drinking too much can be harmful and could cause a condition called hyponatremia. This is where excess water dilutes the salts in your body and your cells swell up, which can cause a number of health problems. Symptoms of hyponatremia include confusion, disorientation, a headache, feeling sick, vomiting, impaired co-ordination and muscle cramps. In a worst case scenario, severe hyponatremia can lead to a coma, heart failure and even death.

Sports drinks

The array of different sports drinks can be overwhelming and you could be forgiven for not knowing which to choose or whether they really offer any benefit. These drinks not only replace lost fluid but they also contain carbohydrates and electrolytes (sodium, potassium, magnesium and chloride). These provide fuel for workouts, help to keep you hydrated and simply make the drink taste better. There are three main types of sports drinks – hypotonic, isotonic and hypertonic.

Hypotonic, isotonic and hypertonic – what’s the difference?

Hypotonic drinks are low in carbohydrates (less than 4g per 100ml) and are designed to replace the fluids you lose during exercise. Isotonic drinks contain a moderate amount of carbohydrate (4 to 8g per 100ml) and as well as helping to replace fluid, they will also go some way towards replenishing your body’s carbohydrate stores. This is important if you’re exercising for more than an hour. Hypertonic fluids have a high concentration of carbohydrate (over 8g per 100ml). This means your body absorbs them more slowly than plain water but it will give you a real boost with refuelling.

If you’re reluctant to shell out for sports drinks in the shops, you can easily make your own. Why not try these recipes for cheaper versions. Don’t drink apple juice because it’s high in fructose which can cause stomach pain.

Action points

• Make sure you’re well hydrated before you begin exercising by drinking little and often during the day. You may find that carrying a bottle of water with you helps remind you to drink.
  
• During exercise drinking small amounts of fluid often is the best way to stay hydrated.
  
• If you’re exercising for longer than an hour, drinking fluid that contains added carbohydrate will keep you going for longer and help you recover quicker.
  
• After exercise you need to replace the fluid you have lost by drinking 1.5 litres of fluid for every litre of fluid (1 kg of body mass) you lose.

For sources and links to further information, see Resources.