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Keeping hydrated for exercise

It might surprise you to know that when you exercise you can lose up to a litre of fluid in an hour, although the amount lost varies from person to person and depends on how long and how hard you’re exercising. You mainly lose fluid 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 ability to continue exercising. Therefore, it’s important to make sure you get the right amount of fluid before, during and after exercise.

Keeping hydrated
Alex McKinven, Physiotherapist, shares some advice on staying hydrated.

Details

  • Before you start Before you start

    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 effect on your performance and can even lead to serious conditions such as heat stroke.

    Provided you keep yourself topped up with fluid during the day, and you haven’t exercised for eight to 12 hours, you should be hydrated enough to exercise at any time of the day. One quick way to test this is to check the colour of your urine. It should be pale yellow – the darker it is, the more dehydrated you are. If your urine is dark or you think you’re dehydrated, slowly drink fluids at least four hours before exercise. If you don’t pass urine or it’s still dark, drink more fluid about two hours before exercise. This will allow enough time for the fluid to be absorbed into your body. You may need to experiment with different amounts and timings of fluid to get it right but as guide, aim to drink 400 to 600 millilitre (ml) in the two hours before you exercise.

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  • During exercise During exercise

    If you’re even just a little dehydrated, it can have a negative effect on your performance so it’s important to keep an eye on your fluid levels.

    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 and how long you exercise for. You can work out how much fluid you lose in a typical exercise session and how much you will need to drink during and afterwards by weighing yourself after exercise (do this before passing urine). Compare this with how much you weighed before exercising (weigh yourself after passing urine). For every kilogram of body weight you lose, you need to drink about 1.5 litres (l) of fluid.

    If you’re exercising for less than an hour, water is all you need to keep you hydrated. If you’re exercising for longer than an hour, sports drinks that contain carbohydrate or even just squash, can help you keep going as the sugar provides extra fuel.

    If you’re training for a race or other event, it’s a good idea to practise drinking while you exercise. This will help you get to know how much fluid you need and when to drink.

    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 After exercise

    Once all the hard work is over, no doubt you will be ready for something to drink. Not only will this be refreshing, but it’s important for restoring your fluid levels, which helps your muscles to recover.

    Remember that you need to be replacing the fluid you lose while you’re exercising – don’t wait until you finish. The sooner you start to replace the fluid, the sooner you will recover. Sports drinks or 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 thirstier so you will drink more and therefore rehydrate more quickly. Don’t drink alcohol or caffeinated drinks straight after exercise because they are diuretics, which means they remove water from your body by increasing the amount of urine your kidneys produce.

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  • Drinking too much Drinking too much

    Although you may be so parched 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 rare condition called hyponatremia. This is when 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 feeling confused or disorientated, a headache, feeling sick, vomiting and muscle cramps. In a worst case scenario, severe hyponatremia can lead to a coma, seizures (fits) and even death.

  • Sports drinks 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. Most people doing moderate amounts of exercise won’t need them, but if you’re doing a lot of strenuous training, they may be useful. As well as replacing lost fluid, these drinks contain carbohydrates and electrolytes (sodium, potassium, magnesium and chloride). These provide fuel, 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. Try these recipes for cheaper versions.

    Hypotonic Isotonic Hypertonic
    20 to 40g sugar (from squash)
    1 litre water
    Small pinch of salt
    40 to 80g sugar (from squash)
    1 litre water
    Small pinch of salt
    80 to 100g sugar (from squash)
    1 litre water
    Small pinch of salt

    When you’re working out how much squash to include, 20 to 40g sugar is about 100ml concentrated squash. However, check the label as different drinks contain different amounts of sugar.

  • Action points Action points

    • Make sure you’re well hydrated before you begin exercising by drinking little and often during the day.
    • Work out your sweat rate so that during and after exercising you can accurately replace the fluid you lose. Aim to drink 1.5l of fluid for every 1kg of body mass you lose.
    • During exercise try to drink small amounts of fluid often to stay hydrated.
    • If you’re exercising for longer than an hour, drinking fluid that contains added carbohydrate may help you to keep you going for longer.
  • Resources Resources

    Sources

    • Fluids and electrolytes during exercise. The Dairy Council. www.milk.co.uk, accessed 21 August 2012
    • Effective hydration strategies for sweat loss. Australian Institute of Sport. www.ausport.gov.au, published July 2009
    • Personal communication, Christina Merryfield, Lead Dietitian, Bupa Cromwell Hospital, September 2012
    • Sports nutrition. American Academy of Orthopedic Surgeons. www.orthoinfo.aaos, published August 2007
    • Fuel your body. British Dietetic Association. www.bda.uk.com, accessed 27 September 2012
    • Hyponatremia in emergency medicine. eMedicine. www.emedicine.medscape.com, published April 2010
    • Hyponatremia. The Merck Manuals. www.merckmanuals.com, published May 2009
    • Selecting and effectively using sports drinks, carbohydrate gels and energy bars. American College of Sports Medicine. www.acsm.org, accessed 21 August 2012
    • Cohen D. The truth about sports drinks. BMJ 2012; 345:e4737. doi:10.1136/bmj.e4737
    • Sports drinks. British Soft Drinks Association. www.britishsoftdrinks.com, accessed 21 August 2012
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