Around 60 per cent of your body is water and it plays a vital role in every bodily function. You can lose a lot of fluid when you exercise – as much as a litre or two an hour – mainly through sweating and breathing.
If you don’t top this fluid back up, you can get dehydrated. This can affect both your general health and how well you can exercise. You’ll feel tired more quickly if you’re dehydrated, and you won’t be able to control your temperature as well as usual.
Water helps fuel your muscles, so drinking before, during and after exercise will boost your energy levels, and may help to prevent cramp.
It may not cross your mind, but making sure you’re well hydrated before you exercise is really important, especially in hot conditions.
If you’re dehydrated before you start exercising:
- your core temperature will rise faster
- your heart will have to work harder than usual
This will affect your performance and can even lead to heat stroke. Drinking enough will help you get the most out of your exercise session and feel good while you’re doing it.
One quick way to test if you’re hydrated is to check the colour of your urine.
It can take time for fluids to be absorbed into your body. So drink steadily during the day and aim to drink around 500ml of fluid at least four hours before you exercise. In the 10 to 15 minutes before you exercise, top up your fluid levels by drinking about half this amount again.
It’s important to drink water during a workout – take a water bottle with you on a run, for example. Being dehydrated can affect your energy levels. Your muscle cells are almost three-quarters water so if you're short on fluid, you’ll feel the strain. Drinking little and often rather than a lot less often will give you the best chance of hitting your exercise targets.
How much you need to drink will depend on how much you sweat and how long you exercise for. How much you sweat and lose water is influenced by your:
- size – larger people tend to sweat more than smaller people, and men sweat more than women
- fitness – fitter people sweat more and earlier in exercise because their bodies are accustomed to needing to cool down
- environment – you sweat more in hot, humid conditions
- exercise intensity – you sweat more as you exercise harder
The best way to figure out how much to drink is to respond to what your body tells you. If you feel thirsty, you really need to drink as your body is already showing signs that it needs to take on more fluid. Here’s another way to work out how much fluid you lose while exercising and how much to drink to compensate for it.
Make sure you always have a bottle of water handy when you exercise so you don’t get dehydrated.
Once all the hard work is over, no doubt you’ll be ready for something to drink. Not only will this be refreshing, but it will also restore your fluid levels and help your muscles to recover. The sooner you start to replace the fluid, the sooner you’ll recover.
Don’t be tempted to reward yourself after exercise with a pint or a glass of wine. Alcohol is a diuretic, which means it removes water from your body by increasing how much urine your kidneys produce.
The array of sports drinks on the market can be overwhelming and they can contain lots of added sugar. So it’s hard to know which to choose and if they really offer any benefit.
If you’re doing moderate amounts of exercise, you won’t need them. Simple water is often the best thing to drink during and after a workout. But if you’re doing strenuous training, sports drinks may be useful.
- If you’re exercising for less than an hour, water is all you need to keep hydrated.
- If you’re exercising for longer than an hour, sports drinks or even just squash can help you keep going for longer. As well as replacing lost fluid, they contain carbohydrates (sugar) and electrolytes like sodium, potassium and magnesium, which you lose though sweat. These drinks provide fuel and help to keep you hydrated.
Make your own sports drink
To avoid spending lots of money on sports drinks which may contain lots of sugar and additives, try making your own at home. Mix 200ml squash (not a low-sugar variety) with 800ml water and add a large pinch of salt.
If you’re training for an endurance event like a marathon or triathlon, speak to a dietitian for advice that’s personalised for you. For more information, take a look at our guide on nutrition for sports and exercise.
Drinking too much can potentially be harmful as it can cause a rare condition called hyponatraemia. This is when you drink more fluid than you lose through sweating and weeing. The excess water dilutes the salts in your body and your cells swell up, which can cause a number of problems. The amount you have to drink to get hyponatraemia varies hugely from person to person. The symptoms include:
- feeling confused or disorientated
- difficulty balancing
- a headache
- feeling sick or vomiting
- feeling bloated
In a worst-case scenario, severe hyponatraemia can lead to a coma, seizures (fits) and even death.
If you have any of the symptoms above while you’re drinking while exercising, seek urgent medical advice.
Get to know what your sweat rate is (see our Hydration during exercise section above) and track how much you’re drinking. If you’re using a refillable bottle, make a mental note of how many times you top it up.
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- Water and sodium balance. The MSD Manuals. www.msdmanuals.com, last full review/revision November 2016
- Renal symptoms during exercise. Brukner & Khan's clinical sports medicine (online). 4th ed. MH Medical. csm.mhmedical.com, published 2012
- Dehydration. Medscape. emedicine.medscape.com, updated December 2017
- Muscle cramps. The MSD Manuals. www.msdmanuals.com, last full review/revision July 2016
- Fluid. British Dietetic Association. www.bda.uk.com, published March 2017
- Dehydration. Medscape. emedicine.medscape.com, updated 31 December 2017
- Selecting and effectively using hydration for fitness. American College of Sports Medicine. www.acsm.org, published 2011
- Healthy hydration guide. British Nutrition Foundation. www.nutrition.org.uk, revised January 2016
- Sport. British Dietetic Association. www.bda.uk.com, published April 2017
- Hydrate right during physical activity. Academy of Nutrition and Dietetics. www.eatright.org, published 23 March 2015
- Electrolytes and fluid balance. Oxford handbook of nutrition and dietetics. Oxford Medicine Online. oxfordmedicine.com, published January 2012
- European Food Safety Authority. Scientific opinion on dietary reference values for water. EFSA Journal 2010; 8(3):1459. doi:10.2903/j.efsa.2010
- Fluids for athletes. Coaching Association of Canada. www.coach.ca, accessed 11 January 2018
- Selecting and effectively using sports drinks, carbohydrate gels and energy bars. American College of Sports Medicine. www.acsm.org, accessed 10 January 2018
- Hyponatraemia. BMJ Best Practice. bestpractice.bmj.com, last updated 2 November 2017
- Hew-Butler T, Ayus JC, Kipps C, et al. Statement of the second international exercise-associated hyponatremia consensus development conference, New Zealand, 2007. Clin J Sport Med 2008; 18(2):111–21. doi:10.1097/JSM.0b013e318168ff31
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Reviewed by Rachael Mayfield-Blake, Freelance Health Editor, March 2018
Expert reviewer, Dr Leon Creaney, Sports & Exercise Medicine Consultant
Next review due March 2021
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