Summary

  • You need to replace fluids when your body sweats during exercise. 
  • When dehydrated, your mind and body will suffer, and your physical performance will diminish.
  • Your body will show symptoms when it is dehydrated.
  • The best fluid is water.
  • There is no fluid replacement plan that suits everyone.

 Woman exercising with water bottle

What you need to know

Fluids keep your body hydrated. Without them your body won’t function optimally.

Here’s what will happen if you don’t drink enough fluid:

  • Your body temperature and heart rate will rise. That’s because when the total amount of water in your body is below normal level (hypohydration) your body can’t properly regulate heat. 
  • You’ll feel more fatigued than usual.
  • You won’t think clearly – your motor control, decision-making abilities and concentration will decrease.
  • Your body’s functions will slow – this includes gastric emptying, so you may feel uncomfortable in your stomach.
  • Your performance in sport or exercise won’t be as good as it could be. The impact is even worse when you’re active and dehydrated in hot conditions.

The simple solution is, of course, to drink enough fluids, and regularly, when you exercise. Make fluid replacement a priority when you’re physically active. 

Drinking enough fluids will help to maintain your concentration and performance, increase your endurance, and prevent excessive elevations in heart rate and body temperature. It’s all about sufficient hydration.

What hydration means

The amount of water you need depends on a range of factors, such as climatic conditions, your health, your clothing, your exercise intensity and duration. So, being well hydrated will differ per person and situation.

As a guide, you probably need more fluid if:

  • you sweat heavily
  • you have certain medical conditions, such as diabetes or heart disease
  • you have cystic fibrosis, which means you have a high concentration of sodium in your sweat 
  • you are using a medication that can act as a diuretic, causing your body to lose more fluid
  • you have a bigger body size
  • you are fit (because fitter people tend to sweat more and earlier in their exercise)
  • you are doing vigorous exercise
  • you are active in hot or humid conditions.

Thirst isn’t the best indicator that you need to drink. In fact, if you feel thirsty, you are probably already dehydrated.

A good test of dehydration is the colour of your urine. If it’s pale and clear it means you’re well hydrated. The darker it is, the more fluid you need to drink.
Another sign of dehydration is a lack of sweat during vigorous activity, when you expect to sweat. No sweating is a sign that you’re both dehydrated and probably suffering heat exhaustion.

What dehydration means

Dehydration occurs when your body’s water content is too low. Here are some body signals that indicate you haven’t had enough fluid:

  • headaches
  • fatigue
  • mood changes
  • slow reaction times
  • dry nasal passages
  • dry or cracked lips
  • dark coloured urine
  • muscle cramps
  • weakness
  • confusion
  • hallucinations.

If you experience any of these symptoms, immediately drink more fluids.

If you don’t rehydrate, your physical and mental performance is likely to decrease as well. A loss of fluid equal to two per cent of body mass (for example a 1.4 kg loss in a 70 kg person) is enough to cause a detectable decrease in performance. A loss of fluid equal to more than two per cent means you risk nausea, vomiting, diarrhoea and other gastro-intestinal problems. 

It’s not possible to train your body to handle dehydration, so don’t delay fluid replacement to ‘get used to dehydration’. When you need water, you need it.

Sweat and dehydration

When you exercise, your body sweats as it tries to return to its optimal temperature. As sweat evaporates from your skin, it removes heat from the body. In turn, you lose body fluid. 

So, you need to drink fluid during exercise to replace fluids lost in sweat. That way, you’ll reduce the risk of heat stress, maintain normal body function, and maintain performance levels. The general rule is: if you’re sweating, you need to be drinking fluids.

It is possible drink too much during exercise. To avoid over- or underdoing it, it can be useful to know your sweat rate. That way, you can work out exactly how much you should be drinking. The Australian Institute of Sport has some pointers about how to do this

For a fluids plan, talk to your GP or a sports dietitian.

What to drink when exercising?

Water is the best drink to satisfy thirst and replace fluid lost during exercise. You should also drink water before exercise. 

Water boasts a huge list of benefits. It’s natural, free, readily available, contains no kilojoules, and contains fluoride, which is good for your teeth.

About sports drinks

Some athletes use sports drinks that contain electrolytes and carbohydrates, which have concentrations that allow the body to refuel during exercise. Sports drinks may be useful if your activity is moderate to vigorous in intensity for more than 60 minutes (see the Australian Dietary Guidelines). However, sports drinks can be high in sugar, so consume them only if necessary. 

Remember that fruit and vegetables contain a high proportion of water, so a fruit snack (such as oranges) can help your fluid replacement.

What not to drink when exercising

  • Avoid cordial, soft drink or juice. These are usually high in carbohydrates and low in sodium. 
  • Avoid caffeine, which can be a diuretic (which means it makes you pass more urine, and so lose more fluid).

Remember…

  • You need to replace fluids when your body sweats during exercise. Otherwise, your mind and body will suffer, and your physical performance will diminish.
  • The best fluid for your body is water.
  • There is no fluid replacement plan that suits everyone. The best way to figure out how much to drink is to listen to your body. 
  • Athletes should seek advice about fluid replacement from a health professional. 

Where to get help

References

More information

Keeping active

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Keeping active basics

Getting started

Staying fit and motivated

Exercise safety and injury prevention

Keeping active throughout life

Health conditions and exercise

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This page has been produced in consultation with and approved by: Fitness Australia

Last updated: January 2017

Content on this website is provided for education and information purposes only. Information about a therapy, service, product or treatment does not imply endorsement and is not intended to replace advice from your doctor or other registered health professional. Content has been prepared for Victorian residents and wider Australian audiences, and was accurate at the time of publication. Readers should note that, over time, currency and completeness of the information may change. All users are urged to always seek advice from a registered health care professional for diagnosis and answers to their medical questions.