We are approximately 60% water!

That seems a lot, but a loss of just 2% will generally class you as dehydrated.

When exercising you lose water, not only from sweat, but also from the water vapour in your breath.

In hot or humid weather, water loses can be at their highest and fluid should be replaced quickly and strategically to avoid dehydration and the subsequent negative effects on your physical and mental performance.

Read on to understand what the experts recommend for keeping hydrated.

Sweat it out!

Why do we sweat and why do we become dehydrated?

Sweating is the body’s defence mechanism preventing our inner body from heating up to unsafe temperatures. The amount of sweat you produce and the amount of fluid lost depends on the intensity of your workout, the length of your workout, the temperature and humidity of the environment around you, and your body’s individual workings.

On average, you can lose anywhere between 0.5 and 2.5 litres an hour of fluids. In hot temperatures and humidity, this can be much higher!

The good news is that sweating and losing fluids is completely normal. The bad news is that you can’t prevent or reduce the amount that you lose!

However, becoming dehydrated is preventable.

Dehydration Danger

The most common factors that cause dehydration include:

  • Drinking only when you feel thirsty
  • Not drinking enough fluids (before, during and after exercise)
  • Excessive sweating
  • Exercising in a dry and/or hot environment or at a high altitude

What are the effects of dehydration?

The effects of fluid loss can be severe! It can reduce blood volume, meaning your body has to work harder to pump blood around your body leaving you feeling tired more quickly with impaired performance.

The more serious side effects Include:

  • Muscle cramps
  • Dizziness
  • Heat exhaustion, which if left untreated will lead to;
  • Heat stroke

Interestingly, studies have found that the effects of mild dehydration have less impact on performance in elite athletes. Perhaps due to their body’s adaptations to exercise from rigorous, long term training regimes.

How to tell if I am dehydrated?

You can easily become mildly dehydrated even when not exercising.

Hydration and dehydration builds up over time. It is not so much a case of having a glass of water and you are hydrated again!

The easiest and most effective way to test your hydration level is to check the colour of your urine.

Studies have found that dark yellow, strong smelling urine is a sure sign that you are dehydrated.

Optimum hydration is characterised by very pale yellow and odourless urine.

What and when should I drink?

Before exercise

Prevention is better than cure. The American College of Sports Medicine (ACSM) recommends drinking 5-10ml of fluid per kg of body weight slowly throughout the 2-4 hours prior to exercise. Though if you are still dehydrated (using the urine colour scale), you should continue to drink water.  

Can I overload on water before exercise?

Consuming too much water either by accident, or in an attempt to ‘Hyperhydrate', is not advantageous. The body only retains what it needs, so when consuming large quantities of water or sports drinks, any excess will simply be excreted. This can be an issue due to (a) electrolyte losses through urine. This is especially important with regard to maintaining sodium levels and (b) the impracticality of needing to frequent the toilets (or bushes!) during your training or race.

During exercise

There are no specific recommendations of how much to drink and how often. It varies considerably from person to person.

The main objective of drinking during exercise is to prevent fluid loses over 2% of your body weight.

The ACSM suggest this calculation to help you work out how much fluid you should consume during exercise.

  1. Weigh yourself before and after a hour exercise session. Record how much fluid you consume during the session.
  2. Calculate the difference between your start and end weight.
  3. If you assume that all weight lost is from sweat, then on average, every 1kg of weight lost equates to 1 litre of fluid.
  4. Add this value to the amount of liquid consumed.
  5. This is your hourly sweat rate
  6. Divide this figure by 4 to give you the amount of liquid you should consume every 15 minutes. Note that this will change with a change in your environmental conditions.

What’s best: Water or sports drinks?

Low/Moderate exercise lasting under 45 minutes: Plain water is adequate to replenish any lost fluids.

Intense exercise lasting between 45mins and 1 hour: Isotonic drinks are ideal, containing 40-80g of carbohydrate per litre.

High intensity exercise lasting 1-3 hours: You are likely to require liquid and fuel replenishment. The ACSM recommend 30 to 60g carbohydrate per hour. This should be achievable from many commercial sports drinks, though always read the nutrition table.

If you need more than you can get from a sports drink, or sports drinks just don’t work for you, an option is to couple water with energy gels, energy bars or even just sweets. It’s best to trial different combinations to find what works best for your body and your performance.

The recent 2018 sugar tax law means many sports drinks now have less than 5% sugar, which may not be enough for your requirements. In very hot and humid conditions a more diluted drink (20-40g carbohydrate per litre) may be more suitable as the water is easier absorbed in less concentrated fluids.

After exercise

When you exercise you lose water and salt. Despite efforts to remain hydrated throughout exercise, you still need to make sure stores are replenished after exercise. Water and food (a banana for example) or simply a sports drink (containing sodium and carbohydrate) should do the job.

  • You should consume 1.2 to 1.5 times the weight of fluid lost during exercise.

Though, not all at once and not immediately after exercise!

Another, extensively researched and proven near-perfect recovery drink is milk. It’s combination of glucose, high quality protein and electrolytes make it ideal for a post exercise drink regarding hydration, glycogen replenishment and muscle recovery. Some plant based milks such as soya milk, are also considered good for recovery.

When I’m not exercising?

The European Food Safety Authority diet reference values for water, currently stand at 2.5 litres per day for men and 2 litres per day for women. This includes fruit juice, milk, soup, squash and contrary to popular belief, coffee and tea!

Caffeine containing drinks can be as hydrating as water, up until a certain level. Consuming over 600mg in a short space of time may contribute to dehydration. This is equivalent to approximately 6 cups of fresh coffee or 15 cups of tea. If you do consume more than 600mg a day, ensure you drink a glass of water in-between cups!


Every day you need to consume 2.5 – 2 litres of water as standard. This should be spread out across the day to ensure constant hydration.

When exercising, you don’t always have to take water with you, especially if you’re out running as a bottle can be pretty inconvenient! But if you are out for a long time or doing an intense workout, ensure you’re smart about how much and what type of fluid you consume.

Straight after, and in the hours after exercise, make sure you recover well and keep topping up your fluids!

Over to you!

Now you know what to do, get out there and test the equations. See how differently a week training in optimum hydration can change your performance and maybe more (to name a few, mood, digestive tract, sleep and concentration).

Remember one size doesn’t fit all so it may take time to get to your optimum, but it’s worth it!

Further Reading

For more training and nutrition advice ake a look at our other blogs including;

How to supercharge your Marathon Recovery

5 Leg Exercises Runners Should Be Doing In The Gym

8 Dynamic Stretches To Enhance Every Training Session

September 03, 2018 — Lisha Armstrong

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