The weather outside is frightful, it’s cold and wet, your motivation to get out the door is at an all-time low and all you can think about is that drastic temperature change as soon as you step through your front door. Once you do venture outside, and even upon your return, the last thing on your mind is slurping on some ice cold water from your sports bottle. But it’s beneficial and here’s why….

A Liquid Asset

You probably already know that water makes up the majority of body mass in a human being. For a 75kg man it equates to approximately 33-53 litres of water – that’s about the amount of fuel you’d put in an average car’s tank. Two thirds of body fluid is found within your body’s cells with the remainder being “extracellular” (found within the blood and gut for example). The fluid consists of electrolytes. These are salts designed to regulate fluid balance, blood acidity and muscle function.

Footprint in water

Why is fluid important during exercise?

Swim lane with swimmer
Maintaining the correct fluid balance during exercise helps sustain your performance in multiple ways:

  • Attenuates increased heart rate and core temperature
  • Improves stroke volume (the blood pumped by your heart in each beat)
  • Improves cardiac output (amount of blood pumped by your heart per minute)
  • Improves skin blood flow
  • Controls sodium levels and levels of adrenaline
  • Reduces muscle glycogen use and therefore preserves these energy resources

When fluid balance strays from healthy levels there are problems that can occur, the most common being hyponatremia.

What is Hyponatremia?

Hyponatremia is a condition that results from excess fluid in the body relative to normal amounts of sodium. It can be caused by drinking too much (consuming fluid in excess to losses) or by simply lacking sodium intake. Side Effects:

  • Muscle weakness
  • Muscle uncoordination
  • Disorientation
  • Seizures
salt shaker of sodium

This is confusing….should I be drinking more or less?

Pouring bottle of water into glass
Drink too much and you can disturb your ideal fluid balance, drink too little and you run the risk of dehydration. However, the body is a remarkable machine when it comes to intake and expenditure and as long as you don’t go to the extreme ends of consumption then you’ll be fine.

In general, depending on body mass, age and sex, we require an average of 2-4 litres water per day. This can come from foods and/or beverages – did you know that 96% of a cucumber is made up of water? Even a carrot is 87% water! Listen to your body, drink when you are thirsty.

Exercise and Fluid

Your fluid requirements also depend on physical activity. You sweat to regulate body temperature through the evaporation of water from your skin. During intense exercise you can lose up to 3 litres an hour! It is quite normal among athletes to go from daily water output of 2.5 litres one day to 6.5 litres the next when competing. Your rate of loss is influenced by:

  • Temperature and humidity
  • Clothing
  • Body size
  • Level of fitness

1 litre of fluid loss is the same as losing 1 kilogram. Before all you fell runners think “Blimey! If I don’t drink for 24 hours then I’ll be flying up that hill tomorrow”, there are some seriously detrimental effects of dehydration that far outweigh the benefits of weight loss. The example below is for a 68kg person:

  • 1% Dehydration / 0.7kg lost = Increased body temperature
  • 2% Dehydration / 1.4kg lost = Impaired performance
  • 5% Dehydration / 3.4kg lost = G.I problems, heat exhaustion
  • 7% Dehydration / 4.7kg lost = Hallucinations
  • 10% Dehydration / 6.8kg lost = Circulatory collapse

So, how can I tell if I’m starting to become dehydrated?

There are some early symptoms of dehydration that you should pay attention to, particularly during exercise when it is sometimes easy to confuse symptoms resulting from other physiological processes (you’ll probably ignore a dry mouth if you have got to mile 20 of a marathon experiencing runner’s knee!) Here’s what to look out for:

  • Dry or sticky mouth
  • Headache
  • Stomach upset
  • Dizziness
  • Muscular weakness
Pills, syringe and temperature

How do I avoid dehydration in the first place?

  • Pre exercise:Ensure you are fully hydrated but be careful not to overhydrate (think back to hyponatremia). If you need a number then 5-7mls per kg of bodyweight within the 4 hours before exercise would be something to aim for (that’s 400-560mls of fluid for someone weighing 80kg).
  • During Exercise:Ensure you replace fluids lost from sweating and try to include a source of energy along with this fluid. If you aim to replace 80% of losses during exercise then this would be sufficient. Including a bit of salt with this fluid ensures your “drive to drink” and can also help to prevent hyponatremia.
  • Post Exercise:If you have been exercising for 1 hour then it is likely that you are dehydrated (even if you have been taking fluids onboard). Post exercise rehydration is vital for regular exercisers so always aim to have a drink close to hand when you have finished your workout. Again, drinks containing carbohydrate and sodium are more effective than plain water.
    • Consume as much fluid that can be comfortably tolerated immediately following exercise (in the region of 400-600mls)
    • Try to consume 250mls every 15minutes for the 3 hours following exercise
    • Include carbohydrate and sodium in your “rehydration strategy”
    • Understand roughly how much fluid you have lost during your session

I’m curious, how do I know how much fluid I have lost?

It’s relatively simple. 1 litre of water equates to a weight of 1 kg. Therefore, weigh yourself before exercising and then again on completion. Subtract the weight after from the weight before (accounting for any fluids that have been consumed).

This gives an approximation of the volume of fluids lost during that workout.

Measuring water using a spoon

What about all these sports drinks?

Simply, drinks can be placed into one of three categories:

  1. Hypotonic – This is a thirst-quencher and is ideal for recreational sports, ensuring that fluid is absorbed quicker than plain water.
  2. Isotonic – Another thirst-quencher, this drink also provides energy and is absorbed at the same rate as water. It’s ideal for use during endurance sports.
  3. Hypertonic – The main focus is to supply energy, therefore the fluid is absorbed slower than water. This is ideal for less strenuous long duration sports.


It’s pretty well accepted among sports scientists that chronic dehydration is a common problem among endurance athletes, preventing them from performing and recovering at their optimal level. We hope this article helps you to understand the importance of hydration and the effect of fluid on exercise and performance.

So, next time, after getting home from a chilly winter’s run, remember to take a few gulps of your drink before plunging into a hot bath!