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Hydration for Runners

On my 16 mile run on Sunday, I took 700ml of water with me, along with some energy gels and a flapjack. 12 miles in, I’d run out of water. This lack of water for the last part of my run was uncomfortable; it was warm and I was sweating a lot, so I decided I better write this blog.

Whenever I do some writing, the first thing I do is look up the definition of key terms that will be used in the piece. This is a relic from my university days when I had to write an essay every week. So, I looked up hydration, expecting to find something about water. The top hit on google was about a molecule combining with water. I had to go to the 5th or 6th result to get the result I was expecting.


1. The addition of water to a chemical molecule without hydrolysis.
2. The process of providing an adequate amount of liquid to bodily tissues.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.

I have included both definitions here, as both are relevant for this article. Speaking of relevance, this article ought to be relevant to everyone, not just runners.

So, now we know what hydration is, why did my running out of water on Sunday make my run so challenging?

What does water do?

Water for the human body is like oil for the car engine. Fuel, like food, provides energy, while oil, like water in the body, helps everything to work properly.
And, by everything, we mean:

  • Digestion including
    • assisting the efficient metabolism of all nutrients
    • helping to carry nutrients in the blood
    • helping the kidneys to function normally
    • removing waste products
  • Lubricating joints and organs
  • Helping with building and repairing the body
  • Assisting with the regulation of body temperature
  • Acting as a solvent for the body’s constituents
  • Providing a suitable environment for the body’s chemical reactions

When running, the most important of these is thermoregulation – assisting with the regulation of body temperature.

How much water do we need?

This varies from person to person, and is dependent on many factors both environmental and physiological. We will all have heard the common recommendation to drink 2 litres of water a day, and you can’t go too far wrong with that, but if we are looking to be more accurate and personal, we would look to drinking 1ml of water for every kcal burned.

According to my garmin , I burned 1262 calories during my run on Sunday, so if the recommendation above is accurate, then I would have needed 1.2 litres to drink on the run. I only had 700ml with me, which ran out before the end of the run, so no wonder I was thirsty.

However, there is more to it than just not having enough on the run. I know my body well enough to know that 700ml is usually adequate, and often too much for me for a run of that duration. But on further thought, I realise that I probably started the run in a dehydrated state. I had left home late morning, having had 2 slices of toast and a cup of tea before setting off. I’m not even sure that I drank any water before leaving.

The ACSM/ADA/DC (2009) recommendation for active people is to consume 5-7ml/kg water at least 4 hours prior to exercise. This would allow enough time to optimise hydration status and for excretion of any excess fluid as urine.

Another important factor was the temperature. The majority of my run happened right in the middle of the day. Whilst it wasn’t sweltering, it was certainly a high enough temperature to cause significant water loss during the 16 mile run.

Top tips

  • Avoid the mid-day sun if you are a salty sweater
  • After your exercise, aim to replace any outstanding fluid loss by 150% as soon as possible
  • Estimate your rate of water loss, so that you don’t fall short on a long run, but also to help you determine a sensible hydration plan for your important races


Estimating water loss:

  1. Weigh yourself before your run without clothing.
  2. Measure how much water you are drinking on your run.
  3. Weigh yourself after your run without clothing.
  4. Water loss = weight before – (weight after + water drunk)

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Dehydation, its more than just feeling thirsty

Sweating causes progressive depletion of circulating blood volume and a thickening of the blood, which leads to progressive strain on the cardiovascular system with a rise in heart rate in order to maintain adequate blood flow to the exercising muscles and vital organs.
The extra strain on the cardiovascular and respiratory systems makes exercise harder and hinders performance.
Dehydration of 2% of body weight impairs performance.
Dehydration of 5% of body weight leads to 30% reduction in aerobic capacity!

So, is hydration just about water?


I left the two definitions in, because in order to achieve a status of hydration, or not being dehydrated, we need to consider the other more chemical definition. We are not just trying to add water, but we are trying to maintain blood volume during exercise, to avoid a reduction in performance and aerobic capacity. This is where sports drinks come in.

There are 3 main types of sports drinks (for during exercise), which are categorised according to osmalality:
Isotonic – provide carbohydrate and maintain hydration. They have the same osmalality as the body’s fluids and are effective at transporting fluid and carbohydrate into the bloodstream
Hypotonic – designed to maintain hydration. They have low osmalality and generally contain less than 4g of CHO / 100ml – These drinks are absorbed faster than plain water and are therefore ideal for rehydrating. However, they will not provide enough fuel for a long workout.
Hypertonic – used to supply carbohydrate.

When we are ‘hydrating’ during our runs, we are trying to maintain the osmalality of the blood, so that various chemical reactions can continue to take place, allowing us to function. This is different for everybody. Some people have low sweat rates and will lose very little in the way of water from running. Others (like me) sweat a lot, and also lose a lot of nutrients in their sweat. I am a salty sweater, so as well as replacing carbohydrate and water, I also need to replace salt. This is where a pre-mixed isotonic drink would come in handy, compared to just water, or just a sweetened drink.

A little note on hyponatremia

So, we’ve spoken about too little water, as dehydration, and the effects it can have on performance. Another risk, which is reallly only a risk during long duration exercise, such as half marathon and beyond, is hyponatremia. This is where the concentration of salt in the blood gets too low, usually due to a combination of salty sweating and taking on too much water during a long distance event. In extreme cases it can cause brain swelling and death. In less extreme cases, it has a lot of similar symptoms to that of dehydration.

The best way to avoid it, is to go into a run or race well hydrated (not over-hydrated), and then during the race, only drink to thirst. And if you can, make those drinks isotonic or hypotonic.
The hypertonic drinks are best saved for after a workout, to avoid gastric distress during your run.

In summary

  • The key to hydration is to work out what works best for you.
  • Start your runs and races well hydrated
  • Prepare for the weather conditions on the day
  • Drink to thirst
  • Use isotonic drinks for very long workouts
  • In hot conditions use hypotonic drinks instead of water to quench thirst
  • Know the warning signs of dehydration and hyponatremia so that you can avoid them

If you have any questions about the topics raised in this blog, or anything at all related to running, then please contact me.

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Angela Isherwood

Angela Isherwood is the founder of I Run Success
She is a REPs Level 3 Personal Trainer, a Run England Running coach, and a multiple marathon runner. She is a London Marathon Good for Age runner, a Boston Marathon Qualifier, a parkrun Run Director and Trainer for Goodgym Colchester.

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