Symptoms and Effects of Dehydration While Running

Slim young woman drinking water after training
Slim young woman drinking water after training

Just walking in the mountains or simple long-distance run leaves you vulnerable to both dehydration and heat stroke/heat exhaustion.

So when you go out running where you can be hours from civilization, prevention of both these debilitating conditions must ALWAYS be your number one priority.

Below is a short summary of the symptoms and effects of dehydration as well as some tips on how to avoid them.


Dehydration occurs when there is a 1% decrease in body weight as a result of loss of body fluid. The symptoms of dehydration can be caused by loss of water or the loss of key electrolytes like sodium (salt).

Normally, however, the symptoms of dehydration whilst running in a hot environment are caused by a mix of both the loss of water AND the loss of electrolytes because you are sweating excessively and sweat contains both water AND sodium.

Always remember, if you are feeling thirsty, you are already dehydrated. The NHS defines three different levels of dehydration as mild, moderate and severe dehydration. The symptoms of all three levels are shown below.

When you are out on the hill, the best way keep check of your hydration levels is firstly to think about whether you’re feeling thirsty, but the best indicator of dehydration is to keep an eye on the colour of your urine. Ideally your urine should be nearly clear or at least a pale, straw colour.

Light yellow means that you are dehydrated, dark yellow means you are very dehydrated and dark brown means you are extremely dehydrated and should seek emergency medical attention.

If you have not been to the toilet at all over a several hour run, this should also be a warning sign. The best cure is always prevention. The NHS recommends drinking at least 8 cups of water per day even if you are not exercising.

When trail running in a hot environment you need to be drink little and often, small mouthfuls of water on a regular basis is the key. It’s no good waiting until you are thirsty and then stopping to down a 500ml bottle of water because firstly it means that you are already dehydrated and secondly, you will get stomach pains when you try to continue your run with a belly full of water sloshing around.

This is why hydration bladders are great for running because you can sip on the go. Another good tip for prevention of dehydration is to replace lost electrolytes such as sodium by adding electrolyte supplements such as Elete to your water.

Mild to Moderate Dehydration:

The first sign of dehydration is thirst. Other symptoms might ne:

  • lightheartedness or dizziness
  • tiredness
  • dry lips, mouthand eyes
  • headaches
  • concentrated dark urine
  • passing small amounts of urine infrequently (fewer than three or four times per day)

Moderate dehydration (a 3-5% decrease in body-weight due to fluid-loss) causes you to lose your stamina and strength and is the main cause of heat exhaustion.

If dehydration is chronic (ongoing), it can easily affect kidney function and may also lead to the development of kidney stones. It can also cause:

  • harm to your liver, joints and muscles
  • cholesterol problems
  • constipation

Severe Dehydration

Severe-dehydration is a decrease of more than 5% of body weight due to fluid loss.

As well as severe thirst, you may also have:

  • wrinkled and dry  skin
  • a weak pulse
  • a rapid heartbeat
  • cool hands and feet
  • inability to urinate
  • irritability
  • sunken eye
  • low blood-pressure (hypo-tension)
  • seizures

If body weight is reduced by more than 10%, this is extremely serious. If it is not treated immediately, this level of dehydration can lead to death as the blood stops circulating. You may need to go to hospital and be put on a drip to restore the substantial loss of fluids.

Trail Running Dehydration

Hydrating for a long distance run, or any size run should not start on the day of the run but rather the day, or days, prior to the run itself.

Drinking 5 liters of water on the morning of a run is not going to be beneficial to your performance.

You should be drinking small amounts of water, regularly, throughout the days prior to a long (or very hot) run. Don’t go drinking gallons of water a day for several days before a run though because this is actually more likely to make you feel dehydrated.

Over-consumption of water days or hours before a run will cause your body to detect an abnormally large increase in liquid and will release a diuretic hormone in order to counteract this and return your body’s water levels to normal.

This leads to extra trips to the toilet etc. which can be over-compensatory and actually leave you feeling dehydrated just before or during a run. Also, drinking crazy amounts of water in preparation for a run can be dangerous. -associated hypothermia (EAH), caused by an overall increase in the body’s water level, diminishes the concentration of sodium in the blood and in extreme cases can be fatal.

The best advice is to drink small amounts, regularly, in the days prior and the day of a long run or a run in high temperatures.

Have your last proper drink at least an hour before running so that it isn’t sat in your stomach. It is also recommended to supplement your fluid intake with electrolyte and sodium replacements to prevent the onset of EAH.

During a run the same philosophy holds true.

Drink small amounts at regular intervals.

This is why hydration packs are ideal for trail running. Don’t wait until you’re feeling really thirsty then slug down a 500ml sport drink all at once.

This will just slosh around in your stomach and give you a stitch.

Later on, it is also vital to replace lost fluids and electrolyte immediately after the run with sports drinks and/or electrolyte replacement products to help prevent muscle stiffness. Furthermore, you should drink (or eat) something to replenish your carbohydrate stores so your body doesn’t continue burning your muscle stores after the run.

This should also be combined with a dose of protein to help repair damaged muscle tissue from your run. The ideal combination is a drink (or meal) of three parts carbohydrate to one part protein. And don’t forget to use proper running shoes and running gear– it will not help with hydration of course, but will help in general.

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