Written By – Damien Cox Accredited Practising Dietitian


Running for multiple hours on end is not only tough from a physical and psychological  standpoint but also a physiological as well. Ensuring all the boxes are ticked and the big rocks in place will set you up for a positive performance. Hydration falls into one of those “big rocks” and ensuring proper hydration will give confidence that the run goes smoothly and as planned. Too much fluid can lead to overhydration and too little can cause dehydration, both can cause serious consequences.

In this blog a discussion around signs of dehydration and overhydration, tips to avoid, and what can go wrong with both.




Dehydration occurs when the level of fluid required in the body is less than the amount needed for normal function. During exercise sweat and fluid loss is going to be the main determining factor to the total amount of fluid loss within the body [1].

There are some fairly common signs that people experience when dehydrated such as [2]:

  • Thirst increased
  • Dry mouth
  • Dizzy or light headed
  • Dark urine

However entering into an endurance run dehydrated or becoming severely dehydrated during the event is going to hinder performance but also could lead to serious health problems. Severe dehydration can lead to problems such as [2]:

  • Increased hearst rate
  • Decreased urine output
  • Lower blood pressure

During an endurance event (12.3-h Ironman triathlon in a cool environment (3.8-km swim, 180-km bike, 42.2-km run) athletes can lose up to 11-12% (7.8-8.5kg) of their body weight in the form of water [1]. A reminder that only a 2-4% decrease in total fluid loss results in a decrease in performance outcomes [3-4].  Not only is fluid loss something to be concerned about but the amount of electrolytes lost during an endurance event is also significant. During an endurance event the effects on water and electrolyte balance have a moderate to large turnover due to sweating and drinking [1]. Meaning that electrolyte intake through the diet and necessary supplementation should be of high importance when considering in the planning leading up to an endurance event.









Overhydrating during exercise dilutes blood and causes water to move into cells which can result in cell swelling. This can result in something called hyponatremia. Hyponatremia can result in a loss of solutes (sodium, potassium), a relative excess of total body water or a combination of both [5]. This can be potentially fatal even to healthy athletes. Symptoms of this can be [5]:

  • Light-headedness
  • Dizziness
  • Nausea
  • Puffiness (hands and feed)
  • Weight gain from baseline

Exercise that is longer than 4hrs in duration, high sweat rate, high sodium or “salty” sweater and small body size are all risk factors for hyponatremia and overhydration [1]. It is important to have a set out hydration and electrolyte strategy leading into an endurance event. Foods and supplements also play a role on how much sodium can be consumed and if supplements are necessary. Some supplements like glycerol can assist with hyperhydration methods by increasing water retention and plasma volume while decreasing urine output. This can be beneficial for some however should only be used with the help of a nutrition professional. If used incorrectly this can lead to hyponatremia and serious health problems.  


In conclusion there is a lot to think about for an endurance athlete. Not only do you have the physical and psychological part of an event but also the area of hydration and the complications that can arrive with dehydration and overhydration. Ensuring a proper hydration strategy is set up and has been practiced in training before an event takes place is the key to success.



Reference List

  1. doi: 3390/nu13030887
  2. https://www.healthdirect.gov.au/dehydration
  3. DOI:1249/mss.0b013e31802ca597
  4. https://doi.org/10.1249/MSS.0b013e31802fa199
  5. doi: 10.1097/JSM.0000000000000221


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