Dehydration during Multi-Stage Endurance Races

The Marathon des Sable is less than 40-days away and one of the reoccurring questions, and fears amongst the runners, concerns dehydration. This blog posting will hopefully, provide you with the answers you need and set your mind at ease concerning this issue.


Firstly, dehydration represents a state where your fluid input is less than your fluid output. The human body consist mainly of water; as a matter of fact as much as 75% of your body weight represents the water that is contained within your body. Water is used for numerous functions within the body; however, for the multi-stage endurance athlete water is primarily used for cooling. It goes without saying that extreme exercise adds some level of stress to the human body as the additional water required for cooling competes with other functions that also requires water to function. The more extreme the event, the higher the bodies requirement for water, and the more vigilant an athlete must be to ensure adequate hydration.

Adequate Hydration in a Normal State

In a normal state where the temperature is moderate and you do not engage in deliberate exercise your body will need water on the following basis, 1,500ml for your first 20kg of body weight plus 10ml per kilogram over and above 20kg. So an 80kg person would require around 2,1litres of water to function normally.

Adequate Hydration in an Extreme State

However, exercise and an extreme environment, such as higher than normal temperatures, will greatly increase your fluid loss, and will require additional water intake. Research has indicated that losing only 2% of your body weight in fluids results in a 25% decrease in performance [1]. Adequate hydration research by Melton [2] provided the following formula for athletes:

  • One hour pre-workout drink between 444ml and 591ml (average 518ml),

  • Fifteen Minutes before you start drink between 240ml and 295ml (average 268ml), and

  • Drink another 240ml every 15 minutes during your workout.

An additional suggestion is to replace every 454g of body weight lost with between 473ml and 591ml (average 532ml) of water. This latter suggestion is somewhat difficult to ascertain in an outdoor environment, however, some pre-race experimentation might provide a refined indicator of your individual fluid loss during exercise at a specific intensity, duration and environment.

For the sake of this article I will use the Melton formula.

Hydration within the Context of the Marathon des Sables

Using the 2013 MdS results as a basis for illustration, and assuming an average runner weight of 70kg’s:Leaders (18h 59m),Middle of the Pack (41h 24m), andBack Runners (73h 25m).The minimum water that is required by all athletes, in accordance with Carlson’s formula would be 2,000ml (2litres) per day. Using Melton to calculate the exercise based water requirement for the event delivers the following results:

  • Leaders: 6,434ml of water per day,

  • Middle of the Pack: 10,754ml per day, and

  • Back Runners: 16,898ml per day.

At the Marathon des Sables athletes will receive a total of 55,5litres of water over the five days of the event. The average water supply, therefore, comes to 11,1litres per day. However, as can be seen from the table above this provides sufficient water for only about half of the runners, all of whom places in the first half of the field. This would imply a deficiency of around 34,49litres for the last runner, which should translate into a weight loss of around 33kg’s. As this has not been seen in the event, I suggest, that the formula might be somewhat deficient at its most extremes. I am doubtful that the front runners would be able to remain hydrated on only 6,5litres of water per day and am equally doubtful that the last runners would require 17litres of water per day. I do think, however, that the median of 11litres per day as a requirement translates well for all runners across all placements. There might be exceptions, and each runner should request additional water from the organisers if he or she feels that they need additional hydration.

From this median, it seems highly unlikely that a runner, if all water is consumed, would reach a state of dehydration. My own experience attests to this as does the research I conducted during the 2013 MdS. This said, however, I have on occasion reached a mild state of hydration which was more often than not the direct result of extreme heat, or a faster than normal pace.The bottom line is, that with a disciplined drinking regime, an athlete during a multi-stage ultra-endurance event such as the MdS would reach a mild state of dehydration from time to time, but with good management would be able to keep it at that. Bad management and bad drinking habits would quickly see a mild state of dehydration escalate to something more serious.

Signs of Dehydration

Now that we have established that mild dehydration is a possibility at an event like the MdS, we need to recognise the symptoms to ensure that we can steer clear from a much more serious dehydration level.

Initial Indicators (first stage)

  • Thirst,

  • Urine turns Yellow, becoming progressively darker.

These indicators would point to two important actions that an ultra-endurance athlete needs to make part of his/her running regime. The first action is to always drink before you get thirsty and together with this, to increase your water intake when you do become thirsty at any point. The second action is the need to urinate regularly. Inability to urinate or the darkening of urine is a strong indicator and can help you steer back towards an adequate hydration level. There is definitely no place for a shy bladder in any ultra-endurance activity.

Intermediate Indicators (latter stage)

  • Dry mouth,

  • No more tears,

  • Sweating stops,

  • Muscle cramps,

  • Light-headedness, and

  • Decreased urine output.

With any of these symptoms, the dehydration has already progressed to a point where its severity will have a severe impact on your ability to perform. Oral hydration is essential and it might be best to stop your activity for 30 to 60 minutes to allow your fluid intake to stabilise. Rehydrate powder or other rehydration products could be used, as these have proven to be efficient.

Severe Dehydration Indicators (possibility to continue)

A more severe dehydrated state could require a more aggressive approach and even specialised medical attention. The following symptoms are clear indicators of severe dehydration, and yet, even with many of these indicators all is not lost. In ultra-endurance running management plays a significant part in our success, and this includes managing yourself out of trouble.

  • Confusion,

  • Weakness,

  • Nausea and vomiting,

  • Heart palpitations,

The severity of these indicators means that you will have to stop for a while, an hour or maybe even two, might be required. While oral rehydration is still possible this should be used as it remains the preferred means of correcting hydration problems. From the work of Barton [3], Burton [4], Sasson [5] and others [6], rehydration strategies should always first start with oral hydration, even vomiting provides no clear indication that oral hydration cannot be used, as long as the amount of water entering the body is more than what is expelled. These authors note that intravenous (IV) therapy should only be considered in situations where urgent fluid resuscitation is needed (such as in the case of blood loss, burns, etc.). In the event of vomiting, oral hydration should be limited to around 5ml per minute, and can be increased once vomiting and nausea ends. This means that within an hour 300ml can be consumed which should roughly provide 217ml of hydration replacement, which is over and above what the body needs during that hour.

It is my view, that the current practice of medical personnel to administer IV therapy is therefore questionable, especially, in cases where oral hydration is still possible and no other conditions exist that can provide a clear indication that the runner is in a life threatening state. It is, hoever, vital that you discuss your particular dehydration / rehydration strategy with qualified medical personnel before you engage in such extreme activities. Their insight and guidence is vital.

Severe Dehydration Indicators (no-possibility to continue)

No dehydration should be allowed to progress to this point and medical treatment, and most probably hospitalization, would be required.

  • Organ failure,

  • Coma, and

  • Death.


With a disciplined eating and drinking regime, constant monitoring and adaptation to your environment and by responding to your body’s needs, the possibility of becoming severely dehydrated is unlikely. The MdS, as many other races such as the Kalahari Augrabies etc. provide sufficient water to remain hydrated, subject to you drinking the water and not wasting it. Drinking in this context refers to a constant intake opposed to large volumes 60 or 90 minutes apart. Given the environment within which these events are undertaken there are always exceptions and variables that cannot be planned for or, in many cases, even anticipated. It is these factors that will require you to manage yourself very carefully (pace, walk-run ratio, and fluid intake) to ensure that your dehydration remains manageable. It should be a given to each and every participant that mild dehydration will happen and that from time to time you may even touch on, for a brief moment, on something more severe. It is when this happens that you should become more aggressive in your approach to rehydrate. Drink lots of water, ask for more water (even if a penalty is recorded) and get back in control.

To the 2014 MdS runners, I wish you the best of training and preparation in the final few weeks to the run-up towards an amazing experience. And to my other readers, I hope that this article has provided some additional information to assist you in your preparation. If you guys have questions, or would like to discuss your preperation etc. don't hasitate to send me an e-mail.

Thanks for visiting push2extreme and for reading our blog.

#marathondessables #mds #hydration #dehydration #iv #ivtherapy #genispieterse #kalahariaugrabiesextrememarathon #kaem

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