Marathon Hydration


Image by Jana Wersch from Pixabay

What to drink and how much during a marathon is a perenial topic of discussion. Over time we have passed through the “drink sparlingly or not at all” to “drink frequently” to “drink to thirst”. What is a good marathoner to do?

The International Marathon Medical Directors Association issued guidance¹ to both organisers and participants regarding hydration and the treatment of those that get into difficulties during and after events.

The practical recommendations from their position statement are as follows:

  1. Drinking to thirst is the body’s dynamic physiologic fluid calculator and in most cases will protect athletes from the hazards of both over and under-drinking by providing real-time feedback on tonicity.
  2. A static fluid calculator can provide an estimate of body fluid losses thereby providing a numeric range as a generalized strategy for fluid replacement during racing and training.
  3. Athletes are advised to understand their individualized fluid needs through use of a static fluid calculator but ALWAYS defer to physiologic cues to increase fluid intake (thirst) or decrease fluid consumption (increased urination, bloating, weight gain) while running.
  4. Water, sodium and glucose (in foods and or beverages) should be freely available at fluid replacement stations, spaced 1.6km (minimum) to 5 km (maximum) apart. The quantity and amount of food and fluid consumed should be guided by individual palatability and tolerance for such items.
  5. Calibrated scales along a marathon course should be at the discretion of the medical team; a weight loss of >4% or any weight gain constitutes justification for medical consultation.
  6. Exercise in extreme heat (>38C) may require hydration beyond thirst during the initial days of heat acclimatization whereas advanced age (>65 y) and cooler environmental temperatures (<5C) may elevate the operating set point for the stimulation of thirst.

The “body’s dynamic physiologic fluid calculator” comes in the form of thirst. This is a fundamental protection system devised to maintain the correct internal balance within the body. It is highly sensitive and it is worth learning to recognise its signs; a dry mouth, irritation in the mouth and throat, and/or an unpleasant taste in the mouth all indicate that you are thirsty and should take a drink. Drinking according to the sensation of thirst, in the majority of cases, will protect you from the hazards of both under- and over-drinking during your event. This is particularly the case for those towards the rear of an event who are likely to be on course for prolonged periods of time.

“A static fluid calculator” comes in the form of tables and models that indicate a blanket approach to fluid consumption. Fluid requirements are highly individual and so dictating ranges of fluid volume to be consumed during prescribed periods of time can encourage extremes of behaviour. No specified range of consumption can accommodate the range of individual requirements, environmental conditions experienced and running speeds can be encountered during a marathon (or ultra) event.

Hence guideline 3 advises using the various advisory tables for advice on fluid requirements but to ALWAYS defer to what your body is telling you. Algorithms cannot account for the situation that you are experiencing in the here and now of a marathon. Your thirst or need to find the nearest loo both tell you about your hydration status and these should be guiding your approach, not a dogged insistence on this much in over that period of time. This is where using your training runs to learn about your reaction to getting sweaty.

Guideline 4 suggests what should be available to runners during races with the caveat that consumption should be guided by the runner themselves.

Guideline 5 gives a parameter for the medical team in attendance. For a 100kg runner (weighed at the start) this would represent a 4kg loss of weight (over half a stone) during the event. At this point you would probably welcome medical intervention and worries about a DNF might not even register with you.

Extreme environmental conditions do alter your requirements particularly during acclimatisation periods. But you will note that the extremes are unlikely to be experienced during any Spring marathon in the UK. The hysteria over the likely temperature on race day for the London Marathon 2018 seems somewhat out of place. The power of persuasion is strong, especially in the nervous, pre-event runner.

If you know in advance what hydration options are going to be on offer at your event, it makes sense to train with those products in advance if you are planning to use them on race day. Sometimes it is necessary for your system to become accustomed to the particular make up of so called sports drinks. You may find that you need to dilute products from their “off the shelf” offering to make them digestible to your GI tract. This is best discovered before race day as the impact can be volotile to say the least.

If you have a preferred product that you know you can stomach, it’s an idea to work out how to manage your hydration on race day in advance. Do you need to carry a bottle or a hydration bladder? Do you need to carry tablets or powder to keep you supplied? How will you carry them? If you need a belt pack or a race vest, again you want to run with this before race day to iron out any issues of fit or function.

Above all, work with your own body’s needs, take notice of the signs that you need to take action and prepare you approach in advance so you are in control of events on race day.


¹     Clin J Sport Med 2006;16:283–292


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