Stopping To Drink: Is It Worth The Time?

Dayton Kelly

This article was adapted from a combination of speeches given at the European Sports Science Conference 2017, most notably Lewis James PhD (Loughbrough University, UK).

Research in dehydration and exercise has long been clouded in uncertainty despite athletes having been observed to lose three to four liters of water an hour exercising in the heat. It is clear that at some point dehydration impairs performance (dying of thirst would certainly increase your marathon time), but the magnitude of dehydration required for performance impairment remains debatable. Recommendations for fluid consumption may be further obscured as performance impairment will further depend on exercise type and the method of dehydration. It does appear, however, that long distance endurance exercise, such as marathon running, does display the greatest performance decrement with dehydration. So, is stopping to drink worth the time?

Where does the current literature stand?

In its infancy, research in dehydration suggested water loss may actually improve performance. It was found that the most successful athletes at marathon type events were actually the most dehydrated and thus dehydration must equal better performance. It was thought that the accompanying weight loss with dehydration may improve running economy. This conception has been since rejected realizing winning athletes were likely more dehydrated because they were running harder.

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Since then, the body of literature has suggested that dehydration resulting in weight loss that exceeds 2% of the athlete’s total body mass will impair marathon-type performance. While this threshold has been repeatedly demonstrated, it has recently been called into question. It was recognized that no literature to date had actually blinded participants of their dehydrated status. Thus, people may be performing worse because they think dehydration will make them perform worse rather than any physiological mechanism be at play.

This would support the idea that marathon runners do not need to drink during marathon running. To tackle this issue, a sample of athletes performed twice performed testing hooked up to a machine providing an intravenous saline solution. On one of these occasions the machine provided regular saline, maintaining hydration throughout the exercise task, and on the other, hypohydrated saline solution was used to induce dehydration. On both occasions, participants were unaware of whether or not they were being rehydrated. The study found impairment regardless of participant blinding, suggesting a real physiological mechanism of dehydration exists.

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Take away:

The literature suggests a physiological threshold for marathon-like exercise performance at a 2% body mass loss of water. Thus, marathon runners are advised to maintain hydration during marathon running to prevent too much fluid loss.

NOTE: Runners should practice like they perform, should you choose to rehydrate during your marathon you should practice grabbing cups of water while running, as you will have to do in competition.

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