Many Runners Drink Too Much Fluid During Exercise

Reading time: 6 – 9 minutes

Popular hydration options among runners and endurance athletes include water and carbohydrate-electrolyte solutions, also known as sports drinks. Sports drink manufacturers, in an effort to sell more product, have convinced a large number of athletes — including accomplished runners — that the key to avoiding medical problems during exercise and racing is to drink as much as possible.

According to new research published in the British Journal of Sports Medicine [1], many runners have erroneous beliefs about their hydration needs, and thus overhydrate by drinking according to a schedule, or drinking “as much as possible.” This increases the risk of exercise-associated hyponatremia, an electrolyte disturbance in which the sodium concentration in the serum is lower than normal. Hyponatremia can cause nausea and vomiting, headache, confusion, fatigue, muscle weakness, cramps, seizures, decreased consciousness and/or coma. With events from community 10K races all the way up to marathons and ultramarathons increasing in popularity among non-elite athletes, understanding public belief about hydration and subsequent hydration behavior is an important public health topic.

Runner drinking water

Exercise-associated hyponatremia: decreased concentration of sodium ions (Na+) in the blood plasma and interstitial fluid, which leads to a variety of complications including cerebral edema, seizures, respiratory difficulty, and death.

In the fall of 2009, researchers at Loyola University Medical Center in Maywood, Illinois, USA, had 197 runners from three local races fill out an online survey. Previous research on endurance athletes has indicated that the individual need for replacement fluid during exercise varies tremendously [2]. However, the message from sports drink manufacturers has permeated so deeply into the belief system of many runners that more than one-third of participants surveyed (36.5%) said they drank according to a pre-set schedule, and 8.9% drank “as much as possible” during an event. This, despite the fact that more than 80% of surveyed runners denied being influenced by sports drink advertising. More than half of runners surveyed (57.6%) reported consuming sports drinks because they believed the sodium would help prevent hyponatremia, despite the fact that sports drinks are hypotonic, and increase the risk of hyponatremia if overconsumed [3].

Hypotonic: a solution that is lower in concentration with respect to a particular substance than the reference solution. In this usage, hypotonic (to the blood, with respect to sodium) means that a sports drink contains less sodium than the blood plasma, and therefore contributes to plasma sodium dilution.

In response to their beliefs about hydration needs during endurance exercise, some runners attempt to calculate sweat rate so as to replace all lost fluid. However, research indicates that when drinking according to thirst, athletes replace only up to about 70% of fluid lost [4]. Despite this, drinking to thirst remains the safest strategy for avoiding hyponatremia [5].

Interestingly enough, it was the older, faster, more experienced runners who were most likely to drink to a set schedule, as opposed to drinking according to thirst. Slower female runners were most likely to drink as much as possible, putting this group in the greatest risk category for hyponatremia. In response to the results of the study, the authors recommend a public campaign to help educate athletes on the dangers of overhydration. This is particularly important where it comes to combating the message promoted by sports drink companies, who encourage the belief that electrolyte-containing beverages fight — as opposed to contribute to — hyponatremia. The investigators point out that research on factors contributing to hyponatremia is not new, but that many runners remain misinformed about safe drinking practices despite the well-established science.

It’s all in how you communicate

In his book Hell and High Water: Global Warming–the Solution and the Politics–and What We Should Do, Joseph Romm — a climate expert who studies and writes about the failure of scientists to effectively disseminate well-established scientific information to the public — notes that scientists are simply bad at communicating information [6]. The reason for this, he explains, is that effective communication requires three key factors:

  1. Use of simple language
  2. Repetition
  3. Use of turn of phrase and metaphor

The very nature of scientific writing and communication causes scientists to avoid each of these three. Scientists tend to communicate in jargon, which simplifies the transmission of information from one researcher to another, but prevents effective transmission of information to the public. Scientists tend to avoid repetition, as most state their case concisely and then move on to something else. Scientists often avoid turn of phrase or metaphor, in the interest of sticking with pure and unadulterated factual presentation. This severely cripples the ability of scientists, including physicians and other medical practitioners, to combat the pervasive messages disseminated through marketing, as advertising agencies understand and utilize all three principles of effective communication.

The scientists refer to the success of the “Back To Sleep” campaign, designed to raise public awareness about safe infant sleep, and suggest that a “Drink To Thirst” campaign — repeated over and over through a variety of media so as to increase message effectiveness — would help to prevent unnecessary injury and death following endurance events.


  1. Winger et al. Beliefs about hydration and physiology drive drinking behaviours in runners. Br J Sports Med. 2011 Jun;45(8):646-9. Epub 2010 Sep 28.
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  2. Hew-Butler et al. Updated fluid recommendation: position statement from the International Marathon Medical Directors Association (IMMDA). Clin J Sport Med. 2006 Jul;16(4):283-92.
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  3. Barr et al. Fluid replacement during prolonged exercise: effects of water, saline, or no fluid. Med Sci Sports Exerc. 1991 Jul;23(7):811-7.
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  4. Greenleaf, JE. Problem: thirst, drinking behavior, and involuntary dehydration. Med Sci Sports Exerc. 1992 Jun;24(6):645-56.
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  5. Dugas et al. Rates of fluid ingestion alter pacing but not thermoregulatory responses during prolonged exercise in hot and humid conditions with appropriate convective cooling. Eur J Appl Physiol. 2009 Jan;105(1):69-80. Epub 2008 Oct 14.
    View abstract
  6. Romm, J. Hell and High Water: Global Warming–the Solution and the Politics–and What We Should Do. William Morrow Pub Co. 2006.
About the Author

Kirstin Hendrickson, Ph.D., is a science journalist and faculty in the Department of Chemistry and Biochemistry at Arizona State University. She has a PhD in Chemistry, and studied mechanisms of damage to DNA during her graduate career. Kirstin also holds degrees in Zoology and Psychology. Currently, both in her teaching and in her writing, she’s interested in methods of communicating about science, and in the reciprocal relationship between science and society. She has written a textbook called Chemistry In The World, which focuses on the ways in which chemistry affects everyday life, and the ways in which humans affect each other and the environment through chemistry.