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The Real, And Simple, Truth About Dehydration And Performance

We examine what might be worse than dehydration — over-hydration.

We examine what might be worse than dehydration — over-hydration.

As the temperatures this summer begin to creep up, runners are bombarded with the message to drink early and drink often when training and racing. However, is it possible that sports drinks companies have overhyped the message about the performance impact of dehydration? Moreover, is it possible that over-hydrating is actually a greater threat to health and performance than dehydration?

According to renowned exercise scientist Dr. Timothy Noakes, this is exactly the case. After critically reexamining the existing research and conducting his own studies, Dr. Noakes has found that runners are more likely to be over-hydrated than dehydrated. More importantly, compared to dehydration, over-hydration is a more serious condition and may have life-threatening complications.

In this article, we’ll examine the signs, symptoms and causes of over-hydration, why dehydration isn’t as dangerous as we fear, and provide research-backed and practical hydration protocols for training and racing.

RELATED: Running 101: Hydration During Running

What Is Over-Hydration?

Over-hydration, scientifically referred to as hyponatremia, is the result of having low blood sodium levels. Despite the marketed hype of dehydration, hyponatremia is actually one of the most common medical complications in long-distance training and racing.

Novice and slower runners who may take four hours or more to finish a marathon and who are drinking mainly water are at extreme risk of hyponatremia.

Slower runners often have an easier time drinking while running since they’re running at a slower pace. In addition, they are more likely to walk through water stops since they are more crowded and the time loss is less significant. As such, they tend to fill up on water quickly and are thus more likely to be at risk for hyponatremia.

The exact mechanism behind hyponatremia is not clear, but it is known to be associated with the over-consumption of hypotonic fluids (like water). It is important to note that hyponatremia can develop from drinking too much fluid before, during, and even after the race. Furthermore, hyponatremia can also develop from over-drinking sports drinks, despite the fact that most contain sodium.

Signs And Symptoms

Although the signs and symptoms of hyponatremia have been described as similar to those of dehydration. Noakes points out that the only true symptom of dehydration is thirst. Symptoms associated with hyponatremia include:

— Disorientation
— Confusion
— Headache
— Muscle weakness
— Nausea and vomiting

If left untreated or misdiagnosed as dehydration, hyponatremia can progress to seizure, brain swelling, pulmonary edema (fluid buildup in the air sacs of the lungs), comatose, cardiorespiratory arrest, or death.

Is Dehydration Really Detrimental To Performance?

The current recommendation for hydration is four to eight ounces of fluid every 15 to 20 minutes of running. However, these recommendations were based on early studies conducted in the 1960’s, which suggested that dehydration determines the body temperature response to exercise and thus, the development of heat-related illnesses.

RELATED: Sports Drinks Vs. Water

However, more recent studies have now shown that dehydration, or rather a reduction in body weight due to exercise, is a normal part of exercise.

A recent meta-analysis of laboratory-based studies examining the impact of dehydration on performance resulted in the following conclusions:

— It was found that a reduction of body weight of 2.2 percent was not associated with a decrease in performance.
— It isn’t dehydration itself that is responsible for any decrease in performance, but rather not drinking in response to thirst.
— Drinking enough to satisfy thirst resulted in a 90 percent performance advantage compared to drinking below thirst and a 63 percent performance advantage over drinking above the thirst response.

The only symptom of dehydration is thirst and often, this thirst becomes so overwhelming that the athlete is compelled to drink when fluid is available.

Furthermore, studies have disproved the claim that a reduction in body weight of less than 2 percent results in impaired performance. For example, a recent study confirmed that Haile Gebrselassie lost 10 percent of his body mass due to dehydration during his world record marathon run in Berlin.

The results of this latest research show, for the first time, that drinking according to thirst is the superior hydration protocol to maximize performance.


The maximum rate at which the intestines can absorb fluid is, on average, about 600 mL (or 20.3 fl oz.) per hour. The kidneys can only excrete fluid at a rate of about 800 mL/hr in males and closer to 600 mL/hr for smaller females. If fluid is ingested above these rates, it will be retained and may cause a number of problems associated with hyponatremia.

Therefore, based on the latest research and findings, hydration for training and racing isn’t as complicated as we’ve tried to make it. Simply drink according to your level of thirst and you’ll be maximizing your performance while also keeping yourself safe.


1. Noakes, T. Waterlogged: The Serious Problem of Overhydration in Endurance Sport. Champaign, IL: Human Kinetics; 2012.

2. Rosner, MH, Kirven J. Exercise-associated hyponatremia. Clinical Journal of the American Society of Nephrology, 2007;2(151-161).

3. Goulet, ED. Effect of exercise-induced dehydration on endurance performance: evaluating the impact of exercise protocols on outcomes using a meta-analytic procedure. British Journal of Sports Medicine, 2012.

4. Wyndham CH, Strydom NB. The danger of an inadequate water intake during marathon running. South African Journal of Medicine, 1969; 43(893-896)