The Debate on Dehydration and Endurance Performance?
It is a pretty commonly held belief among athletes, coaches, and researchers alike in the endurance space that dehydration negatively affects endurance performance. However, despite this widely held belief, a belief that does have substantial evidence to back it up by the way, many researchers and coaches still debate this belief and argue that dehydration might not negatively impact endurance performance.
This topic is one I am keenly interested in as I have had some personal experience with dehydration and it affecting my endurance performance. I could not imagine being able to perform my best without having an adequate hydration strategy during a race. I have seen the deleterious consequences for both myself and athletes I coach when a proper hydration plan is not adhered to, particularly in long-distance races like Ironman and Ironman 70.3 events. I had always believed, based on what I learned in school, what I read in the research, and what I read in sports nutrition textbooks, that dehydration had a negative impact on performance and endurance athletes should avoid huge losses in body water during competition. So, I was shocked to see that some coaches and researchers tried to argue against this idea!
This shock has led me to dive a bit more into the research supporting and refuting the notion that dehydration impacts performance, and I have come across a recent article (1) in which the authors discuss some of the potential methodological considerations of dehydration research that might impact the interpretations and conclusions drawn from it. The aim of this paper was essentially to critically analyze the methods of seminal work in the area of dehydration research and endurance performance in order to have a better understanding of why some might think dehydration negatively affects performance and why others dismiss this notion as false. Because, when you do dive into the research, some shows that dehydration impairs endurance performance while some shows the opposite.
So, does dehydration really affect endurance performance like most of us have been taught? This is an incredibly important question to answer as it influences how athletes and coaches approach hydration strategies and plans during training and in competition. After all, many endurance athletes are very particular about finding an individualized hydration strategy in order to avoid a significant loss of body water and to maintain performance. Let’s discuss this recent paper to see what the authors found.
What Is the Impact of Dehydration on Performance, In Theory?
Dehydration, particularly dehydration of greater than 2-3% of body weight, has long been thought to impair endurance performance. It is thought to negatively impact performance primarily through a reduction in blood plasma volume and a greater competing interest between the muscle and the skin for oxygen-rich blood flow. As the body warms up, one of the primary mechanisms by which athletes cool themselves is through sweating. Blood flow to the skin is increased and sweating dissipates heat as the water moves from the blood to the skin and finally evaporates from the skin.
As more and more body water is lost, this reduces the overall plasma volume of our blood, which has a negative impact on cardiac output during exercise, essentially making the heart have to beat faster to keep up with the oxygen demands of exercise. Additionally, as blood is shunted to the skin to help with sweating, less blood gets diverted to the muscles, thereby placing an overall greater demand on the heart to keep up with the increasing demand for blood to the muscles and to the skin. These two factors, among some others not mentioned here, greatly impair endurance performance output as heart rate and perceived exertion rise in this situation. It is for these reasons above that endurance performance is usually optimized in very cool conditions and when an athlete is well hydrated and has minimal fluid loss. So then, why is it debated that dehydration does NOT have an impact on performance?
What Does the Research REALLY Suggest?
Like I mentioned earlier, there is conflicting research out there that shows that dehydration does not impair performance in select studies. However, as the authors of a recent 2019 paper point out (1), this research may have some flaws that has caused some coaches or researchers to draw incorrect conclusions. I’ll refer you to read this paper in its entirety to get the exact specifics of these potential methodological flaws, but essentially participant blinding to hydration status, the route of rehydration of study participants, and the methods of dehydrating participants may have confounded the outcomes in these studies showing no impairment of dehydration on endurance performance.
Other studies that do not have these potential methodological flaws do in fact consistently demonstrate that dehydration, particularly dehydration of greater than 2-3%, impairs endurance performance. This is important, as it eludes to the fact that hydration status does seem to matter when it comes to maintaining optimal endurance performance. The authors point out some areas in which more research is needed. However, the research does seem to be pretty clear that dehydration of greater than 2-3% body weight impairs performance in most athletes and that hydration strategies/plans should be in place to avoid body mass losses of this magnitude during prolonged endurance activity if optimal performance is the desired outcome, such as in a race or competition.
So, it might be that the hot debate amongst professionals as it relates to dehydration and performance shouldn’t be quite as hot as it is given what the authors of a recent 2019 paper discussed (1). Dehydration does seem to impair endurance performance and hydration strategies aiming to minimize body weight loss to less than 2-3% should likely be a goal for all endurance athletes during competition, particularly prolonged endurance competition (i.e., >90 minutes) where hydration status starts to play a larger role in maintenance of performance.
It would also not be correct to completely dismiss those arguing that dehydration plays little role in impairing endurance performance as more and more research is indeed needed. It is important to consider their argument and use this to investigate the role of hydration status on endurance performance even further so that we have clearer cut findings. However, if you come across a book, article, or professional in the field that is adamant in arguing that hydration does not matter very much and that we should all ‘just drink to thirst” in prolonged endurance competitions, I would take this with a very large grain of salt as the research that backs up this claim is very shaky and fragile to say the least.
What is the main take-home from this brief write-up? Keep focusing on dialing in a hydration strategy that works for you, with the ultimate goal being to minimize fluid losses in competition if optimal performance is your aim!
1. James LJ, Funnell MP, James RM, Mears SA. Does hypohydration really impair endurance performance? Methodological considerations for interpreting hydration research. Sports Medicine. 2019 Dec;49(2):103-14.
Happy training and racing!
-Ryan Eckert, MS, CSCS
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