This article was adapted from a combination of speeches given at the European Sports Science Conference 2018, most notably Mark Funnell (Loughborough University, UK).
Decades of research have been dedicated to determining the amount of dehydration necessary to disrupt athletic performance. Consideration of hydration levels is important to a variety of athletes who may lose water sweating during exercise or intentionally to achieve a weight class in sports like rowing and boxing. As this dehydration may impair an athlete’s performance, it is important to know at what point this detriment begins so hydration protocols can be designed accordingly. Concerningly, our current stance on what this threshold might be has been intensely criticized as inflated by a placebo effect.
How much dehydration is supposed to be too much dehydration?
In the literature, dehydration is most commonly quantified by the associated amount of weight loss that occurs with altered body water content. Current guidelines suggest dehydration affects anaerobic (short, intense exercise) performance when the weight of water lost is equivalent to approximately 4% of body mass. Comparatively, aerobic (long duration, steady state exercise) performance is impaired when only 2% of body mass is lost via water. While these reference ranges have been determined by huge bodies of data, near all studies which were used to generate them have an important flaw: participants have knowledge of their hydration.
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Addressing a fatal mistake in research design.
It is problematic that participants in most dehydration studies have knowledge of their dehydrated status. Because this is the case, it is impossible to ensure that studies demonstrating declines in performance are detecting true declines in performance due to inadequate body water. Participants may instead demonstrate reduce performance simply because they believe dehydration should be compromising their abilities; that is, they are experiencing a placebo effect. This argument against the validity of previous experimental designs has called into question the commonly held guidelines for anaerobic and aerobic performance. Some have even gone as far as to suggest dehydration at low levels does not compromise performance at all and that this placebo effect can be trained out of elite athletes.
Is there some effect of dehydration on performance?
Researchers have begun to address this issue using feeding tubes that run through the nose and into the gut to supply water to participants during exercise. With this kind of design, participants are completely unaware of whether they are receiving water or experiencing dehydration. Such studies have demonstrated preserved negative effects of dehydration suggesting dehydration does reduce performance at least to some extent. However, this still does not demonstrate whether findings in previous studies used to develop hydration guidelines were inflated by a placebo effect.
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A Study to Solve This Issue
Recently, investigators from Loughborough University have compared the effects of dehydration at 3% body mass loss via feeding tube methodology to that in which participants have explicit knowledge of their hydration condition. On a subsequent cycling time trial, they found that dehydration by a feeding tube and more explicit means had an equivalent negative effect on performance time, the heart rate response, and core temperature. This study suggests that knowledge of dehydration minimally affects performance and previous findings and guidelines should stand despite previous arguments against it.
Given this study, we can have far more confidence in previously proposed hydration guidelines. Endurance athletes should take rehydration measures to ensure dehydration does not exceed 2% of their body mass while those relying more primarily on their anaerobic systems should attempt to keep dehydration levels below 4% of body mass. Current athletes can rest relieved that the current guidelines they follow remain supported by evidence despite recent controversy.
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