Fatigue is a key sign of dehydration. But does dehydration cause fatigue, or are they just related? And if dehydration does cause fatigue, why?
Here, we look at three scientific papers written in the last ten years that focus on the causes of fatigue in three different scenarios, to help you get to an answer. The three tiring scenarios researched and reviewed are physical fatigue, mental and cognitive fatigue, and all-over hangover fatigue.
1. Dehydration in Elite Soccer - Sports Medicine 2009
This 2009 paper suggests that ‘no single metabolic factor is a causal of fatigue in elite soccer’. Translated out of science speak, this means that in high-level soccer, we can’t say fatigue is due to any one single thing. However, the conclusion of the paper does suggest that dehydration is one of the physiological, subconscious factors, that makes us feel dehydrated after long periods of intense exercise.
This paper looks at a specific technique called ‘pacing’ used by elite soccer players, and focuses on whether dehydration is a sign of fatigue (feeling wiped out all at once), or of a measured, sustain effort (continually getting more tired and more winded). This key difference adds an interesting voice to the medical conversation regarding dehydration and fatigue. There are flaws in the traditional ideas of how dehydration causes physical fatigue. Newer models, such as that in this paper, tell us about how the brain continually takes information from a large number of factors including blood acidity and skin temperature, to tell how tired and thirsty you are. Rather than reaching a state of severe dehydration, and then taking action once you start to feel the associated symptoms, such as fatigue, your brain is constantly working out what conditions your body is in, and what needs to happen in order to maintain optimal performance.
2. Hydration Status, Mood and Mental Performance - Nutrition Reviews 2015
This paper reviews the findings of several other scientific papers, in an attempt to connect dehydration to mood and cognition. For example, is drowsiness a consequence of dehydration? The paper wisely points out that it is scientifically difficult to observe small changes in brain function and cognitive abilities, so proving a direct relationship between hydration status and mental-fatigue is hard, particularly when we are looking at the relatively small changes in hydration levels that occur in environments such as air-conditioned offices or schools.
However, this research specifically describes several facts related to dehydration that have been shown to affect brain function, including brain volume (which changes depending on how hydrated you are), body temperature (a cause for dehydration) and age (elderly are more susceptible to dehydration). This evidence suggests an intricate relationship between dehydration and how tired you feel, or how well your brain is performing. There’s a bit of a chicken and egg situation going on here, but this paper helps us make some important steps!
3. Dehydration and Hangovers
Who do you feel so tired and drowsy when you have a hangover? One of the initial aims of this paper was to establish the characteristics of a hangover. After searching through the scientific literature, the authors concluded that headaches, nausea and fatigue, amongst other things, are common symptoms of a hangover. However, over the course of their research they identified the breakdown products of alcohol (acetaldehyde), dysregulated cytokine (cell messenger proteins) pathways, and hormonal changes as also involved in the intensity of hangovers. We cannot say that dehydration alone is the cause of a hangover-related fatigue.
Interestingly, the authors point out that hydration can limit, but not totally overcome, the symptoms of a hangover - suggesting dehydration is related but not entirely responsible.
So what’s the answer?
It's difficult disentangle all the related symptoms of fatigue and their direct causes. After exercise, a long day at work, or the morning after the night before, it’s usual to feel tired and achy, have a headache, feel drowsy and unfocused, and maybe even feel nauseous. Dehydration is a common factor in all these situations, and all these things are symptoms related to dehydration, but in each there are other other things in play, as seen in the papers above.
So in short, does dehydration cause fatigue? Yes, and no. Yes, it certainly contributes to the physiological processes that make you tired, and no, it’s not exclusive. Even the most hydrated people in the world need to rest, so that their body can recover from the other things going on that make us fatigued. So don’t rely on good hydration or good sleep alone to keep you awake and alert and performing at your best. Build patterns of good sleep and good hydration (such as with Hydrant!) into your day to allow yourself to be the best you can be.
Writer: Ailsa McKinlay
Editor: Elizabeth Trelstad, www.hellobeaker.com
 Dehydration; Cause of Fatigue or Sign of Pacing in Elite Soccer? Edwards, A.M. & Noakes, Dehydration; Cause of Fatigue or Sign of Pacing in Elite Soccer? T.D. sports med (2009) 39:1.
 Do small differences in hydration status affect mood and mental performance? David Benton, Hayley A. Young; Do small differences in hydration status affect mood and mental performance? Nutrition Reviews, Volume 73, Issue suppl_2, 2015), Pages 83–96
 The Alcohol Hangover Jeffrey G. Wiese, MD; Michael G. Shlipak, MD, MPH; Warren S. Browner, MD, MPH , The Alcohol Hangover. Ann Intern Med. 2000 Jun 6;132(11):897-902.