How to tell if you’re dehydrated.

Hang around with a bunch of athletes long enough and you’ll hear them chat about dehydration. It’s a go to scapegoat when performances go wrong and, any time the weather forecast has those sunny symbols on the map, it becomes a big locker room discussion point.

In fact, the term ‘dehydration’ is so deeply embedded in day to day sporting parlance that few athletes ever stop and consider what it actually means from a scientific standpoint and how it’s actually measured.


We need to talk about ‘dehydration’.

The interesting thing about dehydration is that it's really hard to define and measure in an accurate and meaningful way, even if you have access to some pretty sophisticated technologies.

For example, the ‘gold standard’ lab measurement for total body water involves ingesting radioactive isotopes that are diluted into body fluids and traced over time. Yes, you read that correctly, I said radioactive! This is hardly something most athletes can (or would want to) do on a regular basis. And it doesn’t even give you the full story on your hydration status, as total body water is not the only piece of the puzzle. This method tells you little about the distribution of those fluids across the various body compartments, for example.

Bio medical impedance testing isn't the most practical.

Other lab-based methods include the use of ‘bioelectrical impedance technology’ which is notoriously fickle and the measurement of the concentration of body fluids, but these require the fiddly and invasive collection and analysis of blood, urine or saliva. Not something most athletes fancy adding to their routine.

Although the loss of body water above a certain level can definitely have a (potentially very) negative impact on performance, pinning down exactly what that level is is far from a simple task.


So, how can you tell if you’re dehydrated?

Well, to get started on figuring this out, it's worth clarifying a couple of key terms:

Euhydration is the state of the body in optimal fluid balance.

Hypohydration is the state of the body when in negative fluid balance.

I’m going to dig a bit deeper in to what those terms actually mean for the athlete, but if you’re short on time and just want know how to measure your hydration status, you can skip down to the “How to measure your hydration status’ section.

Euhydration - The goldilocks hydration status. 


Let's start with euhydration.

This is the ‘proper’ term for what people would usually describe as being ‘well hydrated’. It means that body fluid levels are optimal.

This is essentially the state you want to be in when you start exercising, to make sure you can perform at your best.

And it's the state you need to return to post-exercise in order to be fully recovered (from a hydration point of view) and ready to go at it again. As a side note hyperhydration — where body fluid levels are elevated slightly above euhydration — is also of interest to athletes, but I'll deal with that in a future post...

Variations in body size from athlete to athlete mean that there's no fixed volume of fluid that can be used as a yardstick to indicate euhydration. Even percentage of total body weight measurements are somewhat useless, as different body tissues contain varying levels of water.

Notably, fat cells contain a lot less water (~10%) than muscle cells (~73%) so an obese person who weighs 100kg will carry a lot less total water than a bodybuilder of the same weight when they are both euhydrated. In other words, someone with high levels of body fat could be as little as 50% water overall and someone very muscular could be closer to 70%.

Aid stations

So your 'euhydrated state' is very personalised and is dictated by both your body size and composition.

To further complicate matters, your body fluid levels are never static either. Water is constantly being lost in sweat, urine, breathing, tears, saliva and so on and being gained through what you eat, drink and as a by-product of chemical reactions in the body (You effectively manufacture some water in your body when two hydrogen and one oxygen atoms bind as an offshoot of some metabolic processes).

Water in motion

Those fluids are also in constant motion within the body itself, moving from the stomach to the gut and into the blood stream, into the kidneys and the bladder, and more generally between intra- and extra-cellular compartments. For this reason, any measurement of body water that is taken, no matter how accurate, is really just a snapshot of your hydration level at a single moment in time and cannot account for where all of the water inside you is, just that it is ‘there'.

The phrase "like nailing jelly to a wall" seems apt right about now.

Despite all of these moving parts, the good news is that your body (presuming it is in a good state of health) is really incredibly adept at working out and maintaining its euhydrated state.

In fact, given free access to water to drink and without suffering uncharacteristically large losses through sickness, diarrhoea, excessive sweating, or blood loss, the body tends to regulate water levels to about +/- 0.5% on a day to day basis. It mostly does this by making you thirsty when you need to top up your fluid levels, and by making you want to pee out any excess that might otherwise build up in the face of excess consumption. That's good old evolution for you.


Then, there's hypohydration...

Hypohydraton is a state that occurs when your body fluid levels drop significantly below the small normal day to day fluctuations that characterise euhydration.

In practice this is when you lose around 1% or more of your total body fluid.

Most people use the term 'dehydration' to describe this situation, but if we’re being picky, dehydration actually refers to the process of losing body water, hypohydration is the state in which we find ourselves when those losses become significant.

Hypohydration is of interest to athletes because we often subject our bodies to high levels of sweat loss; and therefore many of us move between states of euhydration and significant hypohydration on a very regular basis.


Whilst small losses of body water are corrected quickly and easily with normal drinking and eating habits, fluid losses of more than 4% bodyweight (as can easily occur in hard training sessions or races) can take over 24 hours to fully replace. And this has obvious implications for anyone training very hard on back to back days, competing in multi-stage events, or trying to recover quickly from big races. It can also require some level of proactive effort to ensure replenishment happens and a fluid/electrolyte deficit does not start to build up over time and hamper performance and recovery.

Ok, so now we understand what euhydration and hypohydration are, the more important thing is to understand how to figure out what state we’re in at any given point in time...


How to measure your hydration status.

Accurate measurement of body fluid levels to identify euhydration or hypohydration (without the use of radioactive isotopes or taking blood samples) is essentially impossible. Instead, there are various methods employed in the ‘real world’ that can give a good enough estimate of hydration status...


1) Body weight change.


Short-term changes in body weight are commonly used to evaluate fluctuations in hydration level from day to day.

This method presumes that acute weight loss (especially that which occurs around bouts of exercise and sweating) correlates closely with acute fluid loss. This assumption has proved to be pretty accurate much of the time. Where it can sometimes fall down is in the face of very large losses and intakes of fluid in short spaces of time. For example, you might consume 2 litres of fluid, during or after a bout of heavy sweating, to try and replace what you lost. But all of this may not actually be absorbed by the body even if it sits in your stomach and temporarily shows up on the scales before it is peed out without actually impacting your hydration status in a meaningful way.

This method also relies on you having reliable data on what your normal, euhydrated, body weight is. So it does require some regular weighing and recording of that to keep tabs on the number. Overall this means of assessing body weight change can be a helpful indicator of hydration status, even it's not completely foolproof.

paint (urine) colour swatches 


2) Urine production and concentration/colour.


Monitoring the amount you pee and its colour is often recommended to athletes looking to monitor their hydration status. It assumes urine production will increase and will be more dilute at times when the body is closer to euhydration and that production will drop (and concentration will increase/colour darken) as hypohydration kicks in.

Like bodyweight change, when all other factors are equal, urine colour can indeed be a reasonably good indicator of hydration status, but it too can be confounded in certain circumstances.

Things that decrease the reliability of urine colour as a marker include: consumption of diuretics like caffeine or alcohol that artificially ramp up urine production; excess urine production that can occur with sudden, large increases in fluid consumption (where intake vastly exceeds absorption rates); and the lag time that can occur in the body with release of hormones that control how much you urinate, at times when large fluid losses happen in short spaces of time.

One important thing to mention about using urine colour and volume on its own to monitor hydration status is that it often fails to address, or highlight, the downside of drinking too much.

As urine production just ramps up and the pee becomes more dilute the more you drink, some athletes seem to think that passing more and more clear urine is a positive sign. In fact, they could be diluting their body’s electrolyte stores and encouraging the dangerous condition of hyponatremia if they take things too far.

For this reason, we often find ourselves talking to athletes about the fact that urinating very clear liquid all the time and in large quantities could be a sign of excessive fluid consumption, and that dialling intake back a bit in those circumstances can be an excellent idea.


3) Thirst.


The role of thirst in the assessment and management of hydration status has long been a subject of debate in the world of sports science, with wildly varying opinions and no firm consensus in sight.

The 'conventional' viewpoint, based on many lab studies from the 1980s and 90s, was that thirst is a very unreliable indicator of hydration status as it typically doesn’t kick in until most people are around 2% hypohydrated. Some argue that by the time this happens, performance is already compromised.

More recently this idea has been vigorously challenged with evidence collected 'in the field'. And quite rightly too. More recent data shows that when athletes start exercising when euhydrated (in a balanced state) and can choose when and how much to drink, many develop levels of hypohydration (negative balance) that go beyond 2%, without a noticeable decline in performance.

However, although drinking to thirst is a much better way to manage hydration status and performance than once believed, it is not sensitive enough to be relied upon fully, or in all circumstances.

This is especially apparent when fluid losses are extremely high over extended periods. Or in novice athletes, who are generally less attuned to listen to their bodies during exercise and to responding appropriately to their instincts during stressful situations.


WUT you looking at?

So, the good news is that there are three reasonably good and practical markers available to you to help you monitor your hydration status.

But, because none of these indicators are entirely accurate on their own, some clever people in the US Army came up with the idea of combining all three measures to produce a more reliable rating scale called the WUT system (Weight, Urine, Thirst). This establishes the likelihood of you being euhydrated (‘well hydrated’) or hypohydrated (‘dehydrated’).

Essentially their suggestion is to monitor your body weight, the colour of your urine and how thirsty you are first thing each morning. The ‘first thing in the morning’ element is important as it limits the influence of other factors that interfere with hydration status as the day progresses.

You then feed the results into a simple Venn diagram to give you an indication of whether hypohydration is unlikely, likely or very likely as you begin that day.

The data you need to collect each morning is:

  1. Your body weight. Ideally as soon as you get out of bed, before eating, drinking or going to the bathroom. A loss of 2% or more of your body weight is deemed significant.
  2. A rating of the colour of your urine (is it light or dark in colour)
  3. A rating of your sensation of thirst (thirsty or not thirsty)

If 1 or less of the 3 scores you collect are ‘positive’ (i.e. body weight is within 2% of normal and/or urine is light and/or and you're not thirsty), then hypohydration (‘dehydration’) would be deemed unlikely.

If 2 out of the 3 are positive, then hypohydration would be considered ‘likely’ and this might impact your fluid intake and training plans for the day, especially if you were planning very hard or prolonged exercise in the heat.

If 3 out of the 3 are positive then hypohydration is very likely and therefore strong consideration should be given to correcting that before you undergo strenuous exercise or expose yourself to further large sweat losses.


WUT system. Water, Urine, Thirst.

The beauty of this ‘WUT’ system is how easy it is to implement. It pools quite basic information to give you a much firmer indication of hydration status than any single measure can on its own. Using this approach regularly and over time, it’s possible to tune into your body’s hydration status and become more adept at predicting the outcome of the WUT score before you undertake the test. In the long run, this helps you to become more accurate in your perception of your own hydration status and this makes it more likely that you'll be able to maintain it within parameters that’ll allow you to perform at your best when training and competing.

For a deeper dive into the original literature and scientific background, this paper is a great resource:


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2 comments from fellow athletes

  • Gabriella: June 12, 2017

    This post was a little disappointing as I hoped for a precise hydration measure. It tells me when I need to rehydrate but is it impossible to measure your hydration levels accurately?

    I’m interested in learning how my hydration levels fluctuate depending on the amount of intake. I feel that my body has difficulty absorbing fluids.

    How else could I know how effective my body is using its fluids and how to improve retention?

  • Robin Faulkner: February 19, 2017

    firstly why are there no other comments to such a straight forward and informative post.

    Thank you very much.

    We have started a conversation in our club as recently an “expert” in table tennis has been talking about how to eat and drink during a tournament and we think they are giving bad advice and so we are looking for back up to our opinion and along with that sound and appropriate advice we can pass on to,in particular, our young players who regularly compete in tournaments that can typically last all day with periods of intense competition followed by recovery time before another intense period and of course hopefully finishing with the most intense period of competition the final and so having taken on the correct fuel and hydration can become increasingly important to reduce brain and physical fade.