When considering hydration, we are generally referring to the balance between how much fluid someone is taking in versus how much they are losing.
Fluid In
On the “fluid in” side of the equation, we can include almost all fluids consumed through the diet.
I say ‘almost’ because alcoholic drinks cause excretion of more fluids than they provide, and therefore shouldn’t be included.
However, contrary to popular belief, caffeinated drinks can usually be included, since even though they increase excretion somewhat at high intake levels, they generally provide more fluid than any extra fluid excreted.
Fluid is also provided through foods that contain high water content like certain fruits and vegetables, so they technically also contribute to hydration status (even though counting them as part of your daily fluid intake targets may not be practical).
Fluid Out
On the “fluid out” side of the equation, the first element to discuss is sweating.
When body heat increases (including as a result of exercise), the body produces sweat, which evaporates from the skin, carrying heat with it, and cooling the body as a result.
When body heat increases excessively, it leaves the body less able to carry out certain biological processes, not least those involved in athletic activities, so the cooling effect of sweat allows the body to operate at a given output for longer.
Multiple litres of sweat can be lost during exercise, especially exercise done at higher intensities, durations, ambient heat, and humidity.
We also lose fluid through excretion.
This excretion allows the body to remove waste products, and regulate fluid and electrolyte levels in the body.
A major implication here is that the amount of fluid we lose through excretion, particularly urination, is regulated by the body without much conscious effort, and is based mainly on the amount of fluid and electrolytes we’ve consumed and/or lost through other processes.
We also lose fluid through breathing water vapour, which is increased or decreased based on activity levels and ambient weather conditions.
As all of the above processes occur and fluid levels in the body decrease, the extent to which we replace them (the “fluid in” side of the equation) will determine our current level of hydration.
Why is hydration important for athletic performance?
There is an often-repeated stat that a dehydration level corresponding to as little a 2% of body weight loss is associated with decreased athletic performance.
That means that if an athlete who walks around at 80kg is sufficiently dehydrated so that his body weight is 78.4kg (2% of 80kg is 1.6kg), he is likely to experience poorer performance.
We can assume this figure isn’t exact or the same for each person/activity, and that there is a spectrum of dehydration where higher levels of dehydration lead to increasingly poorer levels of performance.
The main underlying reasons for this are outlined below.
Temperature Regulation
As previously mentioned, sweating allows heat to be dissipated from the body, cooling us in response to the increase in body heat that happens as a result of exercise.
If the body is dehydrated, and therefore has less fluid available, it will protect the existing resources by decreasing how much fluid it will allocate to sweating.
This decreased sweating will result in less of a cooling effect, causing body heat to rise to the point where physiological and psychological factors decrease our ability to continue to exercise at the same level of intensity.
Nutrient Delivery and Waste Product Removal
As we exercise, the working muscles need nutrients and oxygen to be delivered to them.
The muscles also produce fatigue-inducing waste products, which need to be removed.
Both of these processes rely on blood being pumped by the heart for transport.
The increased requirements for both of these during exercise is why heart rate increases as exercise intensity and duration increases.
Given that blood is made up mostly of water, less water in the body (dehydration) leads to a decrease in the amount of water available to be used for blood, and as a result, blood volume decreases and the blood becomes more viscous - simply put, thicker.
Similarly to if you were pushing toothpaste out of the tube, versus if you were pushing water out of it, the heart requires more effort to pump this now thicker blood around the body.
The result is that the supply of oxygen and nutrients and the removal of waste products from the muscles decreases, and the muscles can no longer sustain the same output as compared to when proper hydration allows for less thick blood to do the job.
Electrolyte Levels
When we sweat and urinate, we don’t only lose fluids - we also lose electrolytes.
These electrolytes serve key functions in the body, including sending electrical signals from the brain to the muscles, and regulating fluid levels in the body.
So if we don’t replace them, the body’s levels of electrolytes can decrease to the point where the body’s ability to carry-out those tasks - and as you can imagine, performance - is negatively affected.
Optimising Hydration for Athletes
General Hydration
Generally, the body is good at regulating hydration levels, as long as we avoid the extremes.
On one extreme, you have very low fluid intakes.
Given that the body will still need to remove certain products from the body regardless of fluid intake, it will continue to remove fluids even at low fluid intakes, leading to consistent dehydration in spite of the body’s efforts to regulate.
On the other hand, we have excessively high fluid consumption, particularly if paired with low electrolyte intake, where the body’s electrolyte levels can become so diluted as to lead to potentially fatal outcomes caused by “hyponatremia” - a considerable number of people have died when consuming extreme amounts of fluids during marathons, for example.
However, these are extremes, and because of the body’s ability to regulate these levels, the recommended range of healthy fluid intake is relatively large, somewhere between 2-4L.
However, given how much this can vary based on body weight, weather conditions, lifestyle etc, a practical recommendation often given is to simply “drink to thirst”.
My own addition to this for athletes is closer to “drink to avoid thirst”.
I add this element because if you are to get to the point of thirst by the time you are required to perform, you may already be at a point of less than optimal hydration.
Additionally, avoiding excessively low or high sodium/salt intakes will allow the body to more easily regulate how much sodium it requires.
Again, a simple recommendation on the sodium/salt side is to season food to taste, avoiding extremely low salt intake while avoiding excessively salty foods for the most part, potentially increasing water intake in times where saltier food is consumed.
Pre-Performance Hydration
Similarly to the general hydration recommendation, a simple first step to address hydration in that pre-performance or pre-training period is to not show up thirsty.
In order to do this, maintaining a solid level of general hydration is a good start.
Additionally, consuming an extra ~500ml of water with either a meal continuing salt, or some salt from an electrolyte tablet or pinch of table salt could be useful in ensuring the body has sufficient fluids, also given that sodium can help retain the extra water consumed.
This would be particularly important for morning sessions where the body will have lost fluids overnight without replacement.
This extra consumed will help top up any potential fluid deficit and/or offset some of the fluid that will be lost in the upcoming performance bout.
Intra-Performance Hydration
Previous recommendations have been to aim to consume sufficient fluids to maintain pre- and post- training body weight during exercise.
This meant that if you typically lose 1.5 kg of bodyweight during exercise without drinking, you should aim to consume the equivalent weight in fluids (1.5L) during sessions.
However, this is often extremely difficult (or sometimes impossible given the sport constraints) to do, especially for heavy sweaters.
Not only that, but in recent years, it has been posited that it’s not clear whether it is required or optimal.
For example, if we assume that significant performance detriments start to occur at 2% bodyweight loss, and someone at 80kg body weight is losing 1kg through sweat, but consumes 0.5L (or 0.5kg) of water during it, and is therefore at a total bodyweight loss of 0.5kg for that session, they are nowhere near that 2% loss, so is it fair to assume that athlete will see significant performance downturn?
The same previous logic was also applied to sodium intake, where it was thought that it was optimal to replace all that sodium lost during exercise.
However, this might only be required if we are also replacing all the fluid lost - which as discussed, may not be possible or even necessary.
Otherwise, the higher proportional increase in net sodium loss versus fluid loss would lead to increases in blood sodium concentration, rather than maintenance.
Instead, maintaining sodium levels may only require a small amount of sodium intake, if any, especially when we consider that the body also has reserves of sodium from which it can pull to regulate levels.
However, with all of this said, the longer the duration and higher the intensity (and therefore the more sweat and sodium loss) the more these factors become an issue.
For example, someone training for 4-5 hours who isn’t focussing on replacing fluids and sodium could end up in a place where low fluid and/or low sodium levels become their limiting factor.
Still, it can be a good idea to consume some amount of sodium during the intra-performance period, since it can help with fluid absorption.
The amount required will vary per individual and event, but somewhere in the range of 0.5g-2g per litre of water will be close to optimal - this could be as simple as adding a pinch of salt to your intra-training drink.
To ensure you actually end up consuming the drink, it’s important to not add so much salt that the drink ends up tasting undesirable.
In the same way, carbohydrate intake with fluids can also assist fluid absorption.
Adding enough carbohydrate to comprise 7-10% of the total drink content is usually a good recommendation.
This can also assist in the fuelling of sessions, and the amount of carbohydrate that the gut can tolerate can be increased by “training the gut” through progressively larger intakes, if needed.
The combination of what has been mentioned with electrolytes and carbohydrates here is the reasoning behind isotonic sports drinks.
Given all of this, a simple recommendation is to drink fluids where possible (during breaks in play, at water stops etc) aiming to avoid thirst, consisting of some combination of water, electrolytes, and carbohydrates.
This will need more planning and accuracy the longer the duration and intensity of the event.
The exact combination of these will be dependent on the event, duration, intensity, and individual’s sweat rate, and will be discussed in future articles.
Post-Performance Hydration
The post-performance or post-training period is less important the less dehydration that has occurred during exercise and the further away the next session is.
Still, it makes sense to start replacing the fluids and electrolytes lost as soon as is reasonable, especially given the continued recovery benefits of having a sufficient supply of nutrients and removal of waste products to and from the muscles and other areas of the body.
In this vain, it can be a good idea to assess body weight change from before to after the exercise session, and aim to consume the equivalent amount (or slightly more) in the 1-2 hours following exercise, before then resuming the normal hydration recommendation of drinking to thirst.
For example, this would mean that if someone weighed 80kg before training, and after training weight 79.5kg, they would drink an extra 0.5L of water soon after training.
This would, at least in theory, allow any dehydration caused during exercise to be offset right away, offsetting any downsides of ongoing dehydration.
Article Written By Conor O’Neill
You can find out more about The Everyday Athlete Program by going to everydayathleteprogram.com/coaching.