Hydration 101

hydration race nutrition Dec 11, 2020

How Much Should I Drink and How Can I Monitor it?

There are different approaches when it comes to hydration. The reality is that there are some many factors playing into one’s circumstances that it becomes a very personal situation. It is important to understand some best practices when it comes to hydration, as well as to learn how you can take a more personalised approach to fluid replenishment at home.

Fluid is both personal and often not as straightforward as a text book answer.

There are some guidelines, but new (sometimes conflicting) research is coming out all the time!

The two main schools of thought are ‘drink to thirst’ and ‘drink to plan’.

This might seem intuitive, and more comfortable for you too, but with nerves and other race conditions coming in to play, this may not be effective. Whilst in shorter races or training this may not be so much of a problem, take a long race like Ironman especially- if you are out there for 10-17 hours, then the amount your drink will be crucial.

Having an idea of how much you should be drinking may be a very good idea - particularly for longer races like Ironman, when things get ‘a little fuzzy’ by the time you get to the marathon! What is more, in challenging conditions - like Kona for example - you do need to be aware of how much you sweat in that environment and how much fluid you MAY need to aim for.

Both methods have pros and cons, but both can also lead to over or under hydration.

Think about very hot or cold environments or altitude. These environments will effect how much you are likely to sweat and also the prompts to drink too. Did you know that tattoos may also influence the amount of sweat lost with higher concentrations of sodium and lower sweat rates?! (Luetkemeier et al 2017)

So What Can you Do?

Collecting personalised data on your sweat and hydration will equip you with knowledge about where to start with practicing hydration strategies. Below is a simple way to do a self sweat loss test. This is a great place to start, however, sometimes there are factors we can not effectively monitor. In particular, if you suffer the effects of losing an excessive amount of salt in your sweat, then it may be worth considering a sweat sodium test so you can start to tailor your nutrition and hydration strategies even further.


At Home Sweat Testing

We know that sweat rates vary between athletes, with the amount of sodium that is lost in sweat mostly determined by our genes. Fluid losses during exercise may vary hugely from 0.3 to 2.4 L/h dependent on exercise intensity, duration, fitness, heat acclimatization, altitude, heat and humidity (Sawka et al 2007, Shirreffs et al 2011, Kenefick et al 2012)

We need to remember that body weight is also going to dictate whether that is classified as a large loss or small loss. A simple way to test the approximate amount of fluid you lose is to do a number of different tests, each lasting 1hour. Use a simple spreadsheet to record an indoor bike session, an indoor run session (if you run frequently inside) an outdoor bike and run session (possibly in different weather conditions too if you can).


The process is simple:

  1. Weigh in before you train (with clothes or without), note the value on the spreadsheet

  2. Train for an hour

  3. If possible try not to pee in the hour of your session!

  4. Towel dry off

  5. Weigh in the same clothes / unclothed as before note the value on the spreadsheet

  6. Add to your original weight the weight of any fluids that you consumed and note this value down too!

  7. Calculate the difference between the weight pre-session + any liquid consumed during MINUS the figure at the end.

  8. The figure you have is the approximate amount of fluids you have lost - using 1kg= 1L approximately.

Whilst we would rarely encourage an athlete to match precisely the amount of fluid lost during training with their fluid intake whilst training (as there are other factors like glycogen depletion to consider), it is a good starting point for being aware how much is being lost and how much you may need to re hydrate with.

It has also been suggested that we try to limit fluid losses during exercise to >2% of body weight.

So what does this mean? Well, Sawka et al 2007 suggested that most athletes would be able to think about consuming between 0.4-0,8L per hour, but this is highly individual and needs to be practiced, avoiding over hydrating and hyponatremia. Hyponatremia is caused from low sodium levels.

As previously mentioned, collecting this data at home will be an incredible place to start when it comes to getting proper hydration strategies implemented. However, as a Sports Nutritionist, I recommend getting an even more personalised test done, whereby you can assess all factors playing into fluid loss- not JUST the amount of it!

Get in touch to find out about our sweat testing services and hydration planning protocols!

Email: [email protected]


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