Estimating your sweat losses is a very useful exercise to estimate how much and what you should drink (both fluids and electrolytes) during training or competition.
The amount of sweat loss varies significantly from person to person and can also vary considerably for a given individual depending on several parameters such as: intensity of exercise, ambient temperature and humidity, your clothing choices, genetic predisposition, heat acclimatization,… All these variables determine how quickly and how much your body sweats.
Measuring sweat losses is not something you should do once, but you should do it in different circumstances and on different occasions. This way you can create an interesting “knowledge database” of your losses. This will make it much easier to create a personal plan that meets all your needs.
What do you need?
A scale
A dry towel
Possibly a kitchen scale to weigh your water bottles.
How to calculate your sweat losses?
The calculation is an adaptation of a calculation described in a blog on the MySportScience website by Asker Jeukendrup.
First, go to the toilet and weigh yourself (to be completely accurate, do this naked).
Do your training or competition and write down exactly what you drank.
If you drink 1 or 2 bottles in the session you can weigh the bottles before and after the training or competition and note the difference in milliliters. If you use our spreadsheet you only have to enter the weight before and after and the calculation is done automatically.
After exercising, dry yourself well and wash yourself again. Because the clothing retains some sweat, it is also advisable to do this naked.
Now subtract your post-workout weight from your pre-workout weight to find out your weight loss.
Now calculate how much you drank by subtracting the weight of the bottles after training from the weight before training.
Now you can calculate your sweat loss per hour by taking into account the time of the training or competition.
It is best not to pee during these sessions but nature can't always stop you. If you do have to pee, take 300ml of fluid into account per pee stop. You can then take this into account when calculating your weight loss (as explained in action no. 4 above).
It is also advisable to do this monitoring for sessions that last at least 45' and a maximum of 3 hours. For sessions shorter than 45' the margin of error is a bit too large and for extremely long sessions the weight loss is not only due to fluid loss but also to burning energy.
Use our spreadsheet
If this is all a bit too complicated, we have created an Excel file for you, where you only have to fill in the data and all the calculations are done for you.
If you would like to receive that excel file, please let us know here and we will send it to you by email.
You can fill in different sessions in the document and do this for different circumstances. Over time you will recognize a pattern that you can then apply in the preparation for your competition(s).
If you have also done a sweat test with us, you can add your sodium concentration and it will also calculate your total sodium losses and sodium losses per hour. So you also have an idea of your sodium losses and can determine how much sodium you need to take in.
What do these results mean in practice? Is your sweat loss low, average or high?
It is difficult to determine what an average sweat loss should be, but there have already been quite a few studies on this subject.
A recent article looked at the sweat losses of approximately 500 athletes from various sports with the following results:
Sweat losses ranged from 0.5 l/h to as much as 2.5 l/h (with some peaks of up to 4-6 l/h). This is broadly consistent with the data from the Precision Fuel and Hydration tests.
Another study , conducted a few days before the 2003 Hawaii Ironman, came up with very similar results.
Based on this data and experience, we can assume as a rule of thumb that anything around 1-1.5 l/hr is a 'normal' sweat loss during prolonged exercise of a reasonable intensity.
Anything much less than 1 l/hr would be on the low side
Anything above 2 l/hr should be considered high.
Keep in mind that your body weight and size play a role in all of this to some extent. A very small female distance runner who loses 1.5 l/hr can easily be considered a big sweater, while a basketball player who is 210 cm and weighs 130 kg can be considered a low sweater for the same amount.
What can you do with the data?
Once you have collected a reasonable amount of data, the obvious question is what can you do with the numbers?
If you lose around 500ml of sweat per hour, there is absolutely no need to drink excessively, as the total fluid and sodium losses will not be particularly high. Drinking when you are thirsty seems to be sufficient.
However, if you lose 1.5 l/hour, a proper hydration plan is still required during prolonged exercise to avoid dehydration or sodium deficiencies.
It is especially important to start your effort well hydrated because during exercise, and certainly with high sweat losses, you can no longer compensate for the losses. If there is also a high sodium concentration, pre-loading (sufficient replenishment of fluid and sodium before the effort) is extremely important. We have already discussed pre-loading extensively in this blog .
But moisture loss is only one piece of the puzzle…
As mentioned several times in the article, there are 2 parameters you need to know to make a solid plan. Not only do you need to know your sweat losses (how much you sweat) but also how much salt is in your sweat (= sodium concentration per liter of sweat).
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