Have you ever wondered how much water you sweat out during a bath? Well, I often did. A few years ago, I had a very regular schedule in the morning because my wife and son had to leave early to make it to school in time.  I got up early to make the green juice for the three of us, but after they left at 7:30, I had plenty of time before going to work.

For a couple months during the winter, I had a bath twice a week.  They were therapeutic baths: I was correcting a long-standing whole body magnesium deficiency from decades of intense physical exercise without ever supplementing with Mg, or anything else for that matter.  I was curious to know, and so I tried to remember to weigh myself before and after each bath, and write that down.  I did it for a while, but I forgot everything about it.

A couple of days ago I did a complete cleanup of my closet, and found the little piece of paper on which I had written all the measurements.  I thought it would be fun to share that with you.

I tried to stick to the same conditions in terms of drinking and peeing, but I wouldn’t call it a tightly constrained scientific experiment. I made 15 measurements of my weight before and after soaking in the bath at about 45-47 C for approximately 45 minutes, and calculated the difference:

1. 59.6  58.9  0.7
2. 59.5  58.9  0.6
3. 59.6  58.8  0.8
4. 60.3  59.8  0.5
5. 59.6  58.8  0.8
6. 60.1  59.5  0.6
7. 59.6  58.9  0.7
8. 60.0  59.4  0.6
9. 60.4  59.6  0.8
10. 59.4  58.9  0.5
11. 59.9  59.3  0.6
12. 60.9  60.3  0.6
13. 60.6  60.1  0.5
14. 60.6  60.0  0.6
15. 60.5  59.9  0.6

This is what the distribution of the differences looks like:

Frequency distribution of difference in body weight before and after bath. (The digital scale had one digit, and assuming the precision on each measurement is half that (0.05 kg), this would make the combined uncertainty on each calculated difference the square root of twice the square of that, and hence 0.07 kg.)

Given that there are various trends in our weight from day to day that depend on a wide range of factors, only the difference between the weight before and after the baths is important for us here. But because they all have the same uncertainty, it has no effect on the mean, which turns out to be 0.63 (9.5/15); the variance, which turns out to 0.01; and the error on the mean, which turns out to be 0.03. Hence the mean difference in weight before and after is 0.63 +/- 0.03 kg.

There would have certainly been variations in the temperature of the water, which could account for the variations in the before and after differences ranging between 0.5 and 0.8 kg. We could say that the hottest baths resulted in a water loss of 800 ml, whereas the more moderate temperatures caused a loss of 500 ml. In any case, as we said, the average of the 15 measurements is 0.63 kg, and this equates to 0.63 litres or 630 ml of water.

I think it is reasonable to consider this is in terms of the fraction of water loss with respect to body weight, which for me at the time would have been equivalent to about 1% of body weight. This is probably not precisely the case, but a good guideline to follow: if you weighed 80 kg, you should consider that a 45 minute bath would cause you to lose about 800 ml of water; if you were 100 kg it would cause you to sweat out a full litre.

And so, that’s it. The answer to the question of how much water we lose during a bath—or actually more specifically, during a bath in which the water is around 45 C, and in which we soak for 45 minutes, and in which we have dissolved 1 cup of baking soda and 1 cup of magnesium chloride—is about 1% of our body weight.  Very easy to remember.

Therefore, ideally, we would be sipping cool alkaline water throughout the bath to make up for that loss and minimize the dehydrating effects of that much sweating.

Have you ever done this on your own: weighed yourself before and after a bath to see how much water you lost? If you have, I’d be very curious to know what you found. If you haven’t but are intrigued, and want to do it, please go ahead and let us know.

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