@PrecisionHydration any feedback you have on this approach would be appreciated. Thanks!
I’ve been interested in getting a sweat composition test, but the cost has put me off, so I’ve looked at how I could do something reasonably accurately myself.
What I have come up with is the following plan, which I’ve tested a few aspects of allready.
For measuring the sweat, I will be using a HM Digital AP-1 total dissolved solids tester (https://smile.amazon.com/HM-Digital-Dissolved-Resolution-Accuracy/dp/B007KDYOMU?sa-no-redirect=1) This the main cost of the experiment, at $24. This measures up to 5000 ppm NaCl. Not that it doesn’t tell you what is in the water, just how much - it does this by measuring how conductive the water is. This unit is calibrated to an NaCl standard, so 1 gram of NaCl in 1 Kg of water will read 1000 ppm. You need a fair bit of liquid (a few ml at least) to measure, so while this can measure sweat directly you have to collect a lot of it.
From the perspiration wikipedia page, the linked paper (https://www.semanticscholar.org/paper/Sweat-mineral-element-responses-during-7-h-of-Montain-Cheuvront/b4f7ba01acb697f281e59dc27ca3b51a707e3ca9) shows that sweat is about 80% sodium, 20% potassium, with the rest all trace amounts.
I will be collecting the sweat from my forearms, as this is easy to access, and there is data on how to convert concentration here to overall body concentration. Since I need to collect a lot of sweat, I will be using paper towels over both forearms to collect enough sweat. Initial testing showed that paper towels added some dissolved solids to the water, so for the real test I have washed/dried paper towels in reverse osmosis water to minimize this.
The Gatorade sports science group has a nice page about sweat testing ( www.gssiweb.org) and this includes an equation for calculating whole body sodium loss from measuring 1 specific site - the forearms. After converting that into units that I am measuring, I get:
Predicted Whole Body sweat [NaCl] (g/L) = 0.57 (forearm sweat NaCl) + .618
What is interesting here is that the whole body sweat concentration is a fair bit less than the concentration measured on the forearms.
My plan is to do the following:
- wrap both forearms in paper towel, and cover those with plastic wrap to prevent evaporation. (wash/dry forearms before this.)
- perform activity until paper towel saturated.
- collect sweat in small plastic bag, as this is a good shape for allowing the meter to operate with a small quantity of liquid.
I’ve done a ‘dry run’ of this skipping some of the steps required for accuracy, and have a number that looks reasonable. My sweat measured 4200 ppm NaCl, so using the formula below and adjusting just for Sodium this gives me: (4200 * .57 + .618)0.3930.8 = 752 mg/liter of Sodium in my sweat. (0.393 is Sodium proportion of Na in NaCl, 0.8 is sweat being 80% sodium.) This is a lower number than I was expecting, as I have always seemed to be a salty sweater.
I plan on soon doing a more controlled test with rinsed paper towels and arms.
Some open issues that I am still working on figuring out that I’d appreciate pointers on:
- What are the negative ions in sweat? Ie is there typically a chloride ion for every sodium ion? What balances the charge in sweat? My initial searches didn’t show much on this.
- How does the above answer and the 20% potassium affect how the TDS measurement works? This is really a matter of calibration, as for instance the Precision Hydration test is described as using electrical current to measure concentration as well.
Even with the above areas of uncertainty, my guess is that this is still going to give me a number with +/-20% accuracy. I think even a number with this type of accuracy can be helpful, as at least my initial numbers are telling me I am less than 1gm/Na per liter.