Hi @toyman ,
Thanks for including us in the conversation here, it’s awesome to see you’re taking such an interest in your specific sweat losses and using some solid ingenuity to do some at-home testing.
Firstly, I’ll try and answer your two questions then I’ll add some thoughts on what you’re doing.
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Negative ions in sweat include chloride, bicarbonate, sulfate and phosphate - though the latter three appear in trace amounts so more often than not go undiscussed. There’s a chloride ion for every sodium ion - sodium chloride (NaCl) is a typical ionic compound and each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions. The equal number of sodium and chloride ions balances the charge - there are other electrolytes (charged particles) and trace minerals in sweat but sophisticated, laboratory-based analytical techniques can exclude these (e.g. chromatography (IC), mass spectrometry, ion-selective electrode (ISE) and flame photometry (FP)). With the absence of eliminating the ‘minor’ electrolytes it may be best for you to simply accept that they’re there but largely insignificant (if you haven’t read it yet we’ve got a great blog talking about minor electrolytes and whether you need to consider them in the big picture, check it out here).
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Unfortunately there’s no good answer here, when you’re measuring potassium content too you’ll always end up with a greater number than if you were measuring just sodium (at least that’s the assumption based on there being a greater amount of dissolved solids total). From what we’ve seen you’d be better off with something like a Horiba kit but they’re pretty expensive and from some testing we’ve done we’ve had variable results and certainly wouldn’t be 100% confident.
Now, some thoughts and comments on the whole process. I’m honestly stoked to see such an effort being made to understand individual needs in sweat losses - it’s literally what got us into this niche area years ago and we’ve done plenty of our own ‘backyard science’ ever since.
A few further things to consider as you continue testing your protocol:
Cleaning of the skin with deionised water prior to collection to avoid additional contamination.
Sweat should ideally be collected at the onset of sweating, not the onset of exercise, as sweat collected at the onset of exercise includes some skin surface contamination from residual sweat in ductal lumen (why we always wipe away the first bit of sweat when we test!). The onset of sweating can take between 20-90 minutes depending on an individual and their sweat rate; you want them to have reached steady state sweating so to confirm any contaminants have been flushed out.
Lastly, @stuart_steele1 is right, ‘patch testing’, especially that covered by clingfilm will create a microenvironment which through increasing local skin temperature and humidity will alter regional sweat rate compared with uncovered skin and hence may impact sweat sodium concentration
The biggest factor we’ve seen when looking at at-home testing of this sort is huge variation in results. It’s incredibly difficult to control all of the necessary variables, eliminate any contamination, and ultimately validate your results. And once you start getting varying results you then have no idea which ones to trust, or if you can truly trust any of them - that’s ultimately the big question in all sport science!
Now, I know you mentioned the price of an Advanced Sweat Test being prohibitive but should you want confirmation of your Sweat Sodium Concentration do please drop us a line on hello@precisionhydration.com and we can lend a helping hand getting you an appointment at your nearest Test Centre, once lockdown/shelter in place measures are lifted obviously!
Feel free to keep us in the loop with your continued testing and reach out if there’s anything else we can help with.