Put two athletes through the exact same session, in the exact same conditions, and you'll get two meaningfully different sweat profiles. That's not a fluke — it's physiology, and it's the direct argument against one-size-fits-all electrolyte dosing.
The range is bigger than people think
Sweat sodium concentration varies enormously between individuals — roughly 20 to 80 mmol/L across the normal population, with some "salty sweaters" exceeding even that range. That's not a small spread. It means a fixed electrolyte dose that's right for one person could be wildly wrong for another, in either direction.
What actually drives the variation
Training status is one of the biggest factors, and it's counterintuitive: trained individuals often reabsorb sodium more efficiently via sweat gland duct adaptation, which paradoxically produces more dilute sweat despite a higher overall sweat rate. Heat acclimatisation state and genetic factors both play a role too.
You already have the data
You don't need a lab to get a useful starting signal. The salt residue left on your shirt after a session, and the change in your body mass from before to after training, are both simple, accessible proxies for how much sodium and fluid you're actually losing. More formal patch-based sweat sodium testing exists too, but weigh-in, weigh-out, and a look at your kit will tell you more than any generic serving size on a label.
The takeaway
Most electrolyte brands sell one dose to everyone and let the marketing imply it's personalised. It isn't. Understanding your own sweat profile — even roughly — is a better starting point than trusting a fixed number on any label, including one we'd eventually put on our own.