Sweat is dilute extracellular fluid whose electrolytes are mainly sodium and chloride, with the rest negligible.Main electrolytes (mmol/L): sodium ~40 (range 20-80, high-sweat-sodium phenotypes 90+), chloride ~40, potassium ~5, calcium ~1, magnesium ~0.2. Sodium and chloride are the main story; the rest is negligible.
2 · Losses per liter of sweat
One liter of heavy sweat mainly costs about 900 mg of sodium, while potassium is easily replaced from meals and magnesium and calcium losses are trivial.One liter of heavy sweat loses: ~900 mg sodium (~2.3 g salt), ~200 mg potassium, ~5 mg magnesium, ~40 mg calcium. Potassium is easily replaced from meals; magnesium and calcium losses are trivial compared to daily intake. So the core question is always: how much sodium did you lose?
3 · Individual variation: salty vs light sweaters
Sweat sodium varies enormously between people, set by genetics and heat acclimation, and 10-14 days of heat acclimation can lower it by about 50%.Sweat sodium varies enormously: high-sweat-sodium types (>60 mmol/L) show white salt stains on clothes and salty skin; low-sweat-sodium types (<30 mmol/L) show almost none. This is determined by genetics (CFTR variants etc.) and heat acclimation. 10-14 days of heat acclimation can lower sweat sodium ~50% — the body learns to conserve salt.
4 · Two common misjudgments
Sodium loss depends on sweat sodium concentration times volume, not volume alone, and sweat magnesium is so low it essentially never needs replacing.Misjudgment 1: more sweat = more sodium lost. Wrong — sodium loss is concentration times volume; a light sweater losing 2 L may lose less sodium than a salty sweater losing 1 L. Misjudgment 2: magnesium is a key electrolyte. Wrong — sweat magnesium is under 5 mg/L, essentially never requiring replacement.