Check electrolyte intake needs.
Electrolyte loss is sweat volume multiplied by sweat concentration, and concentration varies enormously between people — from about 200 mg of sodium per litre in a light, well-acclimatised sweater to over 1800 mg/L in a salty sweater whose kit dries with visible white marks. Potassium loss is roughly a fifth of sodium loss. The replacement volume assumes a typical sports drink at 500 mg sodium per litre.
Electrolyte loss from sweat
Sodium mg = sweat litres x sodium mg per litre; replacement ml = sodium mg / 500 mg per litre x 1000
These figures are population-average estimates for general planning only and are not medical or dietetic advice. Individual energy and nutrient needs vary widely with body composition, activity, medication and health status. Consult a qualified health professional or registered dietitian before making significant changes to what you eat, and especially if you are pregnant or breastfeeding, are under 18, or have a medical condition or a history of disordered eating.
Without a lab patch test, use the salt marks as a guide: no visible residue suggests 300-600 mg/L, faint white marks on dark kit suggest 700-1000 mg/L, and gritty white deposits or stinging eyes suggest 1200 mg/L or more.
Rarely. For sessions under about 90 minutes normal meals restore the losses. Deliberate in-session replacement matters for long endurance work, hot conditions, and anyone who cramps or finishes with a headache.
For most training, yes. Drinking large volumes of plain water after very heavy sodium losses is the situation that risks dilutional hyponatraemia, which is why the sodium-per-hour figure matters more than the total for long events.