How much fluid do you actually need to replace?
The number people expect is a fixed one, like two litres. The number that is actually useful is personal, because sweat rates vary enormously between people and between sessions. A cool strength day and a hot conditioning day are not the same event.
Position stands on exercise and fluid replacement suggest that when you need to rehydrate properly, drinking around 125 to 150 percent of the mass lost works better than matching it one for one, because urine production continues while you rehydrate. Spread it over a few hours. Sinking a litre in five minutes mostly produces a trip to the bathroom.
The five-minute sweat test
- 1Weigh yourself in minimal clothing just before a hard session.
- 2Train as normal. Note roughly how much you drink during.
- 3Towel off and weigh yourself again straight after, in the same clothing state.
- 4Subtract. One kilogram down is roughly one litre of sweat lost, plus whatever you drank.
- 5Repeat on a hot day and a cool day. Now you know your range.
Do you need electrolytes, or is that just expensive water?
Both, depending on the day. Sweat carries sodium, and sodium is largely what allows the fluid you drink to be retained rather than passed. Reviews of fluid and electrolyte needs for recovery consistently note that including sodium improves rehydration compared with plain water when losses have been substantial.
- Ordinary 60-minute class in a cool gym: water is fine, and your next meal supplies the salt.
- Long, hot, or heavy sweating session: sodium helps, whether from a drink mix or from salted food alongside your water.
- Two sessions in one day: worth being deliberate, because you have limited hours to recover.
- You finish sessions with white salt marks on your skin or kit: you are a salty sweater, take the sodium seriously.
A drink mix is convenient. It is not the only route. A glass of water alongside a normal meal does much the same job for most people, most days.
How to tell if you are getting it wrong
Persistent thirst, dark urine through the day, headaches on training afternoons, and feeling unaccountably flat in sessions that used to be easy. None of these is proof on its own. Together, in a week where you trained hard and the weather was warm, they are worth acting on.
The opposite also exists. Drinking large volumes of plain water while sweating heavily, with no sodium at all, is not a safe default. If you are drinking constantly and still thirsty, salt is usually the missing piece.
