Drinking water and hydration in the sauna

How much fluid do you lose?

Around half a kilo in a typical session, which is roughly a pint of sweat. You lose more if the sauna is hotter or more humid, if you stay longer or do several rounds, and if you’re larger. A Polish study of 674 students who spent two 10-minute spells in a dry sauna found that heavier people lost the most weight through sweat, and warned they should take particular care to replace fluids.

Weighing yourself before and after is a simple way to see how much you lose. Every kilo lost is about a litre of fluid to replace.

How much should you drink?

• Before: a glass of water (about 250 ml) in the hour before.
• During: sips from a water bottle if you’re doing several rounds.
• After: one or two glasses, and more after exercise or a long session. Keep going until your pee is pale yellow.

Water, electrolytes or something else?

Plain water is fine for most people. Sweat contains sodium, and long, heavy sweating can deplete it, so after several rounds, a hard workout or a hot day an electrolyte drink or a salty snack can help. Avoid drinking very large amounts of plain water quickly after heavy sweating.

What about alcohol?

Don’t drink before or during a sauna. Alcohol makes you lose more fluid, lowers blood pressure further and dulls the warning signs of overheating. In Finland, about half of the people who died in a sauna were under the influence of alcohol. See alcohol and saunas.

Who needs to be extra careful?

• People taking diuretics (water tablets) or other medicines that affect fluid balance.
• People with kidney disease or heart failure who have been told to limit fluids: ask your doctor how to balance the two.
• Older adults, who may feel less thirsty.
• Anyone who has been ill with vomiting or diarrhoea.