A sauna session typically costs around 500 mL of sweat, and more over several rounds or in a hot room. You lose salts with it, mostly sodium.

The weight you lose in the sauna is this water, not fat: it comes back as soon as you drink.

Before and after

Drink a glass of water before you go in, a few sips between rounds, and another one or two glasses afterwards.

After a long session with several rounds, a drink or snack with a little salt helps you rehydrate.

Signs to watch

A dry mouth, a headache, dizziness or dark urine after the sauna usually mean you need more fluid.

Alcohol makes dehydration worse and makes fainting more likely: don't drink it before or during the sauna, and keep it for later, if at all.

Sources

  1. Podstawski R. et al. Sauna-induced body mass loss in young sedentary women and men. The Scientific World Journal, 2014.
  2. Kukkonen-Harjula K., Kauppinen K. Health effects and risks of sauna bathing. International Journal of Circumpolar Health, 2006.

General information, not medical advice.