Do you lose electrolytes when you swim?
You've just done forty minutes in the pool. You're not hot, you're not sweaty, and you're not thirsty. Surely you haven't lost anything?
You probably have. Water hides fluid loss better than any other sport.
You sweat in the water
Your body still heats up when you swim, and it still sweats to cool down. The water around you whisks the sweat away before you can notice. Studies of swimmers have found they can lose a meaningful amount of fluid in a session.
Being in water makes you lose more
Immersion shifts blood towards your core, and your body responds by producing more urine. This is called immersion diuresis. Cold water strengthens the effect, so a winter swim can leave you needing the loo and short on fluid.
Cold water masks thirst even more
Cool water on your skin keeps you comfortable, so the usual cues that you need a drink don't show up. Swimmers often finish a session having hardly drunk.
A FEW THINGS WORTH TRYING
Weigh yourself before and after a long session. Each kilo lost is roughly a litre to replace, and it shows what the water hid.
Leave a bottle at the end of the lane. Drink at every rest break, even if you don't feel like it.
Take sodium before and after longer swims. An hour or more in the water is when it matters most.
Warm up after cold swims. A warm drink and dry layers help your body recover.
Don't count swallowed water. Pool and sea water don't hydrate you.
If you feel faint, confused or unusually dizzy in or after the water, get out and get help. And never swim alone in open water.
Swimming feels cool and easy. Your body is working harder than it lets on.