Cold makes you pee more, so you’re more likely to get dehydrated. True or false. Well, the first half is true: Cold can create what researchers call cold-induced diuresis (CID), meaning you pee more when your body meets cold air or water. When your skin gets cold, blood is shunted away from your skin and redirected to your core. “With more blood in the thorax, the heart says, ‘I have too much fluid on board and need to get rid of some of it,’” says Castellani. But exercise, even at a moderate intensity, prevents CID.
Moving the blood to the core also makes your body think you have enough fluids on board. You need to be smart about replacing what you’re losing, but don’t go crazy: Unless you’re overdressed, you won’t need as much water as you would on a 90-degree day with 80 percent humidity.