Water and salty, starchy foods.

I live in Southern Arizona, and every year, multiple people die from hyponatremia, low levels of sodium in the blood.

The problem is, they bring plenty of water, but they hike for 3 hours in the summer, and in 3 hours, you can lose 6 liters of fluid, not just from sweating, but just from your lungs as well.

They drink adequate fluid, but they can be losing 1-3 grams of sodium per liter of sweat. Sodium levels in the blood drop, the cerebral spinal fluid has a bit more sodium now, and because sodium makes up most of the osmotic pressure in the blood, and because water will follow solutes, water will rush to the cerebral spinal fluid, causing cerebral edema. Your brain swells, and has no more room to expand into, so you die. Sometimes parts of the brain will even be squeezed out of your skull through the foramen magnum. Very bad indeed.

But salt isn't all, you can't just take a salt tablet, those are ineffective.

The sodium transporters require something else, they require glucose to function. These transporters move sodium and glucose, usually in opposite directions. You cannot move glucose without sodium and you cannot move sodium without glucose.

So you need to eat something that has starch and salt. The perfect snack is pretzels. They have lots of salt and lots of starch.

Also, sports drinks will NOT work. Sports drinks are fine if you want to rehydrate, or get your sugar levels up, but they will NOT replace ELECTROLYTES FAST ENOUGH.