[TalnSG]

The guidance I once got from my doctor regarding "sports drinks" (which I don't like and don't drink) was to drink them if (1) you are sweating to the point that your clothes are soaked, or (2) if your are ill in a manner that inhibits intake of food and drink (vomiting, diarrhea, high fever).

Both of those indicate that your body is using fluid at a rate higher than it can be replenished so you need two things....water and salt (potassium or sodium). The salt in either form is to replace the salt that is being washed out of the system to maintain the salinity of the fluid in the body (blood, etc.).

As for what was done before the days of these specialized formulations - salt tablets. Before that you rested during hot weather and made sure you drank water and most food was salted to preserve it, so you got the salt from the food.