After years working for the Forest Service running forest fire suppression crew all we would use is several grains of salt in our canteens.(if you tast the salt you have used too much) After 5 canteens of water we would drink 1 Gatoraid ( the only thing around at the time) Some time the shifts were 18 Hours in over 100 deg. heat.

Never lost a man to heat stroke.

A lot of conditioning in high heat also helped.