During exercise, your body sweats in order to maintain an ideal body temperature. The amount of sweat obviously depends on environment, type, intensity and characteristics of the person exercising. But here is the kicker: dehydration reduces the body’s capacity for sweating and can impair performance by decreasing strength, endurance and coordination. More severely, it can increase the risk of heat cramps, heat exhaustion and heat stroke.
You should be drinking water before, during and after exercise in order to replace the fluid you will lose when sweating and working out. An ideal method to figure out how much water you lost is weighing yourself before and after the activity. Then for each pound lost, you should drink about 16 oz of water. By drinking enough before and during, it will minimize the amount you need to replace later on.
WARNING: don’t fall into the trap of sports drinks, thinking any activity calls for drinking them during working out, or even during the day. People are frequently taking the idea of “replacing electrolytes” too far. The majority of people that exercise are replacing their electrolytes by eating well. When you exercise in extreme conditions for longer than an hour (continuously), then it might be smart to mix a swig of a sports drink in with the water you are drinking. But really just concentrate on drinking water. Also, before buying a drink, read the label to make sure you are getting electrolytes and not just sugar and food coloring and a nice bottle full of flavored water!