Don't forget as you train in the heat, dehydration stresses the body. Body temperatures rise, heart rate increases, more glycogen is burned, the brain struggles to concentrate and exercise is more difficult. Make sure to become familiar with your sweat rate to know how much hydrating you need. Weigh yourself before and after a workout. If you lost 2 pounds, you need to hydrate 80-100% of that amount so you need 26-32 ounces of water replaced in your body.