By the time you feel thirsty you probably have already lost two or more cups of your total body water. It's best to drink fluids throughout the day. Milk, juice, and some other beverages can account for part of your daily 8 to 12 glasses because of their high water content. Drinks such as caffeinated coffees, teas, and sodas are not good water substitutes. Caffeine acts as a diuretic, causing you to lose water through increased urination.
Alcoholic beverages have a similar effect.
Experts recommend drinking plenty of water before you begin a strenuous workout, especially in hot, humid weather. Then drink 4 to 8 ounces of water every 15 to 20 minutes while you exercise. In addition, you should monitor your weight and urine color after heavy workouts. You need to replace each pound lost with two cups of water to bring your fluid balance back to normal. Dark colored urine indicates that you are consuming too little water and need to increase your intake.