It's not uncommon for larger college football players to lose six to eight pounds of fluid weight during a day's practice. Even very gradual dehydration that becomes chronic will reduce performance and cause fatigue. You don't have to be an athlete to get dehydrated. A very general guideline is that inactive people ought to drink eight to 10 cups of water a day 64 to 80 ounces. Active people should have 10 to 12 cups per day.
Water is the cheapest source of fluid for people working out an hour or less.
Fruit juices and soda are not a good choice during exercise, because their high sugar content can cause cramps and digestive upset. Drinking caffeinated beverages, during a workout or at any other time, depletes the body of fluid because caffeine is a diuretic. Steen tells athletes that it's OK to have a latte or cola during the day, but be sure to drink at least as much water to replace what they've lost. Sports drinks are more effective than water for keeping hydrated, she says. For one thing, they are formulated to taste good during exercise, and so athletes are more likely to drink up.