As summer arrives, temperatures heat up. And as we become more active, so do we. More vigorous activity in this weather generally means we sweat more. How can you replace the body fluids you're losing? And do you really need to? Let's answer the second question first. "Yes!" nutrition experts say emphatically. "Most people are walking around in a moderately dehydrated state," says Susan Kleiner, PhD, RD, author of Power Eating. According to Kleiner, we all need a "bare minimum" of 8 to 12 cups of fluids daily, even more to replace the fluid you lose during exercise. Of these 8 to 12 cups, Kleiner advises at least 5 cups be pure water.

Sweating It Away Kleiner explains that you lose about 4 cups of water per hour of exercise, depending on how much you weigh and how much -- and how quickly -- you perspire. A moderate workout in a mild climate will likely result in a loss of 1 to 2 quarts of fluid per hour through perspiration. The more intense the exercise or the more extreme the temperatures, the greater the fluid loss. "If you don't replenish your fluid losses during exercise, you will fatiguefatigue early, and your performance will be diminished," says Kleiner. "If you don't replenish fluid after exercise, your performance on successive days will decay, and your long-term health may be at risk." According to the National Athletic Trainers' Association, says Kleiner, dehydrationdehydration can impair your physical performance after less than an hour of exercise -- even sooner if you start working out in a dehydrated state. It can also increase your risk of developing symptoms of heat illness, such as heat cramps, heat exhaustionheat exhaustion, and heat strokestroke.