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.

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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 fatigue 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, dehydration 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 exhaustion, and heat stroke.

Not Just for Athletes

It’s not just athletes -- even the weekend variety -- who are dehydrated, says Jacob Teitelbaum, MD, author of "From Fatigued to Fantastic!: A Manual for Moving Beyond Chronic Fatigue and Fibromyalgia."

For even the sedentary he suggests: “Simply occasionally pay attention to your mouth and lips. If they are dry, you’re thirsty and need more water.”

Dehydration can be a serious problem for anyone, but children and older adults are at greater risk, according to the Gatorade Sports Science Institute. It offers these tips to avoid dehydration:

--When exercising, drink early and often. Research shows exercise under warm or hot and humid conditions can cause dehydration in as little as 30 minutes. So it’s important to consume fluids not only during and after exercise, but also before a workout or strenuous activity.

--Don’t wait until you’re dehydrated to start drinking. Drinking in a dehydrated state can cause gastrointestinal distress.

--The American College of Sports Medicine recommends that athletes drink enough fluid to fully replace sweat losses during their activity. At a minimum, drink 8 to 10 ounces of fluid every 15 minutes during exercise.

--When active, don’t rely on your sense of thirst. When you are hot and sweaty, your thirst mechanism can shut off quickly and you may not realize you need fluids.

--Drink on a schedule.

--Check the color of your urine. If your urine looks like the color of apple juice, you are probably dehydrated. If it looks more like the color of lemonade, you are probably well hydrated.

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Improving the Taste

Sugary sodas or even fruit juices are not the best ways to replace fluids. “Beverages with a high sugar content are actually dehydrating and should be avoided as a means of fluid replacement,” says New York nutritionist Stuart Fischer, MD. That includes beer too, he points out.

If taste is an issue, Fischer recommends drinking flavored, zero-calorie mineral water, which mimics the taste of soda but contains no sugar.

California nutritionist Stella Metsovas likes to add mint or mint tea, lemon or lemon balm, or hibiscus tea to water to make it more “exciting,” while fitness author Debbie Mandel recommends creating your own spa water by filling a pitcher with water, adding slices of fruit such as strawberry or peach, and refrigerating until the water is delicately fragranced and flavored.

Adding just a splash of fruit juice (cranberry, pomegranate, or blueberry are good choices because of their antioxidant properties) can also make water more palatable, says Jyl Steinback, cookbook/lifestyle author and designer of the health program Eat Right, Move More and Live Well.

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Cold, Pure Water

If you really don’t like the taste of water, the solution may be as simple as buying a water purifier that filters lead and other contaminants from tap water, says Susan Kleiner. Some purifiers attach right to the faucet; others can be installed as part of the entire water system. You can also buy a pour-through filter that is placed in a special pitcher and put right in your refrigerator.

Cold, rather than room temperature, water may also be more appealing. And serving the water in a glass (rather than a plastic or paper cup) will help it stay colder longer and retain a fresher taste.

Seltzer water is another alternative, says Kleiner. Some people like the bubbly “soda” effect, and a splash of juice or a spritz of fruit such as lemon, lime, or orange might help you think of water in a new light.

Make sure if you buy seltzer already flavored that it’s not loaded with sucrose or fructose -- just other words for sugar. And, Kleiner adds, while seltzer is fine to drink throughout the day, it’s not the best choice while exercising because gas from the bubbles takes up space in your stomach, making you feel fuller and decreasing the amount of total fluid you’ll take in.

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Eating Your Water

Fortunately, during the summer we tend to eat watery foods like melons, plums, and peaches, says Cynthia Sass, MPH, MA, RD, LD/N, spokeswoman for the American Dietetic Association. “If you don't like to drink plain water, eating more watery foods is a good strategy,” adds Sass. You can also freeze 100% fruit juice and bits of real fruit in ice cube trays and add them to water.

Finally, says Sass, if you're trying to drink more, consider upping your water intake gradually -- 1 cup at a time -- to allow your body to adjust. “Otherwise you may feel waterlogged and will be running to the bathroom every 15 minutes,” she says. “And that could cause you to throw in the towel.”

By Carol Sorgen, reviewed by Louise Chang, MD

SOURCES: Susan Kleiner, PhD, RD, author, Power Eating, Mercer, Wash. Jacob Teitelbaum, MD, author, From Fatigued to Fantastic!: A Manual for Moving Beyond Chronic Fatigue and Fibromyalgia, Annapolis, MD. “The Science of Sweat,” Gatorade Sports Science Institute, www.gssiweb.org. Stella Metsovas, BS, certified nutritionist, Laguna Beach, Calif. Stuart Fischer, MD, New York City. Jyl Steinback, author, program designer, Eat Right, Move More and Live Well, Scottsdale, Ariz. Debbie Mandel, MA, lifestyle/fitness coach, Long Island, N.Y. Cynthia Sass, MPH, MA, RD, LD/N, National Media spokeswoman, American Dietetic Association, Tampa, Fla.