JOHN YDSTIE, host:

Today in Your Health: Got water? Heat-related dehydration is a big problem in summer time. And if you're active, even if you're healthy, you're at risk.

NPR's Allison Aubrey explains why thirst isn't always the best clue that it's time to take a drink.

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ALLISON AUBREY: It's high season for sports camps and coach R.J. Johnson(ph) has dozens of elementary school-aged kids out playing ball from nine in the morning till 4:00 or 5:00 in the afternoon.

Mr. R.J. JOHNSON (Elementary Coach): We worry a lot about dehydration, also about body temperature and heat. We try to give the kids, you know, four or five water breaks and hose downs; we hose down the back of their neck with a hose.

AUBREY: This helps overcome what Johnson says he's noticed over the years, which is that kids don't always stop when they're thirsty.

Mr. JOHNSON: A lot of times they don't remember to drink water so we have to remind them. Once you pull the hose out, inevitably the kids will want to get a drink from the hose.

AUBREY: Stanford pediatrician Chris Longhurst said the hoses are probably a smart move, given that children can become dehydrated pretty quickly. He says the answer to one of parents' most common questions about dehydration may sound like an exaggeration.

Dr. CHRIS LONGHURST (Pediatrician, Stanford): That by the time a child feels thirsty, is he or she really already dehydrated? And the answer, actually, is absolutely yes.

AUBREY: All of us, children and adults, have a built-in cooling system. When we sweat, the evaporating perspiration helps our bodies cool down and withstand heat. Longhurst says the obvious trick to replenishing lost fluids is to drink.

Dr. LONGHURST: So in short cases of exercise or exertion where you're losing a little bit of fluid, there's nothing better than water. And water is always sort of the mainstay of rehydration.

AUBREY: With more intense exercise, over an hour in length, there's evidence that replenishing electrolytes with drinks like Gatorade or Power Aid can be beneficial. But Longhurst says it's important to remember that colas and some juices have about twice as much sugar, which is a problem.

Dr. LONGHURST: Solutions with too high of a glucose content or sugar content actually defeat their own purpose, which is that rather than replenishing fluid, they can prevent good uptake of that fluid.

AUBREY: Technically, dehydration sets in when a person's lost two percent of their body weight. How quickly this happens depends mainly on the conditions. Coach RJ Johnson says it's not uncommon for high school and college athletes to have dehydration creep up after three or four days of practice in intense heat. Sometimes parents, coaches, as well as the players can mistake headaches, dizziness or a rapid pulse as signs of something else.

Mr. JOHNSON: I know if I don't drink enough water, I go home and I don't feel very well. Or if I don't drink enough water the night before, when I come back to camp in the morning, I can feel it a little bit. So you have to make a conscious effort to keep yourself hydrated.

AUBREY: The American College of Sports Medicine recommends that athletes drink 16 ounces of fluids - that's two full glasses - a couple of hours before starting practice or exercise. Duke University sports medicine expert Blake Boggess says the body needs that much time to absorb the fluid. Once practice begins, players are instructed to drink even more.

Dr. BLAKE BOGGESS (Sports Medicine Expert, Duke University): Every 15 minutes, drink about 10 ounces.

AUBREY: Boggess says for athletes who don't want to take the time to measure out all these fluids, he gives them an alternative. He tells them to pay attention in the bathroom, just before they flush.

Dr. BOGGESS: I find it easier to tell them that, you know, if your urine is dark yellow, or the color of Lemon-Lime Gatorade, you know, you're not drinking enough.

AUBREY: Urine should be straw-like in color, only slightly yellow. Boggess says for most adults, thirst is a good guide, but it's time to take in some fluids. Though there's research to suggest that by the time we reach our late 30s, our recognition of thirst becomes a bit delayed.

For weekend hikers or bikers, this means drink up before you venture out. And it's okay to make it iced tea or even coffee if you prefer them to water. Researcher Doug Casa of the University of Connecticut has found that caffeinated drinks - long assumed to have a strong diuretic effect - don't actually stand in the way of hydration.

Mr. DOUGLAS CASA (Exercise Researcher, University of Connecticut): If you drank one liter of water, your body might retain, say, 800 milliliters of that water and you might lose the remaining 200 as urine. When you have something that has caffeine in it, say a liter of something has caffeine, you might retain, say, 700 milliliters of it and maybe lose 300 to your urine.

AUBREY: Casa says the net effect is that you're still getting most of the extra fluid you need, even with the caffeine.

Allison Aubrey, NPR News, Washington.

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