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Fun in the Sun Means Proper Preparation

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Of course, there is a way around all this: water. It's important for everyone, but it takes on added importance for people working outside, whether they are paving roads or exercising.

David Martin has completed 29 marathons, and he's never, as he calls it, "bonked." But plenty of athletes, experienced or not, fade as they work out in temperatures high enough to make a kettle whistle.

Martin says he owes his successful track record to proper preparation -- especially the proper regulation of fluids. Of course, he also has an advantage over most people in that he has a PhD in physiology and is the chairman of sport science for U.S.A. Track and Field.

"I've always been sly enough and followed my principles of physiology," Martin tells WebMD. Those principles are quite simple, he says: If you're an athlete -- whether you're training for the Olympics or just jogging around the block -- "you really want to live a healthy lifestyle, which means not being thirsty and not being hungry."

Although avoiding the hottest parts of the day is best, that's not always possible. So week-long laborers and weekend warriors alike should make sure they never let themselves get thirsty.

"As soon as they have reached a point of thirst, they actually are dehydrated," Martin tells WebMD. "We say you never pass up a drinking fountain, or carry a small bottle and take sips to make sure your fluid levels are up."

The most dangerous condition for exercisers, Martin says, is humid heat. The evaporation of sweat helps to cool the body, but humidity limits the amount of evaporation, he explains.

If you're exercising, that can be critical. Most of the perspiration we produce comes from blood plasma. So excessive perspiration can decrease the volume of blood just when it is needed in the muscles that are working.

In times of heat stress, the body also sends blood to the skin surface to cool the body by radiant heat loss. "But at the same time, you want that same amount of blood to go into your working muscles, so that you can provide nutrition and oxygen for metabolism. And if you have to share a smaller and smaller amount of blood, because of sweating, with these two major tissue masses -- the skin for cooling, but the muscles for working -- something has to give," Martin says.

This could hurt the athlete's performance, or it could lead to heat exhaustion or even heat stress. A person racing in the heat, for instance, can lose a liter to a liter and a half of sweat an hour. But the body can absorb about a liter of water an hour, so adaptations are necessary for those who must compete or work in the heat.

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