C just sent me an interesting article: Can Dehydration Aid Performance
And while I agree that sports drink promoters probably over promote the fears of dehydration to sell drinks, I think staying hydrated is an extremely important part of running. Even if elite runners can deal with a greater degree of dehydration than the average runner, I think its a razors edge - and its always better to loose a few seconds for that cup of water that having a DNF because you passed out a mile before the finish line.
The book I was reading, Running Like An Animal, spent some time on water how many animals use water to regulate temperature.
I'm extrapolating from his conversation about how the body uses carbohydrate and fat energy stores, but my guess is that the best runners would be ones who would use every bit of water that they had, starting being able to hold more, and loosing more as they ran. Just not a rate that had them run out of water before the end of the race. That would explain why elite runners have a greater loss of water.
Elite runners more likely, in addition to being better at using and storing water, are better at functioning with a decreased amount of water. "running through the pain". Before you body starts to shut down, the brain starts to go. I know I loose concentration rather than ability when I get dehydrated. An elite runner has spent so many miles and races at the breaking point, they are more likely able to push through that feeling.
Camels survive in the dessert not just because they have amazing abilities to hold huge stores of fats, retain a remarkable amount of water and are built to stay cool and shade vital organs... they survive mostly because their bodies can deal with temperatures that would kill us - not just slow us down, or lead to brain damage. They can drink salt water to re-hydrate, their bodies can function with much less water percentage than ours too. Elites are probably just a bit more on the camel side.
Granted camels need a year to recover from extreme exertion - so maybe they're passed the camel side.
For me, based on my runs in the woods, I'm going to be uber-cautious about hydration and not mess up my races. And if I do find myself dehydrated, I'll keep the article in mind and try to channel my inner camel, minus the salt water drinking of course.