Perhaps health-conscious North Americans need to remember that most of our bodies are composed of water, and stop being so preoccupied with minor changes in body weight/ temperature during physical activity. Doctors everywhere will tell you that the best thing you can do for yourself is to drink plenty of water. And if you're getting your electrolytes (in the form of a simple pinch of salt - NOT brightly coloured sports drinks!) that water will be properly absorbed. Studies have shown that most North Americans are mildly dehydrated, the effects of which show up in asthma, bladder infections, and even morning sickness in pregnant women (which I can personally vouch for).
One of the worst things you can do after exercise is to reach for that beer instead of your water bottle, as beer will eventually take more water out of your system.
Fluid needs do vary from person to person - usually dependent on body weight. I am very scheptical about any study that advises against drinking more water!

I do alot of miles running every year, especially during the summer. This year I've been averaging approx. 30Km a week, 4 or 5 runs since the beginning of Feb - even in the snow. I am 48 yrs old and big for a runner at 6'2", 185lbs. I usually run after work (4:00) and in the summer the temp is never a deterrent. I find that regardless of if i hydrate immediately before, through the day, or during the run, sweating and performance are the same. If anything, how i recover from the run i have today will dictate how i feel in my run tomorrow. The things that do seem to work the best are: All post-run water is body temp - never cold; I sip water as I streach and cool down - one glass will do. Once i have stopped sweating, I have one more glass with something to snack on; then i shower and shortly after eat a good supper with a glass of beer or wine. I dont advocate this to anyone (other runners i know think i'm nuts) but it sure works with me.

It's unfortunate that people, by and large, still can't embrace moderation as a way of life. Maybe it's the media chasing the extremes. Maybe it's people wanting a quick fix for anything and everything. I don't know. What I think, though, is that if people took the time to actually listen to what their bodies tell them, they could figure it out on their own 98% of the time.

The wonder is that the eight glasses of water a day myth was so gullibly accepted despite being inherently foolish (your body has a perfect mechanism for informing you of the need for liquid: thirst). It's like that idiotic claim that the average 60 year old Swede is in better shape than the average 30 year old Canadian. Again, why do we want to believe such nonsense?

There was a key statement from Dr. Byrne at the bottom of the Exeter University article:

"I would encourage those people taking part in today's Marathon to be well hydrated before the race..."

If you haven't consumed sufficient fluids leading up to your race (and this goes back several days at least), drinking lots of water just before or during the race won't help.

If you are well hydrated coming into the competition, you'll get through the competition if you don't re-hydrate during the race, but based on my experience with myself and athletes I coach, performance will be affected. (My performance measurements are combinations of event times, training times, and relative positions of regular co-competitors.) Post-event recovery is definitely prolonged if you don't maintain hydration through the competition.

The amount of fluid intake required for optimal health and performance does vary by individual, and also by activity. The type of fluid matters too -- electrolyte drinks are good at some times, but not all: just plain water is very often the better choice.