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JOE PALCA reporting:

Armstrong says caffeinated beverages have gotten a bad rap because they make you urinate.

Mr. ARMSTRONG: The same thing happens when we drink a large container of water. Does this mean we shouldn't drink water? Of course not.

PALCA: So go ahead, have your coffee, and try to keep your cool.

Joe Palca, NPR News, Washington.

ALLISON AUBREY reporting:

If you're training for a marathon or an iron man, how much you drink is important. Of course, there's the risk of dehydration, but athletes now know they can also get into trouble by drinking too much, which dilutes levels of sodium in the blood.

The death of one 28-year-old woman following the 2002 Boston Marathon led to new research, and experts say to strike the right balance, the best strategy is to replace, or put back in your body through drinking, roughly the same amount of fluid, or water weight, you lose through sweat.

Mr. DOUGLAS CASA (Researcher, University of Connecticut): You don't have to necessarily have a perfect match. I mean we, our goal is try to keep somebody within not dehydrated anymore than two percent of their body weight.

AUBREY: It sounds straightforward - stay less than two percent dehydrated. But how in practice can you tell? Douglas Casa is an athletic trainer and researcher at the University of Connecticut. He's got one strategy.

We caught up with him yesterday morning at 6:00 a.m., just before his morning run.

Mr. CASA: What I'm going to do is weigh myself, nude, before I go running.

AUBREY: With an accurate scale, he'll be able to calculate exactly how much water weight he sweats out during his run.

Mr. CASA: I'm stepping onto the scale, and my pre-weight right now is 63.3 kilograms.

AUBREY: Roughly 140 pounds. With that, Casa was out his front door for an hour-long workout.

The focus on tracking dehydration stems in part from concern over a condition known as hyponatremia. This is when someone drinks so much fluid that concentrations of sodium in the blood drop off and the body can't function properly. A study published in the New England Journal of Medicine last year tracked 488 runners who completed the Boston Marathon and found 13 percent of them had dangerously low blood salt levels. Twice as many women as men were at risk.

Lewis Maharam is a sports physician in New York who chairs the International Marathon Medical Director's Association. He says the first symptom runners may notice is a little swelling in the hands.

Dr. LEWIS MAHARAM (Chair, International Marathon Medical Director's Association): They can't get their rings off and they may get nausea and vomiting. They met get dizzy. They may not remember where they are.

AUBREY: Most runners restore salt to normal levels by eating just one meal after a run, and never need medical attention. But with the spate of reported cases of hyponatremia, Maharam's group has a new, simple guideline for hydrating. It's contrary to the old advice that runners should drink as much as they can stomach to prevent dehydration.

Dr. MAHARAM: The new research has shown that the body is a remarkable machine that actually tells you, via thirst, when you need fluid.

AUBREY: In other words, drink only when you feel thirsty.

For the more performance oriented runners, Doug Casa's method is certainly more exacting. After running one hour and drinking one liter of fluid, Casa hops on the scale a second.

Mr. CASA: I'll do kilograms first again. It is 63.0.

AUBREY: So Casa has lost just three-tenths of a kilogram, meaning that he successfully drank enough during his run to replace most of what he lost.

Casa says there's no need to weigh every time. After a handful of calculations taken under a variety of conditions, runners get a sense of how much to drink to keep a steady weight.

Allison Aubrey, NPR News, Washington.

MONTAGNE: And Doug Casa offers more hydration tips at npr.org/yourhealth.

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