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RENEE MONTAGNE, host:

This is MORNING EDITION from NPR News. I'm Renee Montagne.

STEVE INSKEEP, host:

And I'm Steve Inskeep. Good morning.

And now it's time for Your Health. And in this part of the program we'll hear about consulting your doctor online. Some insurance plans will be pay for that.

But first we're going to hear something about what you might call hydration hype. We've all heard we're supposed to keep healthy by drinking eight glasses of water per day. But a new analysis turns a critical eye on that old advice.

NPR's Allison Aubrey reports.

ALLISON AUBREY: It's not that everyone follows the recommendation to drink eight glasses of water a day, but a lot of people seem to think it's a good idea. I spoke to a bunch of commuters who ride the train into Washington each day.

Unidentified Man #1: Well, I guess it's an essential. Yes.

Unidentified Man #2: I don't see that it can hurt you.

AUBREY: I see you've got your water right here tucked into your knapsack.

Unidentified Woman: Absolutely. I think it's good for the skin and for your system.

AUBREY: Tracy Hummer's getting at a common assumption here. It's that all this water helps to clear the gunk or toxins from our bodies. That's an appealing concept, says physician Stanley Goldfarb, a kidney expert at the University of Pennsylvania. He explains that the kidneys do filter toxins from the bloodstream and clear them out of the body.

Dr. STANLEY GOLDFARB (University of Pennsylvania): So the question is, does drinking more water increase this normal important function of the kidneys? And the answer is no. In fact, drinking large amounts of water actually and surprisingly tends to reduce the kidney's ability to function as a filter. It's a subtle decline, but definite.

AUBREY: Goldfarb says it's not exactly clear why this happens. The going theory is that drinking lots of water may stimulate a part of the nervous system, and this appears to slightly constrict the blood vessels that lead to the kidneys.

Dr. GOLDFARB: It probably is the mechanism of this slight fall.

AUBREY: Goldfarb explains if you take a 200 pound man, a full 120 pounds of body is already water. So could drinking a few extra glasses each day on top of what's consumed in food or other beverages really have measurable effects, say on the quality of your skin?

Dr. GOLDFARB: In reality, it's such a tiny part of the amount of water that's in the body that it's very, very unlikely that one is getting any benefit from that.

AUBREY: So for everyone in the habit of hauling around their (unintelligible) bottles or buying loads of bottled water, is it worth all the fuss? University of Pittsburgh's Madeleine Fernstrom says it may be, particular if you're trying to lose weight.

Ms. MADELINE FERNSTROM (University of Pittsburgh): Water's a great strategy for dieters because it has no calories, so you can keep your mouth busy without food and get the sense of satisfaction.

AUBREY: Fernstrom says as for the amount, there's nothing special about drinking eight glasses. Nor is there anything magical about water alone. She says other zero-calorie options such as diet sodas are fine too.

Dr. Stanley Goldfarb is not fooled on the diet argument. He points out that researchers who've studied this have come up with conflicting results.

Dr. GOLDFARB: There were some studies that suggested that in fact caloric intake was reduced when individuals were given water prior to eating. Other studies suggested that it wasn't.

AUBREY: So on this issue you may be the best judge of whether drinking more helps you eat less. As for people just hoping to feel better, some may take Bill Salvatore's approach. He's one of the commuters we heard from earlier who says drinking a little extra water can't hurt you.

Mr. BILL SALVATORE (Commuter): It's just going to make you go to the bathroom a lot more. That's all.

AUBREY: Goldfarb agrees. He says a few people may benefit from extra fluids, for example, those with kidney stones. And certainly athletes and hikers know it's important to avoid dehydration. But for the most part, Goldfarb says, people shouldn't worry about how much water they've had to drink each day. His advice - just drink when you're thirsty.

Allison Aubrey, NPR News.

INSKEEP: Athletes actually run the risk of over-hydration. And you can read about that at npr.org/yourhealth.

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