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Home HEALTH How much water do you really need? Here's the science: NPR

How much water do you really need? Here’s the science: NPR

Many feel tied to a bottle of water after hearing that hydration is key to being healthy. Here’s what the science says about how much water to drink and when.



MARY LOUISE KELLY, HOST:

Many of us feel tied to our water bottles because we’ve been told that staying hydrated is key to being healthy. Our colleagues from Life Kit and Short Wave joined together to talk about what the science says about hydration. It turns out that much of the lore needs to be updated. Short Wave hosts Emily Kwong and Aaron Scott take it from here.

AARON SCOTT, BYLINE: Actually, I’d like to start with a story.

TAMARA HEW-BUTLER: You know, I’m a marathon runner. I mean, that’s, like, my people.

SCOTT: This is Tamara Hew-Butler. And back in 1999, she was a volunteer podiatrist in the medical tent for the Houston Marathon.

HEW-BUTLER: I’m, like, in the corner, like popped blisters, like bandage, like, ankles. And since it was hot, all these runners were being carried out on stretchers. And we assumed everyone was dehydrated. IVs are going into, like, everywhere because that’s what you did. And so in four runners, after the fourth IV, they started having seizures in the medical tent. They needed to be intubated. They were taken to the hospital and were all in a coma for a week. And the diagnosis came back. All had hyponatremia.

EMILY KWONG, BYLINE: Ooh, that sounds very serious. What is hyponatremia?

SCOTT: Yeah. So your body is constantly in this state of fine-tuning the internal balance between water and sodium. And hyponatremia is when that gets out of balance and there’s too much water and too little sodium. In the ’90s, the focus was much more on dehydration, not the other way around. So the following year, at the marathon, Tamara decided to see how many people were drinking and then measure them for hyponatremia.

HEW-BUTLER: The runners drank between 80 and 100 glasses of fluid during…

SCOTT: Wow.

HEW-BUTLER: …Your marathon. yeah. And so for me, I’m like, oh my gosh. They are drinking all this water.

SCOTT: It was such a wake-up call for Tamara that she actually closed her podiatry practice and went on to get a Ph.D. with one of the international experts on hyponatremia in South Africa.

HEW-BUTLER: We did a series of studies and confirmed that runners were drinking too much fluid during the race. And their brains were swelling, and their lungs were filling with fluid. And they were in a coma, with some of them dying.

SCOTT: Deaths like this are rare, Emily, but it’s a risk for all kinds of athletes. I mean, Tamara says that football season scares a lot of people on her field because kids are drinking water, thinking it will fix things like cramps and headaches. But they don’t realize that drinking too much water can actually cause cramps and headaches.

KWONG: Then tell me. How much water these days do we think we should be drinking?

SCOTT: Well, you’ve probably heard that old saying that you should drink eight glasses of water a day, right?

KWONG: Absolutely. Yes

SCOTT: Yeah, that’s not true.

KWONG: Oh.

SCOTT: No one really knows where that came from. But to many hydration experts, this is the myth that you won’t die because the real answer is that it depends. It depends on your body size, your activity level, if it’s hot and you sweat a lot.

KWONG: Brooches. Listen to your body. Okay, although I will say, Aaron, that I can hardly pay attention to my plants’ thirst, let alone myself.

SCOTT: (laughs).

KWONG: How do I control my body’s thirst needs?

SCOTT: Well, the beauty of it, Emily, is that our bodies do it for us.

KWONG: Oh, that’s good.

SCOTT: Yeah. Tamara says that hydration isn’t just about water. It’s all about the water-to-salt balance of our bodies. That’s what keeps us from shriveling from dehydration or bloating from hyponatremia, either of which can be deadly. And the third plays a central role.

HEW-BUTLER: There are sensors located in your brain, and they’re constantly, like, testing your blood, like, oh, to see, like, if it’s the right salt. So, but if it’s too salty, then it’s like, oh my gosh, I need more water. So when that happens, you get thirsty. So when you’re thirsty, what happens? You put more water in your system.

(POILED WATER SOUNDBITE)

HEW-BUTLER: So the sensors are going to be like, ew. It’s too watery. And it’s going to send a signal to a hormone that will make you urinate all that extra water.

(SYNCHRONOUS SOUND OF TOILET FLUSHING)

HEW-BUTLER: So these sensors are in your brain and connect primarily to your kidney.

SCOTT: And all that back and forth between the brain and the kidneys, it happened so fast that your body knows the minute you drink if you’ve had enough to rebalance the ratio of water to salt in your blood.

HEW-BUTLER: You have this sensor in your brain that has been encoded in the DNA of vertebrates and invertebrates 700 million years ago. It’s in worms.

SCOTT: It works for worms. It works for us with just a few caveats.

KWONG: Of course there is. IT’S OKAY. What’s in the fine print?

SCOTT: Yeah. So there’s some research that suggests older people may have reduced sensitivity and probably should drink more.

KWONG: Okay.

SCOTT: And then some studies found that people who chronically don’t consume a lot of water, like about a liter or less a day, might also have a weaker thirst signal and might see an increase in positive mood and vigil. if they drink beyond their thirst. And then other research has shown that people with certain medical conditions, such as kidney disease, kidney stones, and urinary tract infections, can also benefit from drinking past their thirst.

KWONG: Well, good warning. Let’s move on to other myths about hydration. IT’S OKAY. I’ve heard that if you’re thirsty, you’re already dehydrated.

SCOTT: This is, like, yes and no.

KWONG: Okay.

SCOTT: It kind of depends on who you are. Tamara says that our body is constantly, you know, taking samples of our blood and that our thirst comes on after we lose about 2% of our fluids, which is totally fine for most people. You’re thirsty. You drink water. You are good to go. But if you’re, say, an elite athlete or a fighter jet pilot or something that requires intense focus, there’s some research that has found that mild dehydration is enough to keep you from peaking.

MINDY MILLARD-STAFFORD: When you got to this level of 2% water loss, you saw some impact on what we call executive function, higher-order thinking and judgment, and also sustained attention, the ability to continue to be attentive and focused. in a task even if it is very boring.

SCOTT: This is Mindy Millard-Stafford. She’s the director of the Exercise Physiology Laboratory at Georgia Tech. And she researched this mild dehydration, which she says is also linked to things like worsening mood and decreased alertness.

MILLARD-STAFFORD: And it didn’t really matter if the dehydration was due to exercise-induced sweat loss, if people just weren’t drinking enough, or if they were in a passive warm-up situation that wasn’t related to exercise.

SCOTT: I mean, like, you know, sitting in the blazing sun.

KWONG: That’s my favorite water loss situation. Okay, the ultimate hydration myth: coffee and tea. I heard that caffeine is a diuretic and dehydrates you.

SCOTT: Yeah. And this happens to be my favorite because it’s based on a 1928 study that looked at three people.

KWONG: We can throw that away. It was too small a sample size.

SCOTT: Yeah. And so they’ve done more recent research, and this myth doesn’t hold up at all. But do you know what a diuretic is, Emily?

KWONG: What?

Scott: Booze.

KWONG: Oh.

SCOTT: I mean, really, hydration, like so many good things in life, comes down to balance.

(CHAD CROUCH “INVESTIGATION” SOUNDBITE)

KELLY: That was the hosts of Short Wave, Emily Kwong and Aaron Scott. This was just one podcast exception. You can check out Life Kit and Short Wave wherever you get your podcasts.

(CHAD CROUCH “INVESTIGATION” SOUNDBITE)

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