Life Kit - How much water does your body really need?

Episode Date: September 22, 2022

Do you really need to drink eight glasses of water a day? Can drinking water help you lose weight? Does coffee dehydrate you? Experts explain the science of hydration.Learn more about sponsor message choices: podcastchoices.com/adchoicesNPR Privacy Policy

This is NPR's Life Kit. I'm Mariel Seguera. And I have some friends here with me, Aaron Scott and Emily Kwong. Mariel, it is so lovely to have you here. Welcome to NPR. Yay! Host friends! Yes, I'm excited. Okay, so I was told to have a big glass of water for this, but true to form, I do not. I have a quarter of a cup of lukewarm chamomile tea with me. But, you know, those dregs, they have some water. You should just knock it back.

Yeah, it's pretty gross. Meryl, maybe we need to talk about this. I mean, how much water do you drink a day? Clearly not enough. I am constantly thirsty. I feel like I should be filling up more. I mean, how about you guys? Clearly too much. I pee like 12 times a day. But I don't know. Is that too much? It's a good question, Emily. I mean, I'm in the same boat. I'm constantly refilling my teapot over and over again, which kind of makes sense because if you

listen to headlines or wellness influencers these days, it sounds like we are all super dehydrated and water, it's going to fix all of our problems. A new study says a key to shedding those extra pounds could be as simple as having two cups of water before meals. But it's the best thing for your hair, your skin, your nails, and it's free. Water. You can talk more about all of the benefits, but for me, it just feels good. I heard J-Lo's voice in that mix, and I do as I'm told when J-Lo says it. Like, you can't have too much water. That's my takeaway. End of episode. The trouble is, Emily, you can. Too much water can be deadly, just like drinking too little water.

Oh, geez. Seriously? How much am I supposed to be drinking then? Yeah, how much? Like, science must have a hard number. What is it? You would hope so, wouldn't you? I mean, that is the million-dollar question, or if you're the sports drink industry, the billion-dollar question. But the answer, like most good things in life, is it's complicated. I was worried you would say this. Today on the show, we're going to explore the science of hydration and answer questions about the best ways to quench our thirst.

Welcome to our collaboration, Short Life Wave Kit. No, no, no, no, no. Short Kit Life Wave. Oh, yeah. We might need to work on this one. Fine. That's fine. Sorry. Work in progress. Okay, Erin, so you were saying that drinking too much water can be bad for you, even deadly in some cases. Yeah, Emily, like most things, we can overdo it. I'd like to actually start with a story. You know, so I'm a marathon runner. I mean, that's like my people. This is Tamara Hugh Butler. And back in 1999, she was a podiatrist volunteering at the Houston Marathon Medical Tent. I'm like in the corner, like popping blisters, like taping up like ankles. And because it was hot, all these runners were like being carried in on stretchers. And we assumed that everyone was dehydrated. IVs are

going in like everywhere because that's what you did. And so in four runners, after the fourth IV, they started to have seizures in the medical tent. They needed to be intubated. They were taken to the hospital and they were all in comas for a week. And the diagnosis came back, they all had hyponatremia. Ooh, that sounds really serious. What is hyponatremia? Yeah. So your body is constantly in this state of fine-tuning an internal balance between water and sodium. And hyponatremia is when that gets out of balance and there is too much water and too little sodium. You know, I've run a marathon and now that I think about it, losing electrolytes is something to be concerned about,

which can happen from drinking too much water or not replacing the electrolytes you're losing through your sweat. Yeah, we know that now in part thanks to the worker researchers like Tamara. But back in the 90s, the focus was much more on dehydration, not its opposite. So the next year at the marathon, Tamara decided to look at how much folks were drinking and then measure if they had hyponatremia. Runners were drinking 80 to 100 cups of fluid during the marathon. Yeah, and they were like, why did you do that? They're like, well, I didn't want to get dehydrated. And so for me, I'm like, oh my God, they're drinking all this water. It was such a wake-up call for Tamara that she actually closed her podiatry practice

and went to get a PhD with one of the international experts on hyponatremia in South Africa. And we did a series of studies and we confirmed that runners were drinking way too much fluid during the race. They couldn't pee it all out and their brains were swelling and their lungs were filling up with fluid and they were in comas with a few of them dying. Deaths like this are rare, Emily, but it is a risk for all kinds of athletes. I mean, Tamara says football season scares a lot of people in her field because young folks are guzzling water thinking that it'll fix things like cramps and headaches, but then not realizing that drinking too much water can actually cause cramps and headaches. I had no idea the history of hydration, dehydration research. So tell me, how much water

these days do we think we should be drinking? Well, you probably heard that old saying of, you know, you should drink eight glasses of water a day, right? Absolutely, yes. Yeah, not true. No one really knows where that came from. But for a lot of hydration experts, this is the myth that will not die. Because the real answer is, it depends. It depends on your body size, on your activity level, if it's hot and you're sweating a lot. So our first takeaway, Emily, is to pay attention to your thirst.

It will tell you how much you need to drink. Snaps. Listen to your body. Okay. Though I will say, Aaron, I can barely pay attention to the thirst of my plants, let alone myself. How do I monitor my body's thirst needs? Well, the fancy thing about that, Emily, is our bodies do it for us. Oh, how nice.

Yeah, Tamara says hydration isn't about just water alone. It's about our body's balance of water to salt. That is what prevents our cells from shriveling up from dehydration or swelling up from hyponatremia, either of which can be deadly. And thirst plays a central part. There are sensors located in your brain, and they constantly are, like, tasting your blood. Like, ooh, to see if it's just right salt.

But if it's, like, too salty, then it's like, oh, my God, I need more water. So when that happens, it makes you thirsty. And so when you're thirsty, what happens? You put more water into your system. Then the sensors are going to go like, ew, that's too watery. And it's going to like signal a hormone that's going to make you pee out all that extra water. So these sensors are in your brain, and they mainly connect to your kidney. And that whole back and forth between the brain and the kidneys, it happens so fast that your body knows within a minute of drinking whether you drank enough to rebalance that water-salt ratio in your blood. Within a minute of drinking. That is amazing.

Wow. Go bodies. So people are like, oh, I need an app to tell me when to drink my water. I have to have the gallon container in front of me. No, you've got this sensor in your brain that's been encoded in the DNA of vertebrates and invertebrates 700 million years. It's in worms. So that's how important water is to life, That this gene has existed for so long to preserve water balance. Tamara's like parting the waves

with this knowledge. I mean, it's just, she's saying basically, yeah, your body is not only tracking thirst for you, but it's evolved to do this. It works for worms. It works for us, with only a few caveats. What's in the fine print?

So there is some research that suggests that older people may have a reduced thirst sensitivity and likely should be drinking more. Okay. And then a few studies found that people who chronically don't consume much water, like around a liter or less a day, that they might have a weaker thirst signal too

and might see an increase in positive mood and wakefulness if they drink beyond their thirst. And then other research has demonstrated that folks with certain medical conditions, including kidney disease, kidney stones, and urinary tract infections, they also can benefit from drinking beyond their thirst. Okay, good caveat. Certain population groups could benefit from drinking more. But to reiterate, takeaway number one, Tamara says. If you get thirsty, you need to drink water. If you're not thirsty, you don't need to drink water. I remember drinking a lot of Gatorade back in high school out of peer pressure, maybe. And also just the ads said it would quench my thirst better than water and like replace my salts and electrolytes. Yes, Emily, this one brings us to takeaway number two.

If you are sweating up a storm out there playing field hockey, running a marathon, whatever, salt is essential. But it doesn't need to come from sports drinks. I mean, the experts I talked with said that sports drinks do have their place. Like if you are super active and sweating for more than an hour and you need water, salt, and energy, they're a great option. But if you're not burning a lot of energy, then the sugar that is in sports drinks, just like sugar in soda pop, it can be a problem.

I mean, you know, you've probably heard that whole idea that drinking more water helps you lose weight. Yes, I have heard this. Well, yeah, so that has only been proven to be the case if you're replacing sugary beverages like soda and sports drinks with water. But, you know, Emily, in terms of getting salt to your body to keep it balanced and hydrated, it doesn't matter at all where that salt comes from. I mean, Tamara says that she's involved in organizing ultra marathons, you know, these places where people are running a hundred miles over like 30 hours straight through the desert or the mountains. They're intense. Uh-huh. And she says that the organizers at these events, they just set out tables with all sorts of different beverages and snacks and just let

folks kind of have their pick and follow their cravings, whether that's saying, you know, drink some water and use a salt packet, drink a sports drink, drink a soda, eat some peanuts or slurp some pickles. And then Tamara has actually sampled their blood and their urine and found that their bodies do a fine job keeping the water-salt ratio in balance just by them following those cravings. Our bodies, they really are these finely tuned machines. And just out the other side of thirst, like does pea color give us a clue if we're hydrated or not? Yes. So takeaway number three, urine color is not as good a measure of dehydration as thirst. I mean, if we think back to what Tamara was telling us about how the body balances water and sodium,

then we remember that urine is a reflection of how much water the body needs to release in order to keep its balance. Urine is not a reflection of if your body is hydrated or dehydrated. Tamara prefers actually to measure your blood because that's what your body is hydrated or dehydrated. Tamara prefers actually to measure your blood because that's what the body is measuring. But a lot of research, especially the research that says that lots of people are dehydrated, that looks at just people's urine. We did a study. It was on like 300 athletes. And so we had them pee in a cup and then we drew their blood. And out of the 300, there was like one data point where the blood was like outside normal, but like 50% of the urine was abnormal.

And that person who is outside normal in their blood, it was because they actually drank too much water and were dealing with hyponatremia, not dehydration. Oh, that's so interesting. So among these roughly 300 athletes, about half of them showed up as dehydrated when their urine was measured. But looking at their blood, none of them actually showed up as dehydrated. Yeah, you can see here why hydration is such a complicated topic and why, you know, there's these headlines saying like so many people are dehydrated because there's so many

different ways to measure it. But like, just because your urine is really concentrated, it doesn't actually mean that your body's dehydrated. It just means that it's needing to hold onto the water in order to keep that water sodium balance. So like having dark pee occasionally while you're, you know, working out or on a long hike, it doesn't necessarily mean there's a problem. But, you know, there is a caveat here, and that is that some research has found having dark urine, like, all the time over a long period of time could actually increase your risk for things like kidney disease, diabetes, hypertension, a few other things. So there are hydration experts who recommend that you know you start the day with a glass of water and that you do drink enough to generally keep your pee

kind of a golden straw color which will differ from person to person got it erin you're you're like radically reframing how i see urine so so well done um let's move on to some other hydration myths. Okay, I've heard that if you're thirsty, you're already dehydrated. This one is like, yes and no. It kind of depends on who you are. Tamara says that our body is constantly sampling our blood and that our thirst kicks in after we lose about 2% of our fluids, which is totally fine for most people. You get thirsty, you drink water, you're good to go. But if you are, say, an elite athlete or a fighter jet pilot

or something that requires intense concentration, there is some research that's found that mild dehydration is enough to keep you from your peak game. When you got to this 2% level of water loss, you did see some impact on what we call executive function, higher order judgment and thinking, and also sustained attention, the ability to continue to be vigilant and focus on a task,

even if it's very boring. This is Mindy Millard Stafford. She's the director of the Exercise Physiology Laboratory at Georgia Tech. And she looked into this mild dehydration, which she says is also linked to things like worsened mood and lower alertness. And it didn't really matter if that dehydration was due to exercise, sweat-induced loss, whether people were just not drinking enough, or if they were in a passive heating situation. That was not related to exercise. That is to say, like, you know, sitting around in the hot sun.

That's my favorite water loss situation. So, but you might ask, you know, like what is 2% dehydration? So like for me, I weigh 170 pounds. So that for me is like three and a half pounds of water loss, which I thought was like huge. But Mindy said that you can lose like two pounds an hour if you're being super active. So if you're heading into a sports sun, or I guess like a sauna situation, it might be a good idea to drink before you get thirsty. Just might make everything a little more comfortable and clear.

Yes. So our takeaway number four is you might want to, you know, quote unquote, pre-hydrate or have that glass of water in advance. If you're about to go into a test or a race or somewhere where you need to really have a lot of mental focus. But most of the time in our day-to-day life, drinking to thirst is just fine. Okay, last hydration myth. Coffee and tea? I've heard caffeine is a diuretic and dehydrates you. Yeah, and it turns out that this is kind of my favorite one because it's based on a study from 1928 that looked at three people.

We can toss that out. It's too small of a sample size. Yeah. And so they've done newer research and this myth doesn't hold up at all. But you know what is a diuretic, Emily? What? Alcohol. But Mindy actually says caffeine in tea and coffee can help with things like sports. A little bit of caffeine added to a sports drink actually has an additional benefit for exercise performance. We know caffeine increases your alertness and probably some of your cognitive abilities. Takeaway number five, my favorite so far, coffee and tea count towards keeping you hydrated. Along with, you know, food that contains liquid like fruit, soup, yogurt, gazpacho. I mean, we get about 20% of our water from just food itself.

Aaron, I have a pun for you, but these are some watershed moments you've included in this episode. Let's review our takeaways. One, listen to your body and drink when you are thirsty. Two, salt is as important as water. If you're sweating up a storm, sports drinks are one way to get it,

but so is food. I vote for sports pickles. Plus one. Three, your thirst is a better indicator of hydration than your urine, but some experts say it's good to generally aim to have light yellow golden pee. Four, you don't need to drink before you're thirsty or pre-hydrate unless you're really, really focused on peak performance. And takeaway number five, tea, coffee, and really any beverage count towards hydration.

Except for alcohol. As do many foods. One delicious option? Watermelon with some salt on it. I mean, really, hydration, like so many good things in life, comes down to balance. It's like a happy medium, right? Not too much, not too little. Just the Goldilocks sort of

approach. Thanks so much, Erin. This episode was produced by Summer Tamad and Andy Tagle and edited by Giselle Grayson, Megan Cain, and Rebecca Ramirez. It was fact-checked by Britt Hansen. Engineering support came from Gilly Moon. Giselle Grayson is Shortwave's supervising senior editor. Megan Cain is LifeKit's supervising editor. And Beth Donovan is LifeKit's executive producer.

Anya Grunman is our senior vice president of programming. I'm Aaron Scott. I'm Emily Kwong. Thanks for listening to LifeKit and Shortwave from NPR.