RACHEL MARTIN, HOST:
Summer heat can raise the risk of dehydration. We all know this. Especially, this can be a risk for people who like to cycle, or run, or hike or basically do anything outdoors. And it's not just our bodies that are slowed down by the loss of water. There is new research showing that our mental performance can suffer, too. NPR's Allison Aubrey reports that being just slightly dehydrated can make complicated tasks a little more challenging.
ALLISON AUBREY, BYLINE: Do you have any idea how long it takes to become mildly dehydrated in the heat? Not long at all, says Doug Casa. He's a professor in the department of kinesiology at UConn. It can happen in a half-hour run or a one-hour hike.
DOUG CASA: If I were hiking at moderate intensity for an hour, I could reach about 1.5 percent to 2 percent dehydration.
AUBREY: This equates to sweating out a little more than a liter of water for an average-size person. And at this level of dehydration, the feeling of thirst is just beginning to kick in, but already we can be affected in subtle ways from changes in mood to muddled thinking.
CASA: Dehydration can definitely negatively affect cognitive performance, which is really important and relevant because most people wouldn't even know or be able to perceive, normally, that they're 1.5 percent dehydrated.
AUBREY: So what exactly begins to happen when we're mildly dehydrated? Mindy Millard-Stafford of Georgia Tech says it's long been known that this affects our physical stamina.
MINDY MILLARD-STAFFORD: The heart rate tends to be higher. The core temperature is also greater. And then the work just feels harder.
AUBREY: And there's new evidence about how this level of mild dehydration influences mental performance.
MILLARD-STAFFORD: We find that when people are mildly dehydrated, they really don't do as well on tasks that require complex processing or in tasks that require a lot of their attention.
AUBREY: Take, for example, a recent study that included a bunch of young, healthy, active women. Some were students. Others had jobs. And they all agreed to take a bunch of cognitive tests. Nina Stachenfeld of Yale School of Medicine and the Pierce Laboratory led the research. She says, during the study, the women restricted fluids for one day. They were told to drink no more than 6 ounces.
NINA STACHENFELD: We did manage to dehydrate them by 1 percent just by telling them not to drink for that day.
AUBREY: One of the tests they took is designed to measure cognitive flexibility. It's a card game that requires a lot of attention since the rules keep changing throughout the game.
STACHENFELD: Even in these kinds of tests where there's no physiological effects, really, you can see with mild dehydration effects on executive function.
AUBREY: When they were dehydrated, the women made more mistakes. The complex game seemed to trip them up.
STACHENFELD: When the women were dehydrated, they had about 12 percent more total errors.
AUBREY: But, after they rehydrated...
STACHENFELD: We were able to improve executive function back to normal. In other words, to the baseline day when they rehydrated.
AUBREY: This is just one small study, but a body of evidence points to similar findings. And UConn's Doug Casa thinks it's worth paying attention to.
CASA: I absolutely think that there could be big implications of having a mild cognitive deficiency with just small amounts of dehydration.
AUBREY: Whether you're a pilot, a soldier, a surgeon or a student - think of how many jobs depend on people's abilities to be precise and pay attention. Allison Aubrey, NPR News.
MARTIN: Oh, man. Allison Aubrey joins us now in the studio to talk about this. And you're hearing despondency in my voice, Allison, because after your piece, I realize that I'm basically walking around in a state of total dehydration at all times.
AUBREY: OK. Are you drinking four water bottles' worth of water each day? That's what I have in front of me.
MARTIN: Definitely not. You have brought props. There are all these water bottles. I mean, we hear this all the time, that we're supposed to be drinking water, but what your piece illustrates that it really doesn't take that much to dehydrate us.
AUBREY: That's right. And so the question people ask is, well, how much am I really supposed to be drinking so I don't end up dehydrated?
MARTIN: Right. What's the answer?
AUBREY: So there's no exact daily requirement. I know this is going to be more complicated than anybody likes, but several years ago, the National Academies looked at this issue. And they came up with basically ballpark recommendations. They said women in general need about 91 ounces of fluid a day. So that's what I've got in front of me, these four water bottles, 91 ounces here.
AUBREY: But, here's the thing. We get a lot of our fluid from our diets. Fruits and vegetables have a lot of water weight. On average, we get about 20 percent of fluids from food. And then there are all these other factors that can make my water needs different from your water needs - your size, your muscle mass. Also, weather conditions, your activity level. So if you're out there in the heat and you're sweating and exercising, this will obviously increase your water loss.
MARTIN: What if you're just sitting in your air conditioned office?
AUBREY: Right. Well, you know, in the study that we just heard about, that's exactly what was happening. These women weren't athletes. They weren't endurance athletes. They were people who had jobs, and they just restricted fluid for a day. So you're not going to become dehydrated as quickly. But what the evidence shows is, no matter the setting, no matter if you're exercising in the heat or exercising in a temperate climate or just restricting fluids, no matter how you become dehydrated the effects are the same. You still see these ill effects on cognition.
MARTIN: Selfish question. I drink a ton of sparkling water. Does that count?
AUBREY: You know, absolutely, it counts. So no matter how you get these 91 ounces of fluid a day - you know, it could be from any beverage. It can be from coffee. People tend to shy away from coffee or think that coffee isn't...
MARTIN: That it dehydrates you.
AUBREY: Right. But that the evidence shows that if you're habituated to drinking caffeine, that diuretic effect isn't really there. You can count your daily cup of coffee towards your hydration needs, or your tea or whatever you happen to be drinking.
MARTIN: OK. Really, the big question is how do you know you're dehydrated?
AUBREY: Well, you often hear use thirst as your guide, right? But as we age, we're not as good at sensing our thirst. So I think one easy way to know if you're dehydrated is to pay attention - you ready for this? - when you go to the bathroom. You've got to look at the color - you're giving me this really funny look.
MARTIN: Yeah. Well, you know.
AUBREY: You've got to look at the color of your urine.
AUBREY: The darker the color, the more likely you are to be dehydrated. And the color you want to aim for has been described to me - are you ready for this? It sounds like paint palettes (laughter) - pale lemonade or the color of straw. (Laughter). So we actually have this nice link to a color chart online, if you want to check it out.
MARTIN: You can measure the color of your urine. Thank you, NPR's Allison Aubrey. I'm going to go get some water.
AUBREY: Thanks, Rachel. And I'm going to leave one of these behind for you. You need to drink up.
MARTIN: Let's see if I can get that done. Thank you so much, lady.
AUBREY: Thank you.
NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.