Was it hard to concentrate during that long meeting? Does the crossword seem a little tougher? You could be mildly dehydrated.
A growing body of evidence finds that being just a little dehydrated is tied to a range of subtle effects — from mood changes to muddled thinking.
"We find that when people are mildly dehydrated they really don't do as well on tasks that require complex processing or on tasks that require a lot of their attention," says Mindy Millard-Stafford, director of the Exercise Physiology Laboratory at Georgia Institute of Technology. She published an analysis of the evidence this month, based on 33 studies.
How long does it take to become mildly dehydrated in the summer heat? Not long at all, studies show, especially when you exercise outdoors.
"If I were hiking at moderate intensity for one hour, I could reach about 1.5 percent to 2 percent dehydration," says Doug Casa, a professor of kinesiology at the University of Connecticut, and CEO of the Korey Stringer Institute.
For an average-size person, 2 percent dehydration equates to sweating out about a liter of water.
"Most people don't realize how high their sweat rate is in the heat," Casa says. If you're going hard during a run, you can reach that level of dehydration in about 30 minutes.
And at this level of dehydration, the feeling of thirst, for many of us, is only just beginning to kick in. "Most people can't perceive that they're 1.5 percent dehydrated," Casa says.
But already there are subtle — maybe even imperceptible — effects on our bodies and our mental performance.
Take, for example, the findings from a recent study of young, healthy and active women who agreed to take a bunch of cognitive tests and also agreed to restrict their fluid intake to no more than 6 ounces for one day.
"We did manage to dehydrate them by [about] 1 percent just by telling them not to drink for the day," says Nina Stachenfeld, of the Yale School of Medicine and the John B. Pierce Laboratory, who led the research.
The women took one test designed to measure cognitive flexibility. It's a card game that requires a lot of attention, since the rules keep changing throughout the game.
"When the women were dehydrated they had about 12 percent more total errors" in the game, says Stachenfeld.
She repeated the tests after the women drank sufficient water, and their performance improved. "We were able to improve executive function back to normal — in other words, back to the baseline day — when they rehydrated," the scientist says.
Dehydration didn't hamper performance on all the tests; the women's reaction time, for example, was not impeded. The decline was seen during the complicated tasks.
Though the study was small, and funded by PepsiCo, which sells bottled water, Stachenfeld designed the methods and completed the analysis independently. And other scientists say her findings fit with a growing body of independent evidence that points to similar conclusions.
"I absolutely think there could be big implications of having a mild cognitive deficiency with small amounts of dehydration," Casa says.
If you're a student, for example, a 12 percent increase in errors on a test might matter. And whether you're a pilot, a soldier, a surgeon or a scholar, many daily tasks depend on the ability to be precise and pay attention.
For anyone trying to do their best work, the findings raise a number of questions:
How much water do we need?
There are no exact daily requirements, but there are general recommendations.
A panel of scholars convened several years ago by the National Academies of Sciences, Engineering and Medicine concluded that women should consume, on average, about 91 ounces of total water per day. For men, the suggested level is even higher (125 ounces).
Note that this total includes water from all sources, including food and other beverages, such as coffee and tea. Typically, people get about 20 percent of the water they need daily from fruits, vegetables and other food.
Also, water needs vary from person to person. For example, body weight and muscle mass matter. Also, physical activity and heat exposure can increase the amount of fluid a person needs.
How can you tell if you're dehydrated?
One easy test: The color of your urine is a good guide. As a general rule of thumb, the darker the color, the more likely you are to be dehydrated. Aim for shades that have been described as "pale lemonade" or "straw." A color chart developed by physiologist and University of Connecticut professor Lawrence Armstrong can be a helpful guide, researchers say.
Are older people more vulnerable to dehydration?
As we age, we're not as good at recognizing thirst. And there's evidence that older adults are prone to the same dips in mental sharpness as anyone else when mildly dehydrated.
Don't wait until you're thirsty. A good rule of thumb is to sip fluids throughout the day. No need to chug huge amounts at one time; there are some risks to overhydrating, too.
Can coffee, tea and other caffeinated drinks have a dehydrating effect?
The most recent evidence finds that coffee provides similar hydrating qualities to water. In other words, yes, your morning cup of joe — or whatever caffeinated beverage you fancy, can help to keep you hydrated.
As we reported in 2014, people who routinely drink coffee or tea develop a tolerance to the potential diuretic effects of caffeine.
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. Transcript provided by NPR, Copyright NPR.