How Many Glasses Of Water A Day Do You Actually Need?

KATHLEEN DAVIS: If you follow health or fitness influencers at some point you’ve probably heard the line about a person needing eight 8-ounce glasses of water a day to be healthy. The question of the right amount of water for health and happiness is still an open one, and it varies from person to person.

But a recent study in the journal Science looked at just how much water a human body actually uses each day, and this wasn’t just asking people how many sips they had taken but tracking the amount of water that flowed through the bodies of over 5,000 people around the world using labeled isotopes. Joining me now to talk about the work is one of the authors of that study, Dr. Dale Schoeller is a Professor Emeritus in the Department of Nutritional Sciences and the Biotechnology Institute at the University of Wisconsin, Madison. Thanks for joining me today, Dr. Schoeller.

DALE SCHOELLER: And thank you for giving me the opportunity to talk to your audience.

KATHLEEN DAVIS: Absolutely. So let’s start kind of broad. Can you explain to me why we need water?

DALE SCHOELLER: It’s probably the most important nutrient that we consume on a daily basis. If you’re stranded in a desert, and cannot access any water, and don’t drink anything, life expectancy is about three to five days. You’ll dehydrate, and dry up, and die.

And also now the data is growing that even drinking a fair amount of water per day but not quite enough to meet your needs or drinking too much water, getting a little bit over hydrated, affects your chronic disease status. So it increases the risk of diabetes and other chronic diseases if you have a little bit too little or too much.

KATHLEEN DAVIS: And what is the body actually doing with that water?

DALE SCHOELLER: It’s the milieu in which all our chemistry, all our metabolism occurs. The cells are about 90% to 95% water, so all the chemistry that’s going on, all our metabolic processes occur in the liquid state. It’s also responsible for carrying nutrients– the glucose, the other carbohydrates, fatty acids, and proteins– from where they’re absorbed in the intestine to the cells, where the metabolism goes on, and then taking away the waste products from those reactions, and delivering them to the kidneys to be flushed out in the urine.

KATHLEEN DAVIS: Can you walk me through some of the factors that influence what my water needs may be versus another person?

DALE SCHOELLER: Well, the biggest influence is body size. That old eight glasses of 8 ounces is about in the middle of the normal range, but it doesn’t account for variability from the large to small. It’s not individualized. It’s not what we call personalized nutrition now, what a specific individual needs.

The larger the person, the higher their metabolic rate, the higher their energy expenditure. That has the biggest influence, so it accounts for an extra two-fold variation between a small individual and a large individual. In addition, factors such as physical activity, how much energy you expend, how much CO2 has to be excreted in breath, and that carries water out of the body as moisture, so that affects how much you need to replace it so that you don’t dehydrate over the time of the day.

KATHLEEN DAVIS: Mhm, so I personally think a lot about water when I am sweating and if I’m going for a jog when it’s 90 degrees versus if I’m going for a jog and it’s 40 degrees. How much of this is temperature regulation?

DALE SCHOELLER: A fair amount, especially when it’s warm and you are sweating. Existing in a high temperature, high humidity area, where you do sweat a lot, can add about a quart to your water requirement on a daily basis.

KATHLEEN DAVIS: This is Science Friday from WNYC Studios. In case you’re just joining us, I’m speaking with Dale Schoeller, professor emeritus at the University of Wisconsin, Madison. We are talking about the amount of water that the human body actually uses every day. Let’s talk about this study that you did. So you didn’t just ask people how much they drink. Talk about how you actually measure output with labeled water.

DALE SCHOELLER: OK, the traditional methods and what was used for a long time was to ask people how much water they drank. But people underreport the water consumption. You just forget when you stopped at a water from to take a drink. You forget a cup of coffee. You forgot a cup of tea.

The water turnover that we measured with our technique is measured using the stable isotope tracer, a nonradioactive stable tracer, deuterium, which is present in normal drinking water. But we give a little extra, and that mixes across the body water pool. And then every time you drink some water or eat a food containing water, such as an Apple, that dilutes down the deuterium, and that added water, then, is eliminated from the body through urine or sweat because the body works hard to keep the amount of water in it at about a constant level from day to day, hour to hour, minute to minute.

KATHLEEN DAVIS: So to be clear, in this study we’re not talking here about the right amount of water for people to drink but the amount of water that you really observed people using.

DALE SCHOELLER: Correct. What we measure with is stable isotope methodology is the behavior in terms of how much water each individual is consuming during the day.

KATHLEEN DAVIS: So what did you find? What was the range of the average water use?

DALE SCHOELLER: What we found was a very large range. For example, among women the lowest water turnover was about 1.2 liters per day, well below that 64 ounces a day, but the high end was about 8 liters per day in very active, large women. So it was more than a four-fold range between the smallest and the largest individuals and those who are not physically active and those who are physically active.

KATHLEEN DAVIS: Where did this mythology of eight glasses of water come from? What is the mythology there?

DALE SCHOELLER: Well, that’s an interesting story, and the best we can trace it is that, in 1946, the Federal Government here in the US did a study of the requirement in which they had total control over the intake of individuals. And they came up with this figure that water turnover was about 64 ounces a day.

The media picked up on that and said, well, that must be what you have to drink, and the media missed that that total was the total water turnover from beverages plus food and moisture absorbed from the environment. So in converting that directly to water intake actually was overestimating the requirement for the average individual. So it was a communications error.

KATHLEEN DAVIS: So can we just say drink if you’re thirsty? Is that enough to keep us where we need to be?

DALE SCHOELLER: Most of us, yes. You’re thirsty when you start to dehydrate when your body water pool starts to shrink because you’re losing more water than you’re taking in, and you increase your urine production when you take in more water than you need to replace. Both have an effect on the amount of salts that are dissolved in your body water fluid and can be measured in the blood.

So the other thing you can do instead of just responding to your thirst and drinking when you are thirsty– to look at the urine color when you wake up. It should be a light yellow or a straw color. If it’s lighter, if it’s very pale, you’re probably drinking more water than you need to replace. If it’s a dark yellow or a little bit of the browns, you’re not drinking enough fluids.

It’s a good biomarker of where you’re standing in terms of relative hydration. In the elderly, those over 60, 65 years of age, the thirst mechanism starts to break down so that you don’t get as thirsty as you should be. And in fact, as we get heat waves, when they come through, it’s the elderly that suffer from dehydration. So they have to pay attention to drinking a little bit more than they feel thirsty for, especially when it’s hot and humid or if they’re working outside and sweating.

KATHLEEN DAVIS: Dale Schoeller is a Professor Emeritus in the Department of Nutritional Sciences and the Biotechnology Institute at the University of Wisconsin, Madison. Thank you so much for joining us.

DALE SCHOELLER: Thank you for giving me the opportunity to speak.

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