Do We Really Need 8 Glasses of Water a Day?

Discover the truth about hydration: do we really need 8 glasses of water a day? Tune in as we investigate the science behind this recommendation and whether it holds water.

Episode Artwork by Dina Valkov, CC:BY

Shownotes: yournutritionprofs.com

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Transcript

Thirsty for knowledge? Discover the truth behind the age-old advice of drinking 8 glasses of water a day and uncover surprising insights that challenge conventional wisdom on staying hydrated. Prepare to quench your curiosity.

M: I’m Professor Megan 

S: and I’m Professor Susan, and we’re

Both: Your Nutrition Profs!

M: We are registered dietitians and college professors who have taught more than 10,000 students about health and nutrition. We have answered a LOT of questions about nutrition over the years – 

S: Some questions we get asked every year and some are rarely asked but very interesting.

M: We’re here to share our answers to these common (and uncommon) nutrition questions with you.

S: So bring your curiosity and let’s get started. 

Both: Welcome to our class

S: Hello! How is everybody today? We’re doing great. 

M: Great.

S: And hope you are too. What’s on the menu today Megan?

M: Our question today is quite timely as we’re get into the spring and summer months - and one I get all the time in class – how much water should I be drinking? Is the 8 glasses of water a day recommendation a myth or a must? 

S: Such a common question! And I mean I think we can all agree that water is a pretty important nutrient. 

M: Definitely. It’s considered our most indispensable nutrient. We can’t live for more than a few days without water! 

S: It is extremely important for our bodies. I mean 60-75% of our total body weight is water - we’re basically bags of water with some tisSs and organs and other stuff floating around in it. 

M: True. Blood, brain tissue, and muscle tissue are all more than 75% water, even our bones are about 50% water. 

S: That one always surprises me.

M: I know. 

S: Do we really need to drink 8 glasses of water a day though? Where did that recommendation come from? Is it even true?

M: Well – the answer is…we don’t really know where it came from. 

S: Wow. I mean you’d think this would be an easy yes/no answer….but on this podcast it never is.

M: Exactly and that’s why we’re here! So some people theorize that the 8 glasses per day originated from a 1945 recommendation by the National Research Council (NRC) to drink 1 ml of water for each calorie consumed. So at about 2000 kcals per day that’s about 2 liters or about 64 oz or as it is sometimes called: 8x8. 

S: So eight 8 ounce glasses of water a day - 8 x 8. 

M: Correct.

S: OK. So this information from the National Research Council  is based on research, right? 

M: Well actually…no. There does not appear to be any evidence of this recommendation being the result of any type of research study. 

S: Troubling.

M: Yes. 

S: Another possible source of the 8 glasses a day recommendation is from a 1967 book entitled “The Doctor’s Quick Weight Loss Diet” by a Dr. Irwin Stillman. This was one of those very low carb diet similar to the keto diet that we see today. It also required the dieter to drink at least 8-10 glasses of water every day.

M:  And here’s another possible source - In 1974, Drs. Stare and McWilliams from the Harvard School of Public Health, co-wrote “Nutrition for Good Health”. And in this they recommended around 6 to 8 glasses per 24 hours and this can be in the form of coffee, tea, milk, soft drinks, beer, etc. They also mentioned that fruits and vegetables are good sources of water. 

S: But there is no evidence cited to back up the recommendations that they make, so it suggests that it may just be their opinions. 

M: And this is why you always need to cite your sources!! 

S: Agreed! 

M: So this idea of 8 glasses has been around for a long time – at least since 1945 - but it doesn’t seem to be based on any scientific research.

S: Mmm that makes me a little bit nervous…I mean I feel like we were taught 8x8 in school…

M: Oh definitely, I remember that.

S: It’s still actively promoted online, you see it as the recommendation. And so some say just plain water, but other people talk about it’’s all fluids including even beer

M: Mmmm 

S: Which I kinda like…. and of course the water from fruits and veggies. 

M: And this is actually a point of contention when it comes to the 8 x 8 recommendation. Does it have to be literally 8 oz of water? Or does other fluid count? What about the water found in foods? 

S: I get that question every semester from students - every semester. And it sounds like its even more confusing and complicated than we originally thought.

M: Yes but more recent recommendations from the U.S. National Academies identify the Adequate Intake level of water as 11.5 cups a day for women and 15.5 cups (or 3.7 L) a day for men.

S: Wait. 11.5 and 15.5? I mean that’s way higher than the 8x8! And does that mean just plain water?

M: It is higher than 8x8, but this recommendation is for water from ALL sources, so other beverages and the water in foods. And they estimate that we get about 20% of our water intake  from food. 

S: OK So let’s do a little bit of math. If you deduct 20% from those recommendations, it becomes about 9 cups for women and about 12 cups for men of just plain water – right? 

M: Right.

S: I mean that’s still more than 8x8. 

M: Yes.

S: What about the U.S. Dietary Guidelines? What do they say?

M: Well they don’t give a recommendation for a specific water or fluid amount, but they recommend water and other calorie-free beverages like black coffee, tea, or flavored waters. 

S: Yeah you do have to be careful with beverages like juice and soda, the high sugar and calorie content of these can really add up fast and they’re easy to over-consume. 

M: So true! 

S: I mean you buy one out of a vending machine, it’s 20 ounces already or 2 and a half cups.

M: And 30 

S: grams of sugar. 

M: Grams of added sugar. Well, the Dietary Guidelines do provide additional guidance for two particular groups, infants and older adults. So for infants supplemental water is not needed in the first 6 months. After that, 4 to 8 oz of plain fluoridated drinking water can be provided with the introduction of solid foods and then you would slowly increase water intake after 1 year. 

S: And for older adults, the Guidelines do recommend that they drink “plenty” of water and other kcal free beverages and that they consume fruits and veggies to prevent dehydration. 

M: Well after reviewing the U.S. Guidelines, I went down a bit of a rabbit hole looking into what other countries recommend for their citizens. 

S: Easy to do.

M: Yes. But to be honest, they’re all pretty close to the 8x8 recommendation. Some like Australia and Mexico had recommendations again like “drink plenty of water” or “drink water in abundance”, but overall the specific recommendations basically matched ours. 

S: So, it sounds like the general consensus is that everyone across the globe should have about 8 cups of water a day, right? I have also seen posts on social media that 75% of adults are walking around dehydrated. 

M: Oh I’ve seen those too.

S: I mean people are carrying  water bottles around wherever they go like a security blanket and are like “oh my God I need water!”.  And you can buy devices and apps to remind you to drink water throughout the day. So would 8 cups a day actually be enough?

M: To be honest, we really don’t know. I mean I’ve heard that 75% figure too, but it is difficult to pinpoint where it comes from too. Again there doesn’t seem to be any scientific support for that number. And it’s fear-inducing.

S: Definitely, yeah. And assessing a person’s actual hydration status is notoriously complex. So how are we supposed to know how much to drink every day? And is it the same for everybody except infants?

M: You know I think that’s why the specific recommendations for individuals are either really generic or just don’t exist because it’s so complex. There are several reasons why an individual might need more or less water intake. One is climate – if you’re in areas that are hot and humid, or very dry, or at high altitude, or in cold weather which may decrease thirst – you may need to consciously drink more water than usual.

S: And another reason is exercise. I mean sweating is an important part of temperature regulation. So as you increase your physical activity, that causes more fluid loss, and the need to replenish water for sure, and possibly electrolytes, like sodium, chloride, and potassium. Individual needs vary greatly though. 

M: Yes.

S: Sweat rates and how salty your sweat is can be very different from person to person and from one day to the next. 

M: And some researchers think that the 8x8 exceeds water needs for most people - especially if they’re sedentary and live in a temperate climate.

S: I mean, remember most dietary recommendations, including those for water, are for generally healthy people. There are people with certain medical conditions who might  need more or less water.

M: Right. Those with chronic kidney disease or congestive heart failure may require fluid restrictions. And those who are pregnant, breastfeeding, or who are suffering with diarrhea or vomiting, or other infections - they’re going to need more water. 

S: So other than people with certain medical conditions, can’t we just drink when we’re thirsty to avoid dehydration?

M: For healthy adults the answer seems to be “yes”. 

S: Finally a direct answer! Yeah!

M: Yes! We all get thirsty and have the urge to drink. It’s a natural survival instinct. But for some people, like the elderly and those with some brain injuries, this thirst mechanism doesn’t work very well. So for these people a hydration plan may be required.

S: So thirst seems to be our best indicator of when to drink. But there is a back-up plan. You gotta love the human body!

M: Yup!

S: If the body needs water and we don’t drink, it can tap the kidneys for help. The kidneys save water, which reduces urine output and makes urine more concentrated. 

M: Exactly. And urine color can give you a clue about your hydration levels. It should be a pale yellow, and you should be urinating every 2-3 hours while awake. So if it’s darker than that  you may be dehydrated.

S: We’ve actually posted a urine color chart in the show notes at yournutritionprofs.com - so check it out to see if you’re well-hydrated! 

M: The type of foods and beverages you typically consume can also make a big difference in your hydration. So as we said earlier, many foods like fruits and vegetables in particular, provide quite a bit of water. 

S: They also contain so many other nutrients as well – I mean so many good reasons to consume fruits and veggies.

M: Definitely.

S: What about beverages that contain caffeine or alcohol? I mean some people say they cause dehydration. But what does the science say?

M: Again - There is some mixed information when it comes to caffeinated beverages and alcohol. They both serve as mild diuretics. A sciency word meaning they increase fluid loss through urine. However, most sources say that these can still contribute to total fluid intake when consumed in moderation. 

S: Well according to the CDC, the average adult in the U.S. is already consuming about 11 cups of fluids per day and about half comes from plain water. In older adults it’s a little bit less.

M: So I mean it sounds like most people are already meeting, maybe even exceeding the 8x8. 

S: Right. So what’s the bottom line on the 8x8 recommendation?

M: Well there doesn’t seem to be any scientific evidence for this recommendation. And like a lot of things on the internet, someone said it in 1945, and it just became part of the mythology around hydration. 

S: So it’s a myth. So we should drink when we’re thirsty and consume fruits and veggies with a lot of water content. I mean watermelon and cucumber they’re more than 90% water and even fruits like bananas, oranges, pineapples, carrots, broccoli, and dairy products like yogurt and cottage cheese are between 70-80% water.

M: If you live in a harsh climate, exercise regularly, or have one of the medical conditions we mentioned you may need more or less fluid. 

S: There are some guidelines for infants and the elderly so pay attention to those if it applies to your situation. 

M: And there is no real advantage to drinking more water than you need. Some people believe that drinking more water will flush out toxins or help your kidneys in some way, but that’s just not true. 

S: Right. I mean your kidney filters the equivalent of your blood volume about 36 times per day. So any excess water you drink is just a drop in the metaphorical bucket and it will not  magically improve your health.

M: So that’s it for our deep dive into the 8x8 water recommendation. 

S: Myth, not must. Thanks everybody for joining us today. And remember if you’ve got a nutrition question you want answered - let us know! 

M: Join us next time when we’ll answer the question:  why does spicy food make my nose run?

S: Can’t wait. 

Both: Class dismissed

S: We hope you enjoyed this episode. You can find the show notes and a list of sources on our website, yournutritionprofs.com. 

M: Your homework is to follow us at your nutrition profs on Instagram and to listen to our next episode. You can listen on Amazon Prime, Apple Podcasts, Spotify, YouTube, or anywhere podcasts are found. We’d appreciate it if you’d “like” us, write a review, subscribe, and invite your family and friends to join us too. 

S: If you have a nutrition or health question you’d like answered, let us know! We may even do a show about it! Send an email to yournutritionprofs@gmail.com or click on the “Contact Us” page on our website.

M: Thanks to Brian Pittman for creating our artwork. You can find him on instagram @BrianPittman77

Both: See you next time!