Live Long and Well with Dr. Bobby

Episode #14: eight glasses of water/day-important guidance or urban myth?

• Dr. Bobby Dubois • Season 1 • Episode 14

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Our bodies are 60% water and water  is really important.  But, It is not clear the origin of drinking  8 glasses of water per day. There were no studies that scientifically show that this number is the correct amount.  I vote that it is an "urban myth."  Good report summarizes important information about how our body uses water

But, water is crucial for us, just not that mythical amount.  Our body uses water in the following ways:

  • urine to excrete byproducts of what we eat (perhaps 2-3 glasses of water per day needed)
  • respiratory loss (perhaps 1 cup/day)
  • "insensible loss"-sweating to maintain body temperature (wide range depending upon temperature--2 glasses of water/day up to 1 gallon or more)
  • stool perhaps 1/2 cup of water per day

My take on the scientific evidence:

1.        The body is amazing at controlling our water balance.  Trust it!  There is no evidence based reason to focus on getting 8 glasses of water each day.  And, we get substantial fluids from our food/morning coffee/milk or sodas at meals...

2.        If you are thirsty, drink.  If your urine has gotten much darker, then perhaps drink.  Otherwise trust your body
3. these observations don't apply if you have kidney disease or are elderly.  Ask your doctor.

Tasty water "tidbits":  drinking extra water each day...

  •  does not seem to make our skin look young and full (might hydrate it a little but not clear if it looks better)
  • does not seem to affect our joints/cartilage (no evidence found here...)
  • does not reduce dry eyes
  • probably doesn't help us eat less 
  • DOES reduce the risk of recurrent urinary tract infections


Dr. Bobby Duobis:

Hi, I'm Dr Bobby Duois and welcome. To Live Long and Well a podcast where we will talk about what you can do to live as long as possible and with as much energy and vigor that you wish possible, and with as much energy and vigor that you wish. Together, we will explore what practical and evidence-supported steps you can take. Come join me on this very important journey and I hope that you feel empowered along the way. I'm a physician, ironman, triathlete and have published several hundred scientific studies. I'm honored to be your guide. Welcome everyone to episode 14, drinking eight glasses of water per day important guidance or just an urban myth? Now, this is a fun topic and I hope you will enjoy it as much as I did preparing it. As a disclaimer, what I will share is based upon having normal kidney function and not being very elderly. That situation is different and if it applies to you, you should talk with your doctor. Well, let me begin by sharing how I decided to do this topic.

Dr. Bobby Duobis:

As folks may know, I live in Texas, where it can be really hot. We have a ranch with exotic animals and a bed and breakfast and lots of work to do. I sweat a lot and occasionally get muscle cramps or feel lightheaded. I drink a whole lot of water in the summer to stay ahead of the loss from the heat. Now, when it's not hot, like during the winter, I sweat a whole lot less and I drink a whole lot less water. Well, along with millions of others, I had heard of the eight glasses of water per day and wondered huh, where did it come from? And should I pay attention to it or just drink when I feel thirsty? Over the years, I've also noticed that so many people now carry around a bottle of water with them Now, not just in the gym while working out, but all day long. This wasn't true 20 years ago or before that, when I was growing up. And of course, now we have a choice of fancy bottles and fancy colors. Why this focus on drinking water all the time?

Dr. Bobby Duobis:

The question sent me down the nerdy rabbit hole once again, this time on the origin of the eight glasses per day and whether there was a rationale or scientific evidence to support it. Do I need to carry around a water bottle too? Should I be drinking more water even when I'm not thirsty, or is it just an urban myth? Well, having learned a lot, perhaps more than I wanted. I thought it might be fun to summarize it for you, my listeners. Now I do take my job as a curator of the evidence seriously and hope that is actually helpful to you. Of the evidence seriously and hope that is actually helpful to you. This is not a life-changing episode and likely won't empower you to fundamentally change your life and live long and well, but examining urban myths, which is just an example of what we're doing today, is important and knowing which headlines to believe or discard is a really important skill and hopefully you will learn a bit from today and in similar discussions in the future. If you have other urban myths in the health arena you'd like me to explore, let me know at my website, drbobbillivelongandwellcom.

Dr. Bobby Duobis:

Well, as everyone knows, I start with take-home messages and once again, this does not apply if you're quite elderly or have kidney disease. Take-home message one the body is amazing at controlling our water balance. Trust it. There's no evidence-based reason to focus on getting eight glasses of water each day. That's the punchline. I believe that in fact, it is an urban myth. Take home message two if you're thirsty, drink. If your urine has gotten much darker, then perhaps drink. Otherwise, trust your body. And third take-home message. There's also some tasty tidbits I'll share about drinking more water. Not the eight glasses, but whether drinking more water will help your skin look better or make it easier to lose weight or lowers your risk of a urinary tract infection. I'll explore these as well.

Dr. Bobby Duobis:

Our plan for today begin with where did this eight glasses a day guidance come from? Second part of today share some background about our bodies and water and how our bodies use water. For daily functioning, we certainly need water. The issue is whether it's the eight glasses and the third part to explore the other water-related issues, like I mentioned, about weight loss, skin appearance and improving joint function. Let's dive on in together. Part one where did eight glasses per day guidance come from?

Dr. Bobby Duobis:

Now, despite the thousands of headlines I don't know, maybe tens of thousands, hundreds of thousands of headlines I couldn't find evidence to support the specific eight by eight guidance, meaning eight glasses each eight ounces. Everybody seems to have heard of it, but no one knows where it came from. There's even been articles searching for the origin of this urban myth or this guidance and nobody seems to know, and I couldn't find scientific studies showing the eight glasses per day was the right amount. Clearly, water is important but less clear, whether it's six glasses a day, eight glasses a day, 12 glasses a day or more. Now there were some reports back in 1945 by the US Food and Nutrition Board that suggested we need a fair amount of water, but they didn't define the amount back then and interestingly, they said most of the water you need is already in the foods that you eat, and they were probably thinking about canned soups and other kinds of things. Well, if there isn't evidence to support eight glasses a day, perhaps we can figure out a rationale for that guidance or maybe the amount we actually need. So this is now the next part of our discussion.

Dr. Bobby Duobis:

Well, simplistically, our bodies are 60% water and, simplistically, our bodies are 60% water. Because we are mostly water, it seems logical to say that we need to drink water, and a whole bunch of it. And we also know if you don't drink water, dehydration occurs. And dehydration is bad. It causes weakness, it can cause a headache, low blood pressure, mental cloudiness, even seizures and death. And drinking water when you're dehydrated makes you feel better. But here's the crux of the issue Just because the body is mostly water and just because dehydration can cause real problems does not mean that we need to consciously increase our water intake. This is a similar scenario when we talked about magnesium in the supplements episode. The body absolutely needs magnesium for so many things, but adding more as supplements has really not been shown to be helpful for most folks not been shown to be helpful for most folks.

Dr. Bobby Duobis:

The key point I'd like for you to remember from today is our bodies are really good at fluid management. We get thirsty if we lose just 2% of our body weight, which is about a quart and a half of fluids, and dehydration is really not until you've gotten to three quarts or more. And our kidneys are great at holding on to water or, if you have too much water in your body, excreting it. But we do absolutely need some water every day. And how much might that be? Well, the body uses water in a number of ways and maybe if we added up each of those ways, we'll come up with a number, and maybe that number will turn out miraculously to be the eight glasses a day.

Dr. Bobby Duobis:

If you're interested in being nerdy, there's a good and really complete discussion on the topic of how the body uses water and how much it needs, put out by the US National Academy of Sciences Institute of Medicine. It was back in 2005, but it's really an excellent report. Of course, this study and all the others I refer to will be in the show notes if you wish to look at it. So the first area of water need, and the one that is the most I can't say most important, but uses the most fluid on a typical basis is our urine, and our urine needs about two to three glasses of fluids a day. Now why does the urine need fluids? Well, because the body has a number of byproducts that happen from the food we eat. So we eat the food, we digest it, we break down and there are various byproducts that have to be excreted. Now there's a fair bit of stuff that has to be excreted. If you want to know the number, it's about 650 milliosmoles, which is a number you don't really need to worry about. But if you are nerdy like me, it's fun to know these things. And there's only so far.

Dr. Bobby Duobis:

Your urine can concentrate. It can concentrate a lot, but to get rid of those 650 milliosmoles you still need about 500 cc's a day, or a couple of glasses a day, just to deal with urine. And in the elderly and again my guidance and discussion doesn't apply to the elderly. Their ability to concentrate urine gets worse, maybe 20 to 30% worse, and so they need to urinate more to get rid of that same number of byproducts per day. And thus for the elderly they might need to drink even more than a younger person. And this goes the other way. The kidneys are great. If you drink a lot of water you can urinate five to 10 quarts a day, or maybe even more. So the kidneys can really manage both very little water in the system and a whole lot. So from urine we've got a two or three glasses a day minimum. So we're working our way towards the eight glasses, or at least something approaching that.

Dr. Bobby Duobis:

The second area is respiration. So every time you breathe in and out, out goes humidity. That's been estimated for a typical person about a cup of day. So we've got our couple of cups a day of fluids that go for the urine. Now we've got a cup a day for your respiratory. Now, if you're in colder weather you will breathe out even more fluids, and so that's something to keep in mind. Number three, and here is the wild card, the one that is so hard to know for sure, and that's the water, the fluids we use to keep our heat and temperature balance under control. It's called insensible losses, and these insensible losses could be sweating, it could be moisture coming out through the top of your head, and that amounts to one or two cups a day for a typical person in a usual indoor circumstance. Now, that sweating amount could be many quarts a day, could be a gallon a day, or even more for soldiers in the desert. So the number of cups of water you need that relate to sweating is hugely dependent upon the environment, which is why having any specific number like you should drink eight glasses of water today doesn't make sense, because it could be less than that if you're in a very modest environment in terms of heat, and it could be way more than that if you're like I am in Texas, sweating up a storm outdoors.

Dr. Bobby Duobis:

Here's a fun set of facts. As people know, we sweat. That's our way of getting rid of heat. Now, if you're a desert rabbit or you're an elephant, it's their ears. They have blood vessels in their big ears and that's how they dissipate heat. And you may well know that dogs pant, and they're panting not just because they're short of breath, but really that's their way of dissipating heat. The tongues have water on it and so by panting it's helping evaporation. So I mentioned we have a ranch and we have exotic animals. Our latest addition are kangaroos. So I've had a lot of fun getting to know kangaroos and that could be a whole podcast in and of itself. But kangaroos are remarkable about how little water they actually need and the way they dissipate heat is. They lick their forearms. So you'll see them licking their little tiny arms over and over again throughout the hot day, and that then is like an air conditioner it evaporates, cools them off and then they do it again. So this insensible loss through our skin is hugely important and it could be very modest or it could be huge.

Dr. Bobby Duobis:

Now I should mention that you know if you have a fever, you're going to again need more fluids. You're likely sweating. If you're vomiting, there's all sorts of reasons to think there's more ways you might need fluids. And there's one more. It's a small one, but there's one more and that's in your bowel movements, in your stools. Perhaps a half a cup of water is needed.

Dr. Bobby Duobis:

Now these are all the uses of water in the body, but the body also produces water. So when you eat and you break down sugar and carbohydrate and other things. It actually produces water and CO2. And you breathe out the CO2, but it actually produces water. Maybe it's about a glass a day. So, against all the losses that I just talked about before, you're also making a small amount of water. So where does this all net out?

Dr. Bobby Duobis:

Depending upon how hot it is and how much you're sweating, we might need four to six glasses a day, probably not the eight glasses a day. Of course, if you're in the gym a lot and you're outdoors when you're sweating, you might need more. But also keep in mind that you don't just drink glasses of water during the day. Your fluids come from lots of places. If you have soup or fruit, of course, your morning coffee or tea or sodas at a meal, or milk, that's all fluids. You don't need to drink all of the fluid amounts that I just talked about, because a lot of it will come in your two cups of coffee and maybe a soda at lunch and a soda at dinner or a glass of water at dinner. So a lot of it is going to happen during the course of the day. But ultimately you're going to need a quart or two quarts of water a day, probably.

Dr. Bobby Duobis:

Now what's my take on all this? For otherwise healthy people, if you're basically not 80 years old and you don't have kidney problems, if you're thirsty, drink. If you're not thirsty, there's no need to drink more. I did not find any evidence to differ with this. I didn't find for normal people that they got into dehydration trouble if they only drank water when they were thirsty. Or you can look at your urine and see if it's gotten much darker. So that, to me, is my take on this.

Dr. Bobby Duobis:

But people say, dr Bobby, I have a dry mouth and I feel like I should drink all the time. Like I should drink all the time. And I might answer you that yes, that's possible, that it's your thirst that's saying that and you need more water. But there are many causes of having a dry mouth, which could be a side effect of certain medicines. Or if you talk a lot for example, I'm talking a lot on this podcast my mouth is dry and I'm going to want to take a glass of water, or at least a sip or two, before I continue on. Or if you're a mouth breather, you may have a stuffy nose or whatever. All of that might make you think you need water in your body, but perhaps a throat lozenge or chewing gum might be sufficient, all right.

Dr. Bobby Duobis:

So what do you do in your lives? I try to make this as practical as possible for everyone and, as we say, week after week after week, I love N of 1 trials to figure out what you need. And let's figure out what your water needs are. So how might you do an N of 1 trial? Let's take our baseline. Where are we? Okay, what's my baseline weight? What is my urine color? Is it pretty light colored? And you can even do a dipstick in your urine. You can go to the drugstore and buy it and it'll absolutely tell you how concentrated your urine is. So that could be your baseline.

Dr. Bobby Duobis:

And then for a couple of days you might say I'm only going to drink when I'm thirsty. I'm not going to proactively drink glasses of water throughout the day, I'm just going to drink when I'm thirsty. Now, after a few days, take a look. Has your weight changed? Has your urine color changed or concentration, and see if it's pretty much held constant? I would say trust your body, you don't need to force feed the water throughout the day.

Dr. Bobby Duobis:

Now, perhaps a topic for another day. We could talk about drinking too much water, which can cause hyponatremia and its own set of problems. That's not a very common scenario for most people, but it is something that we might talk about and folks can let me know on my website or Instagram if this is a topic for them. Okay, now let's turn to part three tasty tidbits or other reasons that we might think about drinking extra water. Now, these are other potential urban myths, and again you might have others you might wish me to talk about.

Dr. Bobby Duobis:

So the first one does drinking more water improve the appearance of my skin? Look, we'd all like to have nice, healthy, young-looking skin, especially as we get older. Might drinking extra water help? Fortunately, there's actually been some studies that looked at this. Now, the couple of studies I'm going to mention were pretty small and these were done in women, but they basically said okay, we're going to look at the moisture in your skin, said, okay, we're going to look at the moisture in your skin. And now, during the observation period, we want you to drink two quarts additional fluids each day for the next month. And now what did they find at the end of the month? Well, they did find a little more water in some of the layers of their skin, but they didn't assess whether the skin looked better or fuller or more youthful. So does this say drinking extra water is going to help our skin appearance? I don't think so. I don't think the data really supports it. You may hear it on talk radio, you might hear it on podcasts. I just don't think there's a lot of evidence to support it. Is it going to hurt you by drinking that extra water? No, might you do an N of 1 study here. Take some pictures of your skin, drink the extra water, come back a month later, take some more pictures and see what you think.

Dr. Bobby Duobis:

Next, does drinking more water affect your joints and reduce joint discomfort? Well, the folks that say yes, indeed, would cite the fact that 70 to 80% of your cartilage in your joints is water, and then the synovial fluid, the lubricant that goes around, that is also mostly water. So if you didn't have as much joint lubrication, you're going to be at risk for injury. But will drinking extra water actually help? And the answer is I found no studies. I saw lots of things talking about it, but I saw no studies that support this. If folks can find some, by all means let me know.

Dr. Bobby Duobis:

Well, here's the third one. Will drinking more water prevent dry eyes? Many people have real problems with dry eyes, and certainly in certain times of year. Well, once again, it doesn't appear that drinking more water is associated with less dry eye. They did a population study of 51,000 people and about 9% of them had dry eyes. But then when they correlated or tried to see whether people who drank more water per day or less water per day, whether they had more or less likelihood of dry eyes, didn't make any difference. So this was an observational study, wasn't a randomized controlled trial, but again, no data to support it will prevent dry eye.

Dr. Bobby Duobis:

Well, how about losing weight? People talk about, oh, if you have a glass or two of water before each meal, it'll make you feel fuller and you'll eat fewer calories. This was studied. How wonderful. And in this study they basically said, 30 minutes before a meal, drink 12 ounces of water, which is about a cup and a half, and before a couple of meals a day. And when they then looked at whether people immediately consume fewer calories, in the young people, in this study and again it wasn't a huge study there was no difference Whether you had the water you didn't have the water before, and no difference. But in people who are older, they ate 50 calories less. Well, that's interesting. Now, 50 calories isn't a huge amount, but if you multiply 50 calories by a month, well, that could be a half a pound. And obviously, month after month after month, it might make a difference. But when they gave them a test meal at 12 weeks meaning three months into this there were no differences. So what's my take on this? I think it's a good time to consider an N of one trial and see if it helps you in the number of calories you eat.

Dr. Bobby Duobis:

There was another study I'll just briefly mention, where they had water in addition to a reduced calorie diet. And now we know, if you had a reduced calorie diet, you are going to lose some weight, and both groups lost weight. But they found that the group that had water two glasses of water before meals lost about four pounds more than the other group. So we have one study that didn't show much difference. Another study that seemed to show some differences. Again, we're back to the end of one trial. By all means, give it a go.

Dr. Bobby Duobis:

And finally, does water reduce the risk of a urinary tract infection? As people may know, especially women, it's pretty uncommon for men. Younger men are certainly people who aren't very, very old, with large prostates. Women have a lot more urinary tract infections than men Turns out. Drinking more water actually works. Let me say that one again, because all the other ones I wasn't very excited about making a difference. But if you have recurring urinary tract infections, increasing your water intake can reduce that.

Dr. Bobby Duobis:

So they did a study. This was a randomized control trial. There were 140 women, and these are women that had three or more episodes of a urinary tract infection in the past year and who traditionally had less than about a quart and a half, six glasses of water per day. This was a 12-month study, so a really good amount of time. And they said to these women we want you to increase the amount of fluids by about six glasses a day. So, beyond what you normally do, let's add six glasses a day For the women. In the prior year they had about over three urinary tract infections. In the year after they started the increased water regimen, it was about half that, and they had far fewer courses of antibiotics because they didn't get as many infections. In fact they had about half the need for antibiotics. So this urban guidance actually is supported by the evidence, and so this is a good one to end with, because we have real evidence that suggests it works. And this isn't the only study that showed that, and again, this will be in the show notes, if you're interested.

Dr. Bobby Duobis:

Well, we're at the end and it's time for our take-home messages. Keep in mind once again now, for the third time, it doesn't apply if you're quite elderly or have kidney disease. Take-home message one the body is, in fact, amazing at controlling our water balance. There's no evidence to support we need the eight glasses of water each day. Yes, we need water. It will vary if it's hot out or not, or if it's very cold out and you're breathing out a lot of moisture.

Dr. Bobby Duobis:

I believe it's an urban myth and you may need less than that and you may need more than that. Take a message too If you're thirsty, drink. If you're not thirsty, trust your body. Yes, you can also look at your urine. You could do a urine dipstick if you care, and you can see, and from the tasty, tidbit segments. Extra water seems to reduce UTIs, but doesn't seem to make our skin look young, our joints move better or stop our dry eyes. It may help with how many calories you eat. Let's do an N of 1 trial, share it with me and others and we'll see where it takes us.

Dr. Bobby Duobis:

Until next time, I hope you have a few new tools or ways of looking at topics to empower you and help you to live long and well. As always, I am honored to be your guide on this all-important health journey. Thanks so much for listening to Live Long and Well with Dr Bobby. If you liked this episode, please provide a review on Apple or Spotify or wherever you listen. If you want to continue this journey or want to receive my newsletter on practical and scientific ways to improve your health and longevity, please visit me at drbobbilivelongandwellcom. That's, doctor, as in D-R Bobby live long and wellcom.