“They” say that you and I should drink eight glasses of water a day for optimum health. “It is known.” The origins of this wisdom are shrouded in mystery. This is called the “eight by eight” rule among nutritionists: eight glasses of water, eight fluid ounces each, is supposedly what the average person needs to drink every day to remain healthy and hydrated. The claim is often made, but where does it come from? What’s the basis for it? It is not known — there are few weak possibilities.1
Moreover, hardly anyone actually drinks that much water — as far as I can tell from asking my clients and readers and friends. According to a British survey, average daily beverage consumption is about 56 fluid ounces, which is actually getting close.2 And yet just about everyone seems to feel slightly guilty about it and many believe that they are probably “chronically dehydrated” with unknown but ominous consequences! Could this be? After reading everything I can find on the subject, at least one interesting fact stands out clearly:
I am unable to find any scientific research which either supports or contradicts the claim. If anyone has ever carefully compared the health of people who drink water to people who don’t, I can’t find their report, and neither can anyone else.
It is a debate mostly without science. And yet there are many indirect clues. For instance, contrary to popular belief, we know that dehydration does not cause cramping in athletes3 — and if it doesn’t even do that, it seems unlikely that subtle dehydration can be actually be serious.
The conventional wisdom is often wrong. Crime didn’t keep soaring in the 1990s, money alone doesn’t win elections, and — surprise — drinking eight glasses of water a day has never actually been shown to do a thing for your health. Conventional wisdom is often shoddily formed and devilishly difficult to see through, but it can be done.
Malcolm Gladwell, Freakonomics, 2005
So popular that somebody must be selling it
Many of our habits and rituals are rooted in marketing. Orange juice for breakfast? The creation of a marketing campaign a century ago. Bacon with your eggs? Same: “the result of a marketing idea to help one company’s bacon sales,” based on a bogus “study,” no less.4
It’s the same with the modern habit of hydrating, although it’s not as tidy as the product of a single marketing campaign. But it’s come a long way, to things like this: “smart” water bottles that remind us to drink. SERIOUSLY? Yes, seriously.5
Or look at this fairly sophisticated junk science, supposedly showing that surprisingly mild dehydration can make you a bit pissy and foggy.6 If it could be believed, it would certainly be a great reason to hydrate regularly … but turns out it was funded by a giant corporation that sells bottled water! Conflicts of interest aren’t always deal-breakers, but that one probably is.
And that’s just the tip of an iceberg. There’s much more to read about hydration and dubious industry-funded science. From “Everything You Know About Cramps Is Wrong, And Gatorade Is Full Of Shit”:
… much of the science surrounding exercise and hydration has been underwritten by Gatorade, which obviously has an interest in pushing the notion of dehydration as a performance killer and hydration as the silver bullet. (In their book The Runner’s Body, Tucker and co-author Jonathan Douglas mention one fear-mongering study that suggests that “dehydration of 2 percent causes performance to decline by up to 20 percent.”)
The whole thing is terribly damning and makes you wonder if any good science about hydration has ever actually been done.
The creepier origins of water fever
Purveyors of bottled water and sports drinks didn’t cook up the idea that hydration is healthy — they’ve just exploited it. The original marketing campaign for a water consumption habit was about selling a water cure by convincing people that dehydration is dangerous.
Advertisements for bottled water routinely cite the books of a doctor who believes that chronic dehyration is the cause of many health problems. His ideas about water now dominate the discussion to a surprising degree. In fact, you can’t really read much about the subject without stumbling across his claims in one form or another. I am not naming the doctor here, for legal safety, but you can easily figure out who I’m talking about.
He attributes conditions like asthma, arthritis and chronic fatigue syndrome — and much worse — to chronic dehydration. Also obesity, cancer and depression! He claimed that drinking water was an effective therapy for all of these conditions, and many more. His books and website cite no scientific evidence to support this, and they are poorly written.
Many people encounter these ideas in the form of a piece of widely circulated internet junk mail that begins with the somewhat hysterical announcement, “75% of Americans are chronically dehydrated!” There is no evidence of any such thing — it’s just fear mongering. Snopes.com, a website devoted to debunking netlore and urban legends, dismisses this one as having no basis in fact.7
The belief that we must make a point of hydrating is rooted primarily in this old snake oil sales pitch: another ritual born to sell something.
The other side of the water story
Nutrition and medical experts invariably contradict the evangelical/paranoid form of the 8×8 claim. They dispassionately dismiss the idea that a small difference in our water intake has great importance, and offer clear scientific rationales for their skepticism (but still no direct evidence, unfortunately). Minor related myths are easy enough to debunk — “coffee dehydrates you,” for instance, is totally false.8
They cite no proven health benefits to drinking extra water, because there simply are none. It probably doesn’t even help with headaches, which is a common assumption.9
Drinking a gutful of water upon waking in order to cleanse the bowel, with numerous claimed health benefits — known as the “water cure,” one of several over the decades — became so popular in the mid-19th century that it almost defines old-timey snake oil. It certainly didn’t work any miracles, and may have done nothing at all, and it died out almost completely.
The best of all the popular skeptical articles I found on this subject — “Hard to Swallow,”10 by Benedict Carey of the Los Angeles Times — has been widely reprinted. Unfortunately, it relies only on expert opinion, not on evidence, which does not exist.
In 2002, Dr. Heinz Valtin published a more professional review of the subject in the American Journal of Physiology,11 and this is still probably the most thorough scientific paper on this subject. Valtin was forced to rely more on “extensive consultation with several nutritionists who specialize in the field of thirst and drinking fluids” than on direct evidence, since no direct evidence exists. In 2008, in an interview for Nutrition Action Healthletter,12 he comments, “I searched for 10 months with the help of a professional librarian. There wasn’t a single paper that gave any scientific support to this recommendation.” Dr. Valtin then comments on the origins of the 8×8 idea:
Q: Did you ever find out how the 8 X 8 advice started?
A: No. But what makes most sense to me is that as far back as the 1940s, the Food and Nutrition Board of the Institute of Medicine made recommendations on dietary intakes of nutrients, and one of them is water.
The board wrote that a rough rule of thumb would be one milliliter of water per calorie eaten. That would mean about 2,000 milliliters, or two liters, of water a day.
But then in the very next sentence, the board said, “and much of this can be gained from the solid food that we eat.”
It always surprises people that white bread is more than 30 percent water. And I think people forgot about the second sentence, so it became the rule to drink two liters of water a day.
Q: And people decided that coffee, tea, and soft drinks also don’t count because they contain caffeine?
A: Yes. in large, pharmacological doses, caffeine is a diuretic — it increases urine flow. But in the smaller amounts that one gets in normal amounts of soft drinks, coffee, or tea, it is not diuretic. Alcohol in moderation — say, a cocktail or two with dinner — doesn’t lead to significant fluid loss either.
So those beverages, which constitute one half or more of daily fluid intake for many adults, should count toward the total amount of liquid consumed.
The bottled-water industry is making billions of dollars on this myth.
The whole interview is well worth reading.
Perhaps you trust in Oprah more than any stodgy ol’ doctor? If so, her magazine put its weight behind the “just say no to water” campaign in an article wittily titled “You Need 8 Glasses of Water a Day … and other rules to blow off”.13 Once again, however, no actual evidence was presented: just the weight of expert opinion, including Dr. Valtin’s article, and the opinion of the nonprofit Institute of Medicine: “The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide.”14
Or take the advice baked right into the title of this letter to the editors of British Journal of Sports Medicine, by Dr. Timothy Noakes: “Time for the American College of Sports Medicine to acknowledge that humans, like all other earthly creatures, do not need to be told how much to drink during exercise.”15
Over-hydration actually is dangerous
So there is probably no nutritional hazard from neglecting to drink more than we really want to drink. But drinking more than we really want? In an abundance of caution, for fear of dehdryation? That’s another matter.
Dosis facit venenum.
Paracelsus, the father of toxicology
“The dose makes the poison.” Too much of anything will kill you. It takes a lot of water, but water can be hazardous to your health, even lethal. Consider the tragic case of a California woman who hydrated herself to death.16 That was a weird scenario.
Much more common and worrisome are the huge numbers of runners that are over-hydrating, which is almost certainly a result of “water fever.”17 Ironically, the effects of over-hydration can closely resemble heat stroke: the very thing many athletes assume they are hydrating to prevent.
See Scientific American’s article on this topic, “Strange but True: Drinking Too Much Water Can Kill,” for some good quality independent verification.
Just how much is too much? It takes quite a damn bit. Robbie Gonzalez:
Assuming rat biology and human biology are interchangeable when it comes to the quick consumption of large volumes of water, if 100 people each weighing 150 pounds each drank about six liters of water all in one go, around fifty of those people would die, and the cause of death would probably be water intoxication.
It is, thankfully, quite difficult to flummox a healthy pair of kidneys accidentally. It is a testament to the water-processing abilities of this organ that most cases of water poisoning are restricted to extreme instances of hazing and drinking contests.
What Happens To Your Body When You Drink Too Much Water?18
Let’s do the myth math
Is there anything really wrong with the basic 8×8 rule? Perhaps it’s excessive to get stressed about it, but is it actually unsound? What if it was recommended more matter-of-factly?
Although the Institute of Medicine’s report confidently declares that people can let their thirst be their guide … yet the report did set general recommendations for water intake at 91 ounces and 125 ounces daily, for women and men respectively, which is significantly more than you’d get if you followed the 8×8, which only comes to 64 ounces.
Perhaps we get the extra from our food? Not likely. Although the report claims that we get about 20% of our water from food, it’s not enough to make up the difference, and it’s “a misleading generalization anyway,” according to Naturopathic Physician, Nicole Shortt. “The only people who are getting that much liquid out of their food are the ones who are actually eating an ideal diet, rich in fruits and vegetables. Obviously, most people don’t eat that way, and probably need more actual liquid to make up the difference. Or they could eat more fruits and vegetables!”19
So, adjusting the water-from-food figure to 10%, that implies that about 82/112 ounces of our daily requirement should come from drinking … still significantly more than the 64 ounces recommended by the traditional 8×8 rule. And for men, 64 ounces is almost half — a whopping 48 ounces short — of what the Institute Of Medicine says we need. That’s another 6 glasses of water at least, just to make the minimum!
82 ounces = 10+ 8oz. glasses of water
112 ounces = 14+ 8oz. glasses of water
By these numbers, there is nothing nutritionally incorrect about the traditional 8×8 recommendation — indeed, it actually falls far short of what we are officially supposed to be drinking!
So what’s the problem? Why “blow off” the 8×8 rule, as Oprah’s magazine advises us? Why “let thirst be your guide,” as the Institute of Medicine recommends? Indeed, why not actually upgrade the 8×8 rule to 10×8 for women and 14×8 for men?!
Ask yourself …
Does thirst really drive you to drink … enough?
I felt thirsty just now, so I measured out eight ounces, and slammed it back in five biggish swallows: ah, refreshing!
Eight ounces of water — one cup, 250 millilitres — is a respectable amount of fluid, more than I want to drink in one go unless I am actually feeling thirsty. It’s an average glass of water, not large … but not small, either. I’ve got to want it, and I imagine I am not alone in this regard.
Is your thirst actually driving you to drink at least 10 glasses like that per day, if you’re a woman? More than 14, if you’re a man?
Mine certainly isn’t! We’re talking significantly more than the contents of a 2-litre carton of milk here (and just shy of two of those for men). That’s quite a lot! I might drink that much on an athletic day in the sun, but on a typical day of sitting here writing articles like this? I’m sure I don’t.
I have a sneaking suspicion (still no evidence, I’m afraid!) that thirst does not get people to drink the recommended amount. Dr. Nicole Shortt affirms this: “Perception of hunger and thirst is strongly influenced by personality and emotional factors. Only unusually healthy, self-aware people have accurate appetites and thirst sensations. Most people are eating and drinking more or less than they really need.”
So you’re a little dehydrated … so what?
Obviously, we really do need water, and ideally most of us should probably have the full amount recommended by the Institute of Medicine. However, the consequences of mild dehydration are probably not serious, even if the condition is chronic. The only plausible adverse health effect of mild, chronic dehydration is that it might increase the risk of kidney stones — they form more readily when your urine is more concentrated, and possibly avoiding kidney stones is all the reason I need to make sure that my pee doesn’t get too yellow. Lemon water may be even better at preventing kidney stones. But it’s doubtful that the risk of kidney stones is great with dehydration.
The debate about the 8×8 rule would be dead in the water if it weren’t for a profitable campaign of fear-mongering about chronic dehydration. The main reason that this article needs to exist is to contradict that pervasive and unjustified idea that “chronic dehydration” is some kind of health care epidemic. It is probably not. There may be such a thing as “chronic dehydration,” but the weight of expert opinion is clear: if it exists at all, it is not serious, and easily cured in any event.
There is no research about this question because there is no need for it. The only thing at stake is a clear but minor general benefit to your health. In short, there are more important things to worry about, and more important medical questions to study. Drink your 8 glasses per day (or 10, or 14), and ignore anyone who tries to get you too worried about it … or who tells you it doesn’t matter. It does matter. It just doesn’t matter much!
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About Paul Ingraham
I am a science writer in Vancouver, Canada. I was a Registered Massage Therapist for a decade and the assistant editor of ScienceBasedMedicine.org for several years. I’ve had many injuries as a runner and ultimate player, and I’ve been a chronic pain patient myself since 2015. Full bio. See you on Facebook or Twitter, or subscribe:
- Why Drink Water After Massage? — No reason! Massage therapy does not flush toxins into the bloodstream, and water wouldn’t help if it did
- Extraordinary Health Claims — A guide to critical thinking, skepticism, and smart Internet reading about health care
- Ugly Bags of Mostly Water — The chemical composition of human biology
- Get in the Pool for Pain — Aquatic therapy, aquajogging, water yoga, floating and other water-based treatment and injury rehab options.
- Other articles that critically examine popular ideas about health: Does Epsom Salt Work? and Quite a Stretch and Does Posture Matter?.
- Comedian Lewis Black has a great, cynical piece on water, very Carlin-esque (high praise), about ten minutes long. Not safe for work! Lots of foul language — but very funny.
Other interesting reading:
- “Eight Glasses of Water Per Day?,” David P. Mikkelson, Snopes.com. A readable and critical analysis of whether or not we need to drink extra water, from prominent urban legend debunkers Barbara and David Mikkelson.
- ““Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 x 8”?” Dr. Valtin’s scientific paper, although published for a professional audience, is quite user-friendly and worth reading. Or, read a much more accessible interview with Dr. Valtin.
What’s new in this article?
Four updates have been logged for this article since publication (2005). All PainScience.com updates are logged to show a long term commitment to quality, accuracy, and currency. more
Like good footnotes, update logging sets PainScience.com apart from most other health websites and blogs. It’s fine print, but important fine print, in the same spirit of transparency as the editing history available for Wikipedia pages.
I log any change to articles that might be of interest to a keen reader. Complete update logging started in 2016. Prior to that, I only logged major updates for the most popular and controversial articles.
See the What’s New? page for updates to all recent site updates.
2018 — Cited Vreeman on possible origins of the 8×8 rule.
2016 — Cited Spigt 2012 on headaches. Added a paragraph about the “water cure.”
2016 — Minor update today: just added a couple fun pictures, and a tiny bit of editing.
2016 — Added several examples of other personal habits that have clear origins in marketing rather than science. (Also, finally started logging updates, after years of unlogged updates.)
2005 — Publication.
- Vreeman RC, Carroll AE. Medical myths. BMJ. 2007 Dec;335(7633):1288–9. PubMed #18156231 ❐ PainSci #52815 ❐
One origin may be a 1945 recommendation that stated: A suitable allowance of water for adults is 2.5 litres daily in most instances. An ordinary standard for diverse persons is 1 millilitre for each calorie of food. Most of this quantity is contained in prepared foods. If the last, crucial sentence is ignored, the statement could be interpreted as instruction to drink eight glasses of water a day. Another endorsement may have come from a prominent nutritionist, Frederick Stare, who once recommended, without references, the consumption “around 6 to 8 glasses per 24 hours,” which could be “in the form of coffee, tea, milk, soft drinks, beer, etc.”
- Al-Jalali E, Shirreffs S. Water Consumption in Free-Living Adults. Br J Sports Med. 2010;44(14). PainSci #54838 ❐
How much water do people drink? Researchers studied 80 British people (40 men, 40 women). The results indicated that the women consumed more water than males. However, final results were similar to previous research: water consumption was about 2229 ml/day (give or take several hundred ml), with 1668 ml of that coming from beverages.
- Schwellnus MP, Drew N, Collins M. Increased running speed and previous cramps rather than dehydration or serum sodium changes predict exercise-associated muscle cramping: a prospective cohort study in 210 Ironman triathletes. Br J Sports Med. 2011 Jun;45(8):650–6. PubMed #21148567 ❐ If it only takes a little dehydration to cause a problem, what about a lot? In this simple “just check it!” study, researchers just checked triathletes and could not find a connection between dehydration and cramping, which would be easy to find if it was there. Cramping correlated with intensity of effort … not hydration. A scientific slam dunk.
- How Marketing Created Rituals Several other fascinating examples are discussed in detail.
From the delightfully snarky Gawker.com post about this for-realsies odious Kickstarter project:
HidrateMe is a “smart” water bottle that tells you how much water you’ve had and also glows when it is time for you to drink more water. No longer shall you be forced to constantly pass out from dehydration due to the fact that you didn’t know when it was time to drink water. Now, your plastic water bottle will glow for you. Alternately, you can look at your smart phone, which will clearly display statistics that show: you need to drink more water today. As you can see in the inspirational video above, a whole gaggle of highly educated twentysomethings gave their all to bring this plastic water bottle with a computer chip to market.
Wow. Just … wow.
- Armstrong LE, Ganio MS, Casa DJ, et al. Mild dehydration affects mood in healthy young women. J Nutr. 2012 Feb;142(2):382–8. PubMed #22190027 ❐
This research, funded in part by a giant corporation that sells bottled water, supposedly shows that surprising mild dehydration can make you a bit pissy and headachy.
The level of dehydration studied here is similar to what it takes to provoke thirst, and the effects on mood are presumably milder at the lower end of the range. So if the effect on mood is significant, we are probably also thirsty ... and if we’re not actually thirsty, the effect is probably pretty minor. That said, I might agree with the author’s conclusion that “increased emphasis on optimal hydration is warranted,” but I’m also guessing it’s not that big a deal, and I’m inclined to be rather cynical about it, because the conclusion is just so pitch-perfect for a study funded by a water bottling company.
Mood effects are not to be ignored, for sure, but they are also a lot less serious than the health effects that people tend to believe (mostly based on very successful fear-mongering by people selling ‘water cures’).
- Not a Sop to Drink. Snopes.com. Visited September 26, 2002.
- Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One. 2014;9(1):e84154. PubMed #24416202 ❐ PainSci #53892 ❐
Many people believe that coffee is dehydrating. To test this popular idea, 50 men drank four cups (200ml) of either coffee or water each day for three days while their diet and activity were controlled. There were no differences in their body mass, urine volume, and signs of hydration in the blood and urine (pee clarity, basically). If you can drink almost a litre of coffee a day and have no measurable effect on hydration, then it is not “dehydrating” to any meaningful degree. The authors reasonably concluded that coffee “provides similar hydrating qualities to water.”
- Spigt M, Weerkamp N, Troost J, van Schayck CP, Knottnerus JA. A randomized trial on the effects of regular water intake in patients with recurrent headaches. Fam Pract. 2012 Aug;29(4):370–5. PubMed #22113647 ❐ PainSci #53287 ❐
This study testing the effect of drinking extra water on headaches is the only one of its kind ever done as far as I know. It produced a perception of improvement with an extra 1.5L of water per day, and the way it’s summarized by the researchers you could easily mistake this for a positive result. But perception of improvement is a secondary outcome. As for the primary outcome: well, uh oh, there was actually no objective effect on anything that mattered: “Drinking more water did not result in relevant changes in objective effect parameters, such as days with at least moderate headache or days with medication use. There was no significant effect modification for headache intensity at baseline, age, gender, migraine, migraine with aura and tension type headache.”
So the perceived improvement was almost certainly just wishful thinking on the part of the experimental subjects, who had “significantly more positive expectations.” These results are actually evidence that hydrating does not help headaches. •sad trombone•
- Carey, Benedict. Hard to swallow. Reprinted from Los Angeles Times in The Topeka Capital-Journal. Visited September 26, 2002.
- Valtin H. “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 x 8”? Am J Physiol Regul Integr Comp Physiol. 2002 Nov;283(5):R993–1004. PubMed #12376390 ❐ PainSci #55664 ❐
Despite the seemingly ubiquitous admonition to "drink at least eight 8-oz glasses of water a day" (with an accompanying reminder that beverages containing caffeine and alcohol do not count), rigorous proof for this counsel appears to be lacking. This review sought to find the origin of this advice (called "8 x 8" for short) and to examine the scientific evidence, if any, that might support it. The search included not only electronic modes but also a cursory examination of the older literature that is not covered in electronic databases and, most importantly and fruitfully, extensive consultation with several nutritionists who specialize in the field of thirst and drinking fluids. No scientific studies were found in support of 8 x 8. Rather, surveys of food and fluid intake on thousands of adults of both genders, analyses of which have been published in peer-reviewed journals, strongly suggest that such large amounts are not needed because the surveyed persons were presumably healthy and certainly not overtly ill. This conclusion is supported by published studies showing that caffeinated drinks (and, to a lesser extent, mild alcoholic beverages like beer in moderation) may indeed be counted toward the daily total, as well as by the large body of published experiments that attest to the precision and effectiveness of the osmoregulatory system for maintaining water balance. It is to be emphasized that the conclusion is limited to healthy adults in a temperate climate leading a largely sedentary existence, precisely the population and conditions that the "at least" in 8 x 8 refers to. Equally to be emphasized, lest the message of this review be misconstrued, is the fact (based on published evidence) that large intakes of fluid, equal to and greater than 8 x 8, are advisable for the treatment or prevention of some diseases and certainly are called for under special circumstances, such as vigorous work and exercise, especially in hot climates. Since it is difficult or impossible to prove a negative-in this instance, the absence of scientific literature supporting the 8 x 8 recommendation-the author invites communications from readers who are aware of pertinent publications.
- Nutrition Action Healthletter [Internet]. Valtin H. In the drink: do we really need 8 glasses of water a day?; 2008 Jun [cited 10 Nov 2].
- Wildman, Sarah. “You Need 8 Glasses of Water a Day … and other rules to blow off.” O: The Oprah Magazine. April, 2005:208-209.
- Food and Nutrition Board, Institute Of Medicine Of The National Academies. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. The National Academies Press, Washington, D.C., Feb 11, 2004, full text. The quote here is from a press release about the report, not from the report itself.
- Noakes TD, Speedy DB. Time for the American College of Sports Medicine to acknowledge that humans, like all other earthly creatures, do not need to be told how much to drink during exercise. Br J Sports Med. 2007;41(2):109–111. PainSci #54183 ❐
- News.BBC.co.uk [Internet]. Why is too much water dangerous?; 2007 Jan [cited 10 Nov 2].
- Almond CS, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med. 2005 Apr;352(15):1550–6. PubMed #15829535 ❐ PainSci #55209 ❐
According to this report, over-hydrating (hyponatremia) “has emerged as an important cause of race-related death and life-threatening illness” in marathoners. Race-related death and life-threatening illness! From drinking too much water! The researchers found that hyponatremia does occur in a “substantial fraction” of nonelite runners, and the factors most likely to be associated with it are “considerable weight gain while running, a long racing time, and body mass index extremes.”
- Io9.com [Internet]. Gonzalez R. What Happens To Your Body When You Drink Too Much Water?; 2015 Feb 18 [cited 16 Feb 29].
- Shortt, Nicole, ND. Personal interview. August 25, 2005.