“They” say that you and I should drink eight glasses of water a day for optimum health. The origins of this wisdom are shrouded in mystery. It is known as 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 no one really knows where it comes from or what the basis for it is.
Moreover, hardly anyone actually drinks that much water, as far as I can tell from asking my clients, and yet just about everyone feels 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 doesn’t cause cramping in athletes1 — and if it doesn’t even do that, it seems unlikely that subtle dehydration can be serious.
People who buy into the 8×8 claim tend to be passionate about it … and they are often selling water. Advertisements for bottled water commonly cite the books of a doctor — I won’t name the doctor here, for legal safety — who believes that chronic dehyration is the cause of many 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.
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.2
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.3 The best of all the popular skeptical articles I found on the subject — “Hard to Swallow,”4 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,5 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,6 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 appreciable 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”.7 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.”8 This is quite authoritative, and indicates that there is probably no significant nutritional hazard associated with people neglecting to drink more than they really want to drink.
Meanwhile, there clearly is a hazard in drinking too much. It can be dangerous, even lethal, as was recently seen in the tragic case of a California woman who hydrated herself to death.9 Studies have also shown that alarmingly large numbers of runners may be lethally over-hydrating, which is almost certainly a result of “water fever.”10
But … 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!”11
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 …
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.”
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 I’m rarely thirsty and my pee doesn’t get too yellow. But it’s doubtful that the risk is great. And other health problems? Dr. Shortt explains that she has never seen a case that could be solved just by adding water:
A lot of people probably do need to drink more water for optimum health, but it’s nothing to get anxious about. At best, some extra water might help with constipation or with headaches, it might take the edge off the symptoms of some other condition or help your immunity a bit. I’ve never seen a problem, certainly nothing serious, that could be solved by saying, ‘Oh, you just need more water.’
And she cautions against giving cheap, comforting advice:
People ask me to endorse their pet rules all the time. They want me to tell them they can have a glass of wine every day. I tell them if they want to prevent heart disease, there are much better ways. And if someone wants me to put my stamp of approval on blowing off the 8×8 water rule, I say, ‘No way. You need the water.’ But it’s not a big deal. Worst-case scenario you’re going to be slightly less healthy than you could be.
Hydration may be a factor in people’s health problems, but it’s never the whole story, and every case is different. I wouldn’t ever attempt to generalize and say that most people are suffering from dehydration. It’s just not that simple. Some people definitely are dehydrated, but I doubt that many of them are suffering all that much from it.
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 to worried about it … or who tells you it doesn’t matter. It does matter. It just doesn’t matter much!
I am a science writer, former massage therapist, and assistant editor of Science-Based Medicine. I have had my share of injuries and pain challenges as a runner and ultimate player. My wife and I live in downtown Vancouver, Canada. See my full bio and qualifications, or my blog, Writerly. You might run into me on Facebook and Google, but mostly Twitter.
Other interesting reading:
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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.
According to this report, over-hydrating (hyponatreamia) “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.”BACK TO TEXT