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Hydration 101: How Much Water Do You Really Need?


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Eight ounce glasses of water, eight times per day: this has been the water drinking mantra pushed by health professionals, beverage companies, and popular media for decades. From the number of people sipping on water bottles on a constant basis, it seems like the “drink more water” message is getting through. But is this message accurate? And where did it come from?

It may surprise you to learn that there has never been any scientific evidence to support the “eight by eight” doctrine when it comes to proper hydration. The first recorded scientific endorsement of a water intake recommendation appeared as a brief footnote in 1945, when the Food and Nutrition Board of the National Academy of Sciences published its Dietary Guidelines. (1) And like many other nutrition recommendations, it appears that individual needs for water vary widely, and 64 ounces of pure water every day may actually be too much for some people. (2) So how do you determine how much water you need as an individual?

There is no evidence to support the recommendation to drink 8 glasses of water a day.Tweet This

How much water do you really need?

There’s no question that adequate hydration is important, as water is a critical nutrient, but the eight by eight guidelines are a gross oversimplification. There is no universal requirement for water intake, and your needs will vary widely based on age, gender, body size, health status, and physical activity levels. Numerous environmental factors, such as high temperature and humidity levels, also influence water needs. (3)

It is difficult to estimate an exact amount of water needed for an individual, so thirst should be used as a general guideline for most individuals. In other words, if you are thirsty, drink, and if you’re not thirsty, don’t force yourself to drink simply because you believe it to be a healthy practice.

Additionally, the eight cups a day recommendation typically does not account for the water content of food. Many Paleo staples are surprisingly high in water; besides just fruit and vegetables, foods like yogurt, salmon, eggs, and potatoes are about 75% water, and four ounces of broiled salmon provides about one half cup of water. (4) If your diet is full of water-rich fruits, vegetables, and animal products, you may not need as much liquid water as you think. And if you’re eating soups or broths, that volume should certainly be counted towards your daily fluid intake.

Let thirst be your guide

Thirst is a sufficient indicator for most people of hydration status. From an evolutionary perspective, thirst has done a pretty good job of enabling us to survive as a species. While many believe that thirst is an indicator that a person is already dehydrated, this claim has not been substantiated by any research. (5) Thirst actually begins when the concentration of blood, an accurate indicator of our state of hydration, has risen by around two percent; experts generally define dehydration as beginning when that concentration has risen by at least five percent. (6) So while thirst is a good indicator that a drink would help maintain good hydration, it doesn’t necessarily imply dehydration.

Exceptions to thirst guiding hydration might include athletes engaged in exceptionally strenuous activity, when dehydration is far more common and adequate rehydration is essential to athletic performance as well as good health. Drinking plain water can improve performance in endurance exercise, but there are further performance improvements when carbohydrate and electrolytes are added. (7) Sodium should be included in fluids consumed during exercise lasting more than 2 hours, and is beneficial for aiding rehydration for anyone engaged in moderate activity. (8)

People with health conditions that affect their thirst, such as diabetes or kidney disease, may need more precise estimations of fluid needs on a daily basis. Excessive thirst is a symptom of hyperglycemia, among other diseases, and not necessarily an indication of dehydration.

If you find yourself having constant thirst despite drinking regularly, you may have a serious condition that needs treatment from a medical professional. Again, individual needs will vary greatly, so pay attention to your own health.

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What should you be drinking?

Pure water is probably good enough for recreational athletes engaged in mild to moderate activity. Those doing intense training or those who sweat excessively, however, will need electrolytes in addition to water, and possibly even sodium tablets. Sodium is essential to avoid hyponatremia, a serious condition caused by a lack of salt in the blood, leading to water imbalance and water build-up in the brain. (9) Therefore, sodium is one of the major electrolytes that is essential to include in a rehydration beverage following strenuous exercise.

There are many great natural alternatives to commercial sports drinks for replacing electrolytes during and after strenuous exercise. Bone broth is full of minerals such as calcium and magnesium, which can help with muscle contraction and relaxation, as well as amino acids for improved muscle and joint repair, making it a great choice for athletes. Coconut water has a good mix of electrolytes and simple sugars to aid in sports performance, though extra salt may need to be added to some brands that are lower in sodium to optimize rehydration.

Homemade sports drink recipes can be found around the web; a great one can be found on Wellness Mama’s blog. Fermented pickle or sauerkraut juice is another great way to boost hydration, especially for those who need the extra sodium and electrolytes. An added bonus of drinking these fermented juices is that they also contain probiotics to aid gut health. Even just adding a pinch of sea salt to your water bottle should be sufficient to aid in rehydration after a trip to the gym.

Take home message: Drink what you want!

The best advice I can give when it comes to water consumption is let your thirst be your guide, and adjust your fluid intake depending on your lifestyle and environment.

If you are relatively sedentary, live in a cold climate, don’t sweat much, or eat water-rich foods, you probably don’t need to be drinking eight cups of water a day. There’s no evidence to support those recommendations, and forcing down excess water throughout the day is not only unnecessary, but can cause damage if done too frequently. For most people, the body is its own best guide.

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Join the conversation

  1. I wonder if you have read Waterlogged by Dr. Tim Noakes.

    If not I think you should.

    You may re-evaluate your opinion of the need and benefit of electrolyte drinks during exercise.

    On top of that it contains an absolutely breathtaking evolutionary view of thirst and bipedalism that alone make this book an essential read.

  2. “Think back to biology.” I took biology in college — one semester 50 years ago. I was a music major. So now I just sit at my computer and happily soak up the knowledge of others such as yourself — and not infrequently display my ignorance of the whole subject. 🙂

  3. Thanks, DePaw. No wonder I’ve felt absolutely water-logged on the few times I’ve actually tried to do the 8 x 8 thing! The part about “produced by the metabolism” is interesting. I would never have thought of that as a source of hydration.

    • Think back to biology. Remember respiration? Glucose + oxygen –> carbon dioxide + WATER!

  4. Fibre Menace has a great bit on the origin of the ‘8 glasses a day’ thing.

    Page 48:
    “A person weighing 70kg [155 lbs] requires at least ca. 1,750ml [59oz] water per day. Of this amount ca. 650ml is obtained by drinking, ca. 750ml is the water contained in solid food, and ca. 350ml is oxidation water.” This quote is from ‘Human Physiology by Schmidt and Thews.

    The book goes on to say that 1750ml is about 7.5 ‘glasses’, and was rounded up to 8 glasses for the slogan. But this includes water in food and produced by the metabolism, so only 650ml or 2.5 ‘glasses’ actually needs to be drunk.

    Merck Manual of Diagnostic and Therapy is quoted saying “a daily intake of 700 to 800 ml is needed to match total water losses and remain in water balance”

    This is only 3 ‘glasses’ and includes drunk water and that within food too!

  5. From what I’ve read, the 1945 Food and Nutrition Board recommended 1 milliliter of water for each calorie consumed, which worked out to about 64 ounces of water per day. BUT the rest of that statement: “. . .but most of this quantity is contained in prepared foods,” somehow got lopped off, and the 64 ounces has taken on a life of its own. As with everything else, there are clearly a variety of needs for a variety of individuals, but Chris, you are right to question the conventional “wisdom” of 8×8 per day for everyone.

  6. Thank you for clarifying this Chris. I sometimes hear from clients that they cannot get enough ice water. Is this indicative of something in particular? Thank you for your time.

  7. My husband is an active 75 ad never feels thirsty or hungry.. he does not like water either. I have real trouble getting him to drink at all. I need to make him cups of tea several times a day. He complains about a dry mouth and brown urine. One day last august he was left alone at home for 10 hours, and when I returned he was in a state of collapse – he had neither eaten nor drunk all day. I do not know how to persuade him to take care of himself!!

  8. Dear Chris, a very interesting article this is! I was wondering what your view is in respect of the work of the following people:
    – Dr. Corinne Allen (http://www.brainadvance.org), doctor and proponent for natural health for over three decades, as well as an international researcher. – her 4 part lecture called ‘Brain on Water (part 1’ …2, 3, and 4) is available on youtube.
    – Dr. Batmanghelidj, M.D. (author of many books, one of which is ‘Your Body’s many cries for water’)
    – or Dr. Barbara Wren (www.natnut.co.uk), author of ‘Cellular Awakening’
    And there are many more…

    To comment on what you’ve said, i absolutely agree that the ‘optimum’ water intake does depend on the person’s diet, size, genetic disposition or how much exercise we take or if they have a condition. The question is water just water? And what water provides the best and most effective hydration and is most helpful to your body? I’ve recently invested in a Japanese innovative technology by a company called Enagic that manufactures home units which not only filter your water whilst preserving the minerals, but which also turn your ordinary tap water into a micro-clustered, ionised, alkaline and highly antioxidant water called Kangen Water. Due to its unique properties the water acts as a free radical scavenger (as it’s negatively charged), alkalises your body and provides instant hydration as the molecules of the water are much smaller comparing to ordinary tap water. This kind of water is available by so called ‘healing waters’ or waterfalls where exactly the same process happens naturally. But how many of us live by a waterfall these days? 🙂
    I’ve done a lot of research and my due diligence for the past few months, still reading books and scientific reports and the findings are just incredible, including people’s testimonials and so I love sharing this with people as optimum hydration is a necessity.
    If anyone wants more information feel free to contact me or visit http://www.kangenwateredinburgh.com to sign up for a free newsletter and request a free e-book. There’s a great website put together that explains the rationale behind the technology.
    password: scotland

    I look forward to your comments! 😉

  9. Great post, Chris.

    Now that it’s hot outside, I really like sipping iced green tea throughout the day – some caffeinated and some decaf. I find if I don’t have something to sip with me, then hydrating is out of sight, out of mind, and I don’t get hydrated enough. I don’t think I’m overdoing it now, but I’m quite sure there have been times when I’ve underdone it.

    Several people commented on filtered water. I wonder if you’ve done a post on that. If so, will you please reply with the link, and if not, might you consider doing one?

    I found this link, which rates the various ways of filtering water: http://bit.ly/OeOnf7 It would be great if you would comment on its accuracy.

    Many thanks.

    • Susan,
      I have the whole house system (installed May 2011) that received the “A” grade on your website link. I am very happy with it and know that my water is much improved. I didn’t really study the rating method employed by the website but they didn’t compare very many systems with each other. I had a plumber install it for me under my house (crawl space) and I change filters in it quarterly and annually (different filters). After 3 years or 300,000 galloons I will replace the main unit.

  10. I definitely have something wrong with my thirst signals in that they don’t really exist or alert me when I need to hydrate myself until I am dehydrated. I never feel thirsty. I was having dizzy spells from dehydration and still never felt thirsty. I have to make myself drink water and rely on the color of my urine to tell me if I’ve had enough water or not.

    I do notice that the more water I drink the more I want to drink and the less I drink the less I want to drink… maybe when I drink more of it I activate my thirst signals somehow? Not sure, but it has always been a mystery to me why I’m never thirsty and on the rare occasions I do feel thirsty I can’t believe it. Guess for the most part I am getting enough, but again, even when my urine is darker yellow and I know I’m not drinking enough water I’m still not thirsty.

    • I seem to be losing my thirst too. I’m only 31, not 90. I would like to know why I am losing it. In the past, oh, month I’ve probably been thirsty a dozen times.
      The thing is, I know my mouth is super dry, but drinking water isn’t quenching. I have to force it down.
      I’ve got celiac disease and seem to have issues with sulfites.
      I really want to know what’s causing me to loose my sense of thirst.

  11. I’m convinced that the SAD and lifestyle (i.e. too much light, stress, and not enough sleep) are responsible for the bulk of our health problems (any arguments?). The diets and lifestyles in retirement/nursing homes is often sub-optimal in that regards (poor diets, prescription diuretics, poor sleep, etc). Those living a low-stress life and eating a traditional diet can probably rely on thirst as an intact cue. People living a crappy lifestyle are probably dehydrated, and for those people swilling a couple litres of water will help temporarily, but only by correcting diet/sleep, etc will their systems correct itself.
    I think Chris’ recommendations should be taken in proper context…sort out your lifestyle and you probably don’t need to carry a Nalgene from your belt loop.

  12. This line of discussion reminds me of the “sip all day” versus “drink as you want” at periodic times through the day. This whole hydration system back-pack craze has me in stitches. I see children and adults engaged in light to moderate activities with a tube hanging form their mouth all day. I often wonder if this has an effect on the thirst-signaling system in our body. Do their bodies “forget” how to be thirsty or are they always thirsty being accustomed to a “steady drip” .. I much prefer to carry a canteen or water bottle and drink as I am thirsty.

  13. A dentist told me that people on drugs such as anti-depressants are a group that do need to drink regularly, but in small amounts at a time to keep the mouth moist. Otherwise, they are likely to end up with either gum disease or fillings. This is because the drug alters the saliva flow, and not enough saliva changes the bacterial content of the mouth. Whichever bacteria change determines the gum/teeth status.

  14. Hi, Chris, great article. My question is about well water. When we moved here 20 yrs ago we put in a reverse osmosis system because our well water (the well is over 200′ deep) often has a slight sulfur taste. Now I worry that we should have been drinking the well water straight in order to get the minerals. Which is more beneficial/safer – to drink the well water, get the minerals and risk the bacteria/possible pollution, or to filter the well water and get the minerals somewhere else? We are out in the country but there is a huge military base nearby that could be polluting the groundwater. thx.

  15. Thanks so much for your take on the water recommendation.

    I love kombucha as an ‘electrolyte’ drink while hiking in the hot sun. I’ve been able to do the same hikes without getting dehydrated that I had suffered on while drinking lots of plain water. One bottle of kombucha to one bottle of plain water seems to do the trick for me.

    • Kombucha is awesome! It saved my back when I had a bulging L4-5. Reduced inflammation, gluten-free and McKenzie Press-Ups…..those saved my back. No surgery. I am on my way in a few to get Kombucha at the Farmers Market. Trying a new flavor: Pina Colada.

      Thanks for your reply

  16. I wonder how much water our paleolithic ancestors drank? They certainly didn’t have nalgenes. I bet our ancestors ran more dehydrated than we think.

    • Really. This whole drink-tons-of-water thing doesn’t ring true for me at all. It never has. Our ancestors didn’t have Nalgenes. Well said.

  17. We have a whole house water filtering system and to me the water just taste better because it is so clean tasting. Besides when I bath or wash clothes none of the toxic get into my skin or my clothes. I am all about fresh clean water.
    Great article…I too drink when I am thristy, some days more then others. Go figure.

    • Could not agree more about filtered water. Mine is an Aquasana whole house filter. Stripping out the flouride, chlorine, and who knows what else. The water taste is excellent. It’s just a shame I’m also using that to flush the toilets. 🙂

      • @Rufus, It is good to have it in your toilets too Rufus. If your toilets have treated water in them the Chlorine will evaporate into the air as the water stays exposed to the air. You then are breathing the chemicals instead of drinking them. My unit is a whole house Aquasana also, I am very happy with it.

      • The Aquasana system does not remove fluoride, also be sure to check if your municipality uses chlorine or chloramines to sanitize water because the system will only reduce chloramines not completely remove them.

  18. I have a whole house water filtration system and I keep glass water bottles in the refrigerator. When I go out of the house for work or errands I carry water with me. Throughout the day I sip water and carry my water into resturants when I go out to eat.
    Water filtered with a quality system and consumed on a regular basis (not to excess) is what works best for me.