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8 Glasses Of Water A Day Is The Mother Of All Hydration Myths

We can take a breath and put down our water bottles. A couple of glasses of water in addition to fluids typically consumed in a day and you're good to go.

It's time I bust this one wide open.

Of all the nutritional myths out there, this one has got to be a contender for the mother of all myths.

Telling everyone that they need to drink an extra eight to 10 glasses of water per day, above and beyond what they get from other beverages and food, is ridiculous advice.

On the surface it seems to make sense. The human body is composed of about 60 per cent water and it needs water to function; we can only live for two or three days without ANY water at all.

So what's the problem?

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Water. The miracle nutrient with near magical properties

The reasons for guzzling water are as endless as they are spectacular, and in some cases, downright magical.

In all fairness, there's usually (always?) a speck of truth behind most myths.

Self-stylized "health experts" love to throw around nuggets like these and think that they're actually giving sound nutritional advice. These sound bites are catchy, easy to tweet and, to the unsuspecting person, make perfect sense.

In all fairness, there's usually (always?) a speck of truth behind most myths. It's true legitimate dehydration can affect cognition and we are always excreting toxins (thankfully) as part of urination, but pounding back extra water does not increase the amount of toxins excreted (a simple urinalysis shows this) and objective measures of skin hydration have shown that drinking water does nothing to make your skin dewy.

It's not good enough to simply parrot myths like these over and over again without proof. I want facts, not personal beliefs, and so should you.

Fluid balance 101

To understand how much water we need, a basic understanding of fluid balance, or the amount of water entering the body being equal to the amount of water leaving it, is important. Obviously, individual water requirements vary to some degree depending on the size of the person, age, sex, amount of physical activity/exercise, environmental temperature and humidity. I'm not disputing that, what I am disputing is the notion that everyone needs to consume eight to 10 glasses of added/extra water per day under normal, everyday conditions.

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How much water do we REALLY need?

As a general guideline, the Institute of Medicine recommends the equivalent of three litres (12 cups) of fluid (water) per day for men 19 and older and 2.2 litres (nine cups) for women of the same age to meet the body's normal fluid requirements.


To replace losses, a constant supply of water is needed, which we typically get from:

  • Food, which is 40 to 90 per cent water by weight (500ml to 1000ml, or two to four cups, per day)
  • Added fluids like water, tea, coffee, ice, soups, juices (1,500ml, or about six cups, and more per day)
  • Aerobic respiration, or "metabolism"; water is a byproduct of our biochemical processes (300ml, or just over one cup, per day)

This comes to a total of 2,300ml (2.3 litres) to 2,800ml (2.8 litres) or about nineto 11 cups per day, give or take.


Water is lost through normal physiological activities:

  • Respiration (breathing) and perspiration (we can lose about 500ml to 650ml, or just over two cups, per day)
  • Urination (we lose another 1,500 to 2000 ml, or six or eight cups on average per day; more if you drink more, less if you drink less), even feces (100ml, or just under half a cup)

This comes to a total of 2,100ml (2.1 litres) to 2,750 ml (2.7 litres) or about 8.5 to 11 cups per day, give or take.

In a word: balanced.

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Staying hydrated

Under normal circumstances, people (well, all animals actually — do we have to tell our pets to drink?) can meet their fluid requirements simply through their usual diets and drinking in response to their thirst. We've been doing so for all of human history. We would not have survived as a race if we had to consciously think about drinking water all the time; all that will do is give you urine that is almost 100 per cent water. The kidneys' job is to keep the electrolyte levels in your blood within a very tight range to prevent things like, oh, seizures. The kidneys will purposefully filter out all that extra water and send you to the bathroom multiple times per day.

Of course, there are circumstances when we can lose water faster than we can replace it and become dehydrated; excessive sweating during hot and humid or dry weather; exercising; occupations with increased sweating such as firefighters, landscapers and roofers; as well as the elderly can be at risk due to a decrease response to thirst cues, but for the rest of us, we can take a breath and put down our water bottles. A couple of glasses of water in addition to fluids typically consumed in a day and you're good to go.

As a general rule of thumb, if your urine is pale light yellow and clear, it means you're meeting your fluid needs. If it's very dark yellow and strong smelling, you could stand to top up your fluid intake. However, in my almost 20 years of practice, I've yet to encounter someone who's truly suffering from true dehydration outside of the hospital setting. Don't believe me? Track what you eat and drink in a typical day, do a 24-hour urine collection and you'll see that you are more than likely in perfect fluid balance.

Doug Cook RD, MPH is a Registered Dietitian and Integrative & Functional Nutritionist. Doug's practice at the Donvalley Integrative Digestive Clinic focuses on digestive and mental health.He is the coauthor ofNutrition for Canadians for Dummies(Wiley, 2008),The Complete Leaky Gut Health & Diet Book(Robert Rose 2015) and 175 Best Superfood Blender Recipes (Robert Rose, 2017). Learn more by checking out his website

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