How much water should you drink?

Most people think they should drink more water than they do. I remember when I was little there was an ad on TV telling us to drink 8 glasses of water a day. And the glass she was holding looked bigger than 250mls from memory. A commonly quoted amount of water to drink is 8 x 8 oz glasses a day. 8 ounces = 237mls which is 1.896L/day.
I had a frustrating discussion just the other day during my time away at Christmas. I am fairly sedentary and regularly drink around 2L of water a day and they were saying they thought I should try to drink more. The age-old issue of experience vs academia, or more commonly, anecdotes vs actual science, was brought up.

But do we really need to drink a shiteload of water every day?

A quick google search reveals that, despite this myth still being propagated even today, there’s quite a bit of noise trying to correct it. The majority of the results are trying to dispel or at least clarify the truth behind the myth.

More seriously though, we need to drink water for obvious reasons (homeostasis). If we don’t we either drink too little (dehydration), or drink too much (hyperhydration).

According to Wikipedia, dehydration…

Symptoms may include headaches[7] similar to what is experienced during a hangover, decreased blood pressure (hypotension), and dizziness or fainting when standing up due to orthostatic hypotension. Untreated dehydration generally results in delirium,unconsciousness, swelling of the tongue and, in extreme cases, death

If we drink too much,  one nasty result is “water intoxication“:

first observable symptoms of water intoxication: headache, personality changes, changes in behavior, confusion, irritability, and drowsiness. These are sometimes followed by difficulty breathing during exertion, muscle weakness, twitching, or cramping, nausea, vomiting, thirst, and a dulled ability to perceive and interpret sensory information

Drinking too much also leads to weeing a lot.

But what is the optimal amount?
According to this report by the National Academy of Sciences (American)

The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide. The report did not specify exact requirements for water, but set general recommendations for women at approximately 2.7 liters (91 ounces) of total water — from all beverages and foods — each day, and men an average of approximately 3.7 liters (125 ounces daily) of total water. The panel did not set an upper level for water.

Wikipedia also clarifies – “Food contributes 0.5 to 1 l/day, and the metabolism of proteinfat, and carbohydrates produces another 0.25 to 0.4 l/day,[6] which means that 2 to 3 l/day of water for men and 1 to 2 l/day of water for women should be taken in as fluid, i.e. drunk, in order to meet the Recommended Daily Intake (RDI)”

The Kidney Council of Australia and other sources also clarify that “From the kidney viewpoint, all fluids including those containing caffeine and alcohol should count towards your daily fluid total.”

I’ve been reading a very in depth review found here
This article discusses some interesting points, including:

  • the incidence of cancer of the urinary bladder was reduced significantly by a high fluid intake [for men] … the authors calculated that within this range, the risk of bladder cancer decreased by 7% for every 240 ml of fluid added

    I find this fascinating and I think it makes sense because if the things that can cause cancer are in contact with the bladder for longer if you pee less often. Also,

  • A similar correlation has been reported for colorectal cancer and premalignant adenomatous polyps… In some instances (79,86), the beneficial effects were apparent with as little as five glasses of water a day. As with cancers of the urinary bladder, there may be gender-related differences.


  • They found, at a 6-year follow-up point, that women who drank five or more glasses of water per day (1,185 ml or more) reduced their risk of fatal coronary heart disease by ∼41% compared with women who drank two glasses or less (474 ml or less). The comparable figure in men was 54% less risk. The effect was limited to water; in fact, the drinking of “fluids other than water” (coffee, tea, juices, soft drinks) appeared to increase the risk of fatal coronary heart disease.

These stats are crazy!

So, to answer the question, this excerpt from the conclusion of the review:

Thus I have found no scientific proof that we must “drink at least eight glasses of water a day,” nor proof, it must be admitted, that drinking less does absolutely no harm. However, the published data available to date strongly suggest that, with the exception of some diseases and special circumstances, such as strenuous physical activity, long airplane flights, and climate, we probably are currently drinking enough and possibly even more than enough.

The Kidney Health Australia website says “Drink to satisfy thirst, is a good guide. To satisfy thirst – water is the recommended fluid!”. Note: this seems to indicate another often spoken phrase – that “by the time you’re thirsty, it’s too late.” – is also a myth.

According to all this, drinking a moderate amount of water and drink when you’re thirsty. Who’d have guessed?


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