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I know the consequences of drinking too little water (dehydration then death), and drinking too much water within a period of time (also death), but what are the consequences of consistently drinking slightly too much water?

I'm curious about the long-term health consequences of consistently having 1-3L of unneeded extra water per day (so for example, if I need X litres per day to be healthy, but I have X+1L or X+3L per day consistently). Aside from additional urination there's no obvious short-term symptoms but there may be a long term impact on health which I'm oblivious to.

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    Mind to tell what benefits of extra water do you expect? Beside possible (not firmly proven) decreased risk of kidney stones--mainly in those with personal or family history of kidney stones--there are no known benefits of drinking extra water, as far as I know. Also, I am not aware of any physical harm of drinking "X+3" liters of water per day except of constant visits to the toilet (if I find a link, I'll answer below). – Jan Dec 8 '16 at 13:01
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Your body should tightly control the water concentration (osmolality) via sensors in the brain (osmoreceptors) which send chemical signals (vasopressin) to the kidneys. These receptors are very sensitive and are capable of making you thirsty with only 1% dehydration.

So a normal person drinking extra water will just make more urine and pee it out. An abnormal person (elderly, alcoholic or brain/kidney-damaged) may not be able to produce/react to these chemical messages properly.

However the kidneys need to make sure that this extra urine doesn't take salt (predominantly sodium chloride) with it - the salt concentration is also tightly regulated. The urine it makes must therefore be dilute.

If you drink too much water for a prolonged period of time, you wash concentrated salt from the part of the kidney that works to concentrate urine (the renal medulla).

So when you don't drink all that extra water, your kidneys are less able to conserve water, you will not be able to make concentrated urine and you will actually become dehydrated (volume deplete).

Moral of the story: your body knows what it is doing. Drink when you are thirsty.

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