There is no normal or medically recommended water intake. That youngsters carry around 2L bottles of water (or caffeine-loaded hi energy cyber pop) is, frankly a. silly and b. unnecessary.
To answer this question properly, you need to know three facts.
- Bladder volume – the human bladder volume is 300 (200-400) mls – i.e. quite small – a can of coke
- Frequency of urination – the average adult urinates approximately 5-7 times a day, depending on fluid intake
- Urine output – this will lead to a urine output of say 6 x 300 mls = about 2L (well 1800 mls!), based on drinking about 2.5L a day (500 mls is ‘insensible loss’, i.e. lost through sweat, sputum/mucus and poo).
OK OK. So how much water should you drink?
Answer. As we said, there is no normal (or desired) fluid intake
But about 2.5L is fine, for younger people. That can equate to 8 x 300 ml glasses of water a day. But that ‘water’ does not have to be pure water. It can be water in tea, coffee, mild, squash, fruit, fizzy drinks. The odd beer is fine too.
Better still, drink when thirsty, not when not.
If you drink more than that, you will wee more times a day. What goes in, comes out.
Many older frail people should probably drink less, as more frequent urination can lead to falls (and fractures) especially at night. Many older people learn this, so drink under 1.5L a day and are fine.
If you have kidney problems, do you need to drink a lot to ‘flush the kidneys’?
There is no evidence for this. As we at MyHSN say, drink if you are thirsty, not if not. Now we are getting somewhere. Right .. ?
Duration of urination
Interestingly, mammals empty their bladders on average for 21 seconds (standard deviation 13 seconds), despite a difference in bladder volume from 0.15 ml (mouse bladder) and 300 ml (human) to 18 L (elephant) and 300 L (blue whale).
[“Why don’t you time yours?!” MyHSN Ed]
