The amount of water necessary per day?

Factual Questions
1

I found some sort of advice that said that one should drink a number of ounces of water equal to his body weight in pounds per day. I’m a solid 275, and while I find the “8 glasses a day” reasonable, 2.5 gallons seems like waaay too much. Is it really necessary for me to drink all of that?

2

Basically, if you’re thirsty, you should drink some water. If you’re not thirsty, don’t worry too much about it. Millions of years of evolution have given us a pretty good feedback mechanism for determining how much water we need.

3

ultrafilter is right. Even the old 8 glasses of water a day is based on a misunderstanding, a rather vital sentence is missed from the original recommendation. Here is one discusssion of how it came about.

4

All that said, there may be some good reasons to drink more water than you need.

Conventional wisdom holds that drinking lots of water will prevent your body from retaining water. That’s a good thing.

And it turns out that drinking water aids in weight loss (see here)–not much, but it’s easy, and does help.

5

A new water study just came out: Dietary Reference Intakes: Water, Potassium, Sodium, Chloride, and Sulfate

6

For medical and surgical patients, the time-honoured formula to calculate daily water requirements is:

100 cc/kg/d for the first 10 kg body weight
50 cc/kg/d for the second 10 kg weight
20 cc/kg/d for every kg thereafter

So, an 80 kg person would need 10 X 100 + 10 X 50 + 60 X 20 = 2700 cc/d

For a 125 kg (275 lb) person, we’d get 10 X 100 + 10 X 50 + 105 X 20 = 3600 cc/d

As an aside, there is some evidence that drinking “plenty of fluids” leads to a lowered risk of bladder cancer.

7

KarlGauss gives the necessary recommended fluid requirements per day. This value does NOT include additional fluid for higher than normal work loads or environments. On the other hand, the 100-50-20 formula was designed for children and is presumably less accurate for heavy adults. Also, working kidneys are skillful at extracting water from cola, tea, coffee, juice, fruit and lots of other stuff you eat and drink; so you will survive if you don’t drink your daily gallon of H2O.