The idea that our good health depends on our drinking eight glasses of water a day has been promoted for decades and is still being promoted. Recent evidence shows that this is a myth.
Water is vital for health, yes, but eight glasses is excessive according to the department of dietetics and human nutrition at the La Trobe University in Victoria (Australia). I was told that it could be four eight ounce glasses of water and four eight ounce glasses of another non-caffeinated, non-carbonated beverage.
Unprocessed fruit and vegetables contain a lot of water and this counts toward our daily water intake. Did you know a baked potato is 75% water?
"There is nothing magical about water from a glass of water as opposed to water from a food or any other beverage" says Prof. Susan Barr of the University of British Columbia. There is nothing magical about water from bottles either.
I have been told that if you drink a caffeinated beverage you need to drink its equivalent in water to equalize the dehydration caused by the caffeine. However the researchers involved in this study have found that drinking caffeinated beverages such as tea and coffee do not lead to dehydration and count toward your daily water intake.
How do you know if you need water? Are you thirsty? Then you need water. There is no evidence we need to drink more water than what thirst dictates. Another way to tell is by the colour of your urine. Is it light in colour? You're good. If it's too light you might need to drink less water. If it's dark, you need more. Easy.
There is no evidence that eight, eight ounce glasses of water benefits health in any way and represents an urban myth. [source]