The issue of how much water is sufficient for one's good health has been severely battled and it will continue to encounter strong opinion differentiation for reasons only known to the people involved.
There have been, and surly there will be more studies(26, 27) and reports(28, 29) that question and oppose the commonly accepted recommendation to consume at least 8 glasses of water per day, claiming, people don't have to consume as much as 8 glasses. On the other side of the opposition, one study(11) found benefit in consuming 8 glasses of water per day; another report(30) is listing Adequate Intake (AI) throughout the lifespan of a human for different age groups, from 0-6 months (AI=0.7L/day) to 19-70+ for Men (AI=3.7L/day) and for Women (AI=2.7L/day); yet another report(31) says the average adult man must consume at least 2.9L/day, and the average adult woman at least 2.2L/day. A quick search on the Internet would reveal many more different recommendations for many different life situations for both, children(32) and adults(33).
It is obvious that there is no clear recommendation for water consumption, which is why, perhaps, the strong opposition to it. Though, one clear fact is that the human body constantly loses water through the lungs when we breathe out, in perspiration, in urine production, and in bowel movements.(6, 8, 34) So, the body is indeed in a constant need of water.
However, in today's advanced society, thinking that tea, coffee, alcohol, and other manufactured beverages are desirable substitutes for the purely natural water needs of the daily stressed body is an elementary but catastrophic mistake.(35) As the Beverage Trial showed, no beverage can be a substitute for the purely natural water.
Also, it is true that these beverages contain water, but what else they contain are dehydrating agents. They get rid of the water that they are dissolved in plus some more water from the reserves of the body.(6, 34, 35)
Furthermore, in today's world, our body's need for water is much higher than it once was. Our food is too rich, too salty, and too concentrated, and the use of dehydrating substances such as alcohol, caffeine, and tobacco is very widespread; as it requires more water to dilute the toxins that come into the body. Stress, overheated and artificially ventilated homes, offices, and stores, air and water pollution - all contribute to our increased need for water. As a consequence, large numbers of people do not realize that they are chronically dehydrated, much less that lack of water is the cause of many of their health problems. There is only one solution: drink a lot more water.(34)
Unfortunately, our thirst sensation, as it is understood today - a dry mouth - is not an accurate indicator of the body's actual water needs. If we don't feel thirsty, we tend not to drink water. We wait to become thirsty before we even begin to think of drinking water. The whole problem of health deterioration begins by this very attitude toward water intake - deficit management only - and even that is only in half measures. By the time the body reflects its thirst through invoking its thirst sensation, it is short of two to three glasses of water. We may drink only one glass, leaving the body with two glasses less than it needs. Unfortunately, this gap expands as we grow older. This process results in a slowly deteriorating loss of thirst sensation, so much so that the need for regular water intake as a sensation gradually becomes forgotten.(6)
A good gauge for the water needs of the body is the color of the urine. A well hydrated person produces colorless urine - not counting the color of vitamins or color of additives in food. A comparatively dehydrated person produces yellow urine. A truly dehydrated person produces urine that is orange in color. What this means is that the kidneys are working hard to get rid of toxins in the body in very concentrated urine. That is why urine becomes darker in color. Dark color urine is a good sign of dehydration, and colorless urine is a good sign that a person is well hydrated.(6, 35)
To sum up, many conditions, including modern day food, would affect the amount of water one person may need to consume daily. Hence, a person could drink all the beverages he or she wants, including water, but his or her urine output would show how hydrated or dehydrated he or she is, not considering the color of vitamins and food additives. So, it's up to every single individual based on their diet and conditions they live and work in to decide how much water he or she needs to drink daily until he or she regularly produces clear colorless urine to know that he or she is well hydrated.