Eight glasses of water a day will just give you a lot of extra trips to the bathroom. Drinking extra water does not "flush out toxins" or offer any other useful function.
Why do so many people believe this rule? The number originally came from the National Academy of Sciences of the United States Food and Nutrition Board, which publishes recommended daily allowances of nutrients. The 1945 edition of the Food and Nutrition Board said "A suitable allowance of water for adults is 2.5 liters (about 8 cups) daily in most instances." This amount is based on the calculation of one milliliter of water for each calorie of food. HOWEVER, the Board also noted that most of the water you need is in the food you eat.
All foods contain water. Even the driest nut or seed has a lot of water in it. Furthermore, when food is digested, it is converted to energy, carbon dioxide and WATER. Most people can get the fluid the body needs from food, and they only need to drink enough water to prevent constipation .
Researchers have showed that plain water is not needed as long as enough fluid is obtained from other drinks and food. Twenty-seven healthy men consumed one of two diets for three-day periods and were studied in a lab setting. The first diet included plain water while the second omitted it, relying on only foods and beverages other than water for fluid. None of the nine measures of hydration were affected. Journal reference
A reasonable amount for a healthy human is one cup of water or any other fluid with each meal. If you have a problem with constipation you may not be drinking enough water, but if you are not constipated, you are getting plenty. You'll also want to replace fluids whenever you sweat a lot, particularly when you exercise or in hot weather. Drink water whenever you feel thirsty, but there's no health benefit from forcing yourself to drink eight glasses of water a day.