Drinking eight glasses of water a day just means you will spend a lot of time running to the bathroom.

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 recommended: "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. You can get most of the fluid your body needs from your food, and you only need to drink enough water to prevent constipation.

When you eat, the pyloric valve at the end of your stomach closes to keep food in the stomach. Then the stomach takes fluid that you drink and food that you eat and mixes them into a soup. Then the soup passes to the intestines and remains a soup until it reaches your colon. Only then is the fluid absorbed to turn the soup into solid waste in the colon. If you do not have enough fluid in your body, your body extracts extra fluid from your stool, which makes it hard and can cause constipation. You do not need additional water to "cleanse" or "remove toxins;" healthy kidneys provide that function without any extra help from you.

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 in hot weather. Drink water whenever you feel thirsty, but there's no benefit from forcing yourself to drink eight glasses of water a day.