It is not fully clear how much water intake is needed by healthy people, although some assert that 6 – 8 glasses of water daily is the minimum to maintain proper hydration.
The notion that a person should consume eight glasses of water per day cannot be traced to a credible scientific source.
The effect of, greater or lesser, water intake on weight loss and on constipation is also still unclear.
The original water intake recommendation in 1945 by the Food and Nutrition Board of the National Research Council read:
"An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods."
The latest dietary reference intake report by the United States National Research Council recommended, generally, (including food sources):
§ 2.7 liters for women
§ 3.7 liters for men
Specifically, pregnant and breastfeeding women need additional fluids to stay hydrated.
According to the Institute of Medicine—who recommend that, on average, women consume 2.2 litres and men 3.0 litres—this is recommended to be 2.4 litres (approx. 9 cups) for pregnant women and 3 litres (approx. 12.5 cups) for breastfeeding women because an especially large amount of fluid is lost during nursing.
For those who have healthy kidneys, it is somewhat difficult to drink too much water but (especially in warm humid weather and while exercising) it is dangerous to drink too little.
People can drink far more water than necessary while exercising, however, putting them at risk of water intoxication, which can be fatal in particular, large amounts of de-ionized water are dangerous.
Normally, about 20 percent of water intake comes in food, while the rest comes from drinking water and assorted beverages (caffeinated included).
Water is excreted from the body in multiple forms; including urine and feces, sweating, and by water vapor in the exhaled breath.