There is no evidence supporting this recommendation. "This common advice may be traced back to a 1945 recommendation: A suitable allowance of water for adults is 2.5 litres daily in most instances. An ordinary standard for diverse persons is 1 millilitre for each calorie of food. Most of this quantity is contained in prepared foods. If the last, crucial sentence is ignored it could be interpreted as instruction to drink eight glasses of water a day." Indeed, drinking excess water can be dangerous and may result in water intoxication, hyponatraemia, and even death.