" The eight-a-day recommendation can be traced back to a 1921 paper in which the author measured his own urine and sweat output and determined he lost about 3 percent of his body weight in water a day, which comes out to about eight cups. Consequently, for the longest time, water requirement guidelines for humanity were based on just one person’s urine and sweat measurements. "

― Michael Greger , How Not to Die: Discover the Foods Scientifically Proven to Prevent and Reverse Disease