It turns out the oft-quoted “drink at least eight glasses of water a day” recommendation actually has little underpinning scientific evidence.5 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.6 Consequently, for the longest time, water requirement guidelines for humanity were based on just one person’s urine and sweat measurements.
