The Elemental Guide to Water

How Much Water Do You Really Need to Drink?

Scientists have a totally new understanding of thirst

This story is part of The Elemental Guide to Water, a five-part special report on the health benefits of water, the science behind seltzer, the truth about fancy H2O, the safety of tap water, and how much water you really need to drink.

The well-known “8 x 8” rule — you should drink eight 8-ounce glasses of water a day — is not only daunting, it’s unfounded. In fact, nobody is sure where the idea came from, and science doesn’t support it. “It has no basis in fact,” says Michael Farrell, a professor at Monash University in Australia, who studies how the brain responds to thirst and other sensations. Likewise, the old advice to “drink before you’re thirsty” is countered by the latest research, as scientists finally figure out how the brain knows when you’re thirsty, and when you’ve had enough.

The human body is 55–60% water, varying by individual (muscle has more water than fat). Blood is 83% water, and 70% of your brain is all wet. Water aids digestion, clears toxins from the liver and kidneys, removes excess sodium from the bloodstream, regulates body temperature and blood pressure, protects skin and other tissues, and keep joints lubricated.