Fact: 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.
We lose water constantly, by breathing, sweating, and using the toilet. But water loss is highly variable. On a cool day when a person isn’t active, eight glasses of water could be “well in excess of need, in which case a lot of water will be excreted” along with vital substances like sodium, Farrell says. Alternately, a person exercising on a hot day might need more than eight.
Drinking too much water can be deadly. Water intoxication, called hyponatremia, dilutes the body’s salt level, causing cells to swell. The condition is rare, but at least 14 athletes are known to have died from it, according to a 2015 study published in the Clinical Journal of Sport Medicine, which issued this new advice to athletes: Drink when you’re thirsty.