How much water do we need?

The idea that drinking eight glasses of water a day is the healthy thing to do has been around since the 1940s. It’s not true, but at this point it’s a widely held myth.

On a site called Optimum Health, for example, I found this statement: “The average person needs 8-10 glasses of water daily to PREVENT DEHYDRATION. At Optimum Health we encourage our clients to become properly hydrated. If you want to … allow you body to function beautifully … you must give it a whole lot more water!”

How the eight glasses myth got started

Chinese medicine teaches that people should drink when they’re thirsty. The Chinese find it strange that Westerners strive to drink so much water. Public health recommendations in the West have now come to the same conclusion.

The misinformation originated at the end of World War II. The US Food and Nutrition Board recommended that adults consume one milliliter of water for each calorie of food. Women are smaller than men, on average, but a reasonable daily caloric intake for an adult, disregarding gender, is 1900 calories. 1900 milliliters is 64.2 ounces. There are eight ounces in a cup, so that’s where the eight came from.

Things went wrong when eight cups of liquid became eight glasses of water in addition to the liquid we get from food. The original recommendation of 64 ounces included both food and beverages.

In 2004 the Food and Nutrition Board put out a revised recommendation: 91 ounces (11.4 cups) for women and 125 ounces (15.6 cups) for men. The source of this amount of liquid could be coffee, tea, milk, soda, juice, fruit, vegetables, or any other food. There was no recommendation for how much additional water we should drink: “[T]he vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide.”

There is no scientific evidence that drinking eight glasses of water a day (in addition to liquid consumed in food) is beneficial for health. There are some medical conditions that benefit from increased water consumption, such as kidney stones and a tendency to develop urinary tract infections. But healthy people can stay healthy without the extra water.

Can you drink too much? Water intoxication

It rarely happens, but it is possible to drink too much water. Normal, healthy kidneys can process one liter of fluid an hour (4.2 cups). When the body takes in more than that amount – in a water drinking contest, for example – this throws off the electrolyte balance. This can lead to a potentially fatal disturbance of brain functions called water intoxication or hyper-hydration.

There have been a number of newsworthy cases. I remember reading about a young woman whose mother had died of cancer. She developed a fear of the disease and a belief that drinking water would prevent a similar fate. After standing in the shower and drinking continuously, she died of water intoxication.

Other cases involve, sadly, abuse of children who are punished by being forced to drink large amounts of water, leading to their death. Fraternity hazing has produced a number of deaths from water intoxication. In 2007 a 28-year old woman (a mother of three) attempted to win a Nintendo Wii game console in a radio show contest called “”Hold your wee for a Wii.” The contest involved drinking large amounts of water without urinating. She made it home from the radio station, but was found dead in her home by her mother.

Most healthy behaviors are a matter of common sense (like not texting while you drive). I’ve had acupressure students who believe drinking ice cold water burns calories and leads to weight loss. It’s true that eating cold food requires calories to heat up the food, leading to poor digestion. But the best way to use water for weight loss is to substitute plain water for high calorie soft drinks.