The mysterious origins of the “8 glasses of water a day” rule

by Gillian Mayman on October 22, 2012

An illustrated guide to parenting with science.

My ten year old is a world champion picky eater. His diet is basically limited to frozen waffles, pizza, and cereal with milk.

This made it especially panic-inducing concerning when he came home from school last week complaining that he had to drink water. Did he not like water anymore?

It turns out that his teacher had admonished the class to drink at least 8 glasses of water a day. He was even required to keep a “water journal” to track how much water he drank every day for a week.

The idea that you need to drink at least 8 glasses of water a day is ubiquitous in American culture. I’ve had doctors tell me this. I’ve read it on credible medical web sites. I’ve listened to a professor of public health discuss this as if it is scientific fact.

However, there is no scientific basis for the “8 glasses of water a day” rule. Even more intriguingly, the origins are somewhat of a mystery.

It might be this man’s fault.

Dr. Frederick J. Stare wrote a book in 1974 with Dr. Margaret McWilliams that contains one of the earliest known references in print to this admonition.

How much water each day? .. for the average adult, somewhere around 6 to 8 glasses per 24 hours

This is how most of us understand the issue.

However, what Dr. Stare actually wrote was:

How much water each day? This is usually well regulated by various physiological mechanisms, but for the average adult, somewhere around 6 to 8 glasses per 24 hours and this can be in the form of coffee, tea, milk, soft drinks, beer, etc. Fruits and vegetables are also good sources of water.

You can see that Dr. Stare understood the two most important points that are generally lost when people discuss how much water is needed every day.

  1. This is usually well regulated by our bodies. This is not something the average, sedentary person needs to be overly concerned about. We eat, we drink, and it takes care of itself.
  2. This can be in the form of other liquids and food. It does not need to be actual water. It can come from pasta or yogurt or an apple.

So why do we all think that we need 8 glasses of actual water each day? Dr. Stare seems to blame the rumor on this man.

Dr. Irwin Stillman created the popular “Stillman Diet” in 1967 and it is a precursor to the more modern Atkins diet. Dr. Stillman very clearly specified that eight glasses of water were needed each day and even offered a specious biological argument.

Dr. Stare is clearly dismissive of Dr. Stillman’s advice when he discusses the Stillman Diet:

[Dr. Stillman] insisted on a minimum eight 10-ounce glasses of water daily… The reason for the eight glasses of water is to provide sufficient water for the kidneys to use in washing away the fatty acids resulting from the breakdown of fat, says Dr. Stillman. A bit later, however, he writes, “The ‘why’ of this functioning is not fully understood.”

The origins of the “8 glasses of water a day” rule was explored by Dr. Heinz Valtin in a 2002 article and Dr. Tsindos  in a 2012 article. After extensive searches of the published literature, they found absolutely no scientific evidence for the idea that most people need to drink at least 8 glasses of water a day.

So how much water do we need? Clearly, athletes and people who live in very hot climates need more water. But for the average person, the amount of liquid that they take in just needs to match the amount that they lose through bodily excretions such as urine, feces, sweat, and exhaled water vapor. For women this is about 2.2 liters and for men it’s about 3 liters, according to the Mayo Clinic. [This translates to about 74 ounces for women and 101 ounces for men.]  However, the liquid that is consumed does not need to be water. It can be tea, coffee, soda, juice or other beverage. It can also come from foods that we eat which contain water.

There is an argument, of course, that water is a much better source of liquid than coffee, soda, or juice. There is certainly a case to be made regarding calories, sugar, and caffeine. But in terms of liquid intake that is needed, the average person’s body does not really care about the form that liquid takes.