8 glasses of water a day: Myth or medicine?

By John Murphy, MDLinx
Published October 10, 2018

Key Takeaways

Everyone knows that you need to drink 8 glasses of water a day. This advice has been around forever, so it must be true, right? Probably not. There’s no scientific evidence to support the claim that the average human needs to drink 8 glasses (ie, 8-oz cups) of water each day. Also, drinking a lot of water doesn’t necessarily provide the benefits that many sources claim. In fact, drinking too much water can be a problem.

The claim doesn’t hold water

No scientific study has ever concluded that we must drink 8 glasses of water per day, a 2002 review article emphasized. The likely origin of the oft-quoted advice may be a 1945 publication from the National Research Council’s Food and Nutrition Board that stated that an “ordinary standard” of water for adults is 1 mL for each calorie of food. So, a person who eats 2,000 calories per day would require 2,000 mL of water, or roughly 8 cups.

However, the subsequent sentence by the Food and Nutrition Board—“Most of this quantity is contained in prepared foods”—may have been ignored, and so this recommendation was likely misinterpreted as an instruction to drink 8 glasses of water each day.

Water vs fluid intake

Americans get nearly 20% of their fluid intake from food, according to the What We Eat in America component of the National Health and Nutrition Examination Survey. The remaining 80% of fluid intake comes from beverages, of which only a little more than one-third is plain water. Other beverages include soft drinks, coffee, and alcohol.

“You don’t have to consume all the water you need through drinks. You also don’t need to worry so much about never feeling thirsty,” wrote pediatrician Aaron E. Carroll, MD, in The New York Times. “The human body is finely tuned to signal you to drink long before you are actually dehydrated.”

Advantages of drinking extra water

Drinking extra water doesn’t necessarily provide any benefits for the average person—it depends on the situation and the individual. In regard to weight loss, people who drank water instead of sugary beverages with meals consumed fewer calories, although total fluid intake in study participants didn’t seem to change. In studies assessing the effect of fluid intake on kidney stones, researchers found that drinking more water led to fewer kidney stones, but drinking more tea and alcohol also reduced the risk of kidney stones. For urinary tract infections (UTIs), women who drank more water halved their number of UTIs as well as their number of prescribed antibiotics, but these were women who already had a low daily fluid intake (< 1.5 L).

Disadvantages of drinking extra water

It’s unlikely that the average person can drink too much water—we have a built-in satiation reflex that tells us when we don’t need to drink any more. Drinking too much water, known as water intoxication or hyponatremia, occurs more often in people with illnesses, such as kidney disease, or in people taking certain medicines, such as diuretics.

But sometimes people without illness—indeed some of the healthiest people, such as marathon runners and other extreme athletes—are the ones at risk for water intoxication. Drinking large quantities of water, combined with depleted sodium reserves due to physical activity, have led to fatal consequences in extreme athletes.

How much water is enough?

Just drink when you’re thirsty, advised Michael J. Farrell, PhD, MSc, associate professor, Department of Medical Imaging and Radiation Sciences, Monash University, Victoria, Australia, and one of the investigators of the aforementioned built-in satiation reflex.

“If we just do what our body demands us to, we’ll probably get it right—just drink according to thirst rather than an elaborate schedule,” Dr. Farrell said.

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