Do you need 8 glasses of water a day? Hydration scientists demystify the number

As summer heats up, there are more opportunities for spending time in the sun. But soaking up its rays also means you might dehydrate more quickly. You’ve probably all heard we should be getting eight glasses of water a day, but is that true? How much H2O do you actually need?

Why is hydration important?

“Hydration is the most essential nutrient needed to sustain life,” William Adams, adjunct assistant professor in the Department of Kinesiology at the University of North Carolina at Greensboro tells Inverse.

Between 50 and 70 percent of our bodies are made up of water; it’s essential for many cellular, metabolic, and thermoregulatory functions within our bodies. For example, water helps deliver nutrients to the cells, helps prevent infections, prevents constipation, aids joint health, and helps conduct nerve impulses.

Stavros Kavouras is a professor of nutrition at Arizona State University where he directs the Hydration Science Lab. He tells Inverse, “​​We keep discovering ways underhydration [affects] the body. Primarily during the last decade, we’ve found it can affect things like cardiovascular disease, dementia, glucose dysregulation, and diabetes development. So there are lots of data on chronic diseases you normally you would never associate with low water.”

Even cognition and driving ability can be affected by a lack of water, Kavouras adds.

Should we really be drinking eight glasses of water every day?

While eight glasses of water every day is an oft-repeated bit of health advice, it’s not quite that simple. The U.S. National Academies of Science, Engineering, and Medicine recommends that women consume between 2 and 2.7 liters of water per day and that men drink between 2.5 and 3.7 liters of H2O daily, says Adams.

For those of us not well versed in the metric system, that roughly amounts to nine to twelve, eight-ounce glasses for women and 11 to 16, eight-ounce glasses for men.

In the U.S., those recommendations include water found in food which Kavouras says accounts for about 20 percent of our water intake.

When do we need more water than what’s typically recommended?

Some people may need more than those baseline recommendations.

“If someone is working outside all day or exercising, or living in hot environments, the overall water need may be much greater,” Adams says.

That’s because of how and why we sweat. When we’re exercising or hot, the body produces sweat. Sweat, as you probably know, is mostly water. So if someone is sweating, they are losing water. If they are exercising in a hot environment, they’re losing even more water. The same is true for people working in hot environments and those who are required to wear heavy clothing or uniforms at work.

“Water needs may also increase for individuals visiting places at high altitudes like the Rocky Mountains. When someone is at altitude there is less oxygen in the air, which requires them to breathe at a higher rate,” Adams says.

Humans lose water when breathing, especially when the environment is dry.

“If someone is hiking in the Rocky Mountains — even if it’s not that hot — they are at risk of dehydration due to the increased volume of water lost due to breathing.”

While the immediate loss of water for someone hiking at a high altitude isn’t substantial, over the course of the day, “this can add up throughout the course of the day,” he says.

What are some early signs of dehydration?

If you’re enjoying the spectacular views of a Rocky Mountain hike, you may not be aware of just how hard you’ve been breathing, how long you’ve been walking, and how much water you may have lost. Fortunately, your body has some ways of letting you know that you might be approaching dehydration.

Adams says, “One of the earliest signs of dehydration is the sensation of thirst. It usually occurs when someone loses about two percent of their body weight in water.”

The more dehydrated you are, the stronger the sensation of thirst will become.

One of the best ways to gauge your hydration is how often you’re going to the bathroom and what your urine looks like when you do.

“You should be going to the bathroom every two to three hours throughout the day, so that’s about six or seven times per day,” he says. “And your urine should look more like lemonade and less like apple juice.”

If you’re not meeting that standard, try keeping water close by when you work. “Many people remember to bring water when they go out but forget about it when they’re home,” Kavouras says.

What are some good non-water sources of water?

Adams stresses that drinking water is the best way to maintain hydration, but fruits and vegetables can also be good sources of water. The same is true for beverages like milk, tea, and juice.

That said, keep an eye on the sugar content of your beverages. Sugar-sweetened beverages do contain water, Adams says “the long-term effects of solely consuming these can be detrimental on one's health.”

Can some beverages be dehydrating?

Adams says there’s a bit of a common misconception regarding dehydrating food and beverages. “All foods and beverages contain water, however the water content in these foods varies.”

The notion that caffeine is dehydrating is another misconception.

“While consuming high concentrations of caffeine could have a diuretic effect, consuming a cup of coffee has not been shown within the scientific literature to be a diuretic,” Adams says.

When someone drinks a beverage quickly — as people tend to do with coffee — the body doesn’t need all that fluid at one time, so it gets rid of the excess, causing you to look for the nearest toilet, which may be why many people think these beverages are dehydrating.

Alcohol may be the worst beverage when it comes to water retention, Kavouras says because it suppresses a hormone called vasopressin which helps you retain water. This lack of water retention can lead to dehydration.

Are sports drinks like Gatorade hydrating?

While sports drinks can be hydrating by aiding water retention, Adams says “the retention capacity of these beverages does not make a clinical difference in hydration status.” In other words, they are no better at rehydrating you than water.

“Sports drinks can be advantageous when someone is performing a strenuous exercise that is over 70 to 90 minutes in duration...anything less than that, there is no added benefit, and water should be the number one go-to choice,” he says.

What if you don’t like the taste of water?

Kavouras says people who don’t like the taste of water may want to add slices of cucumber or strawberry to liven up the look and taste of the beverage. “Lemon or lime also can make a difference without really adding sugar or calories in your drink,” he says. “Another thing that seems to help people is carbonated water.”

So drink up; your body will thank you for it.